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september 2007 rev 1 1/35 AN2559 application note system power supply board for digital solutions introduction this document describes a power supply reference board designed for powering digital applications, such as cpus, fpgas, memories, etc. the main purpose of the board is to illustrate the basic principles used for the desi gn of the power supply and to give designers a usable prototype for testing and use. the trend in recent years in the supplying of power to mcus, cpus, memories, fpgas, etc. is to reduce the supply voltage, increase the supply current and provide different voltage levels for different devices in one platform. a typical example of this situation is the fpga. the fpga contains a core part which works at a low level voltage, the interface part placed between the core and the output, the system part, etc. it is important to note that each fpga family has a slightly different voltage level and the trend is to decrease the voltage for each new family. the lowest operating voltage currently available is 1 v, and this can be expected to decrease to 0.9 v or 0.8 v in the near future. a similar situation exists with other digital applications. typically, the main cpu, memory and interfaces require different supply voltage levels. low operating voltages also present another challenge - transient. digital devices are typically sensitive to voltage level. if the voltage drops below or crosses over a specific limit, the device is reset. this limit is typically 3 or 5%. on the other hand, digital device consumption can change very quickly (s everal amps in a few hundred nanoseconds). a power supply must be able to react very quickly with a minimum of over (or under) voltage, especially in cases where very low output voltage is required. there is additional stress placed on power supplies for digital applications in the industrial environment. the industrial standard bus is 24 v, but this voltage fluctuates and the maximum input voltage level required can reach 36 v. additional surge protection is also a mandatory part of power supply input for industrial applications. the goal of the board described in this application note is to cover all of the issues outlined above. it is intended mainly to satisfy industrial input requirements (operating voltages up to 36 v) and generate several output voltages for mid-range power applications (up to several amps). the main output voltage level can simply be set. www.st.com
contents AN2559 2/35 contents 1 main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 input part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3 pm6680a block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.0.1 power management block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.0.2 start-up/enable block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.0.3 step-down parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 dc-dc converters based on the l5970ad . . . . . . . . . . . . . . . . . . . . . . . 14 3.2 reset circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4 pcb layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 6 measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 6.1 pm6680a block - measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 6.1.1 efficiency and light load consumption modes . . . . . . . . . . . . . . . . . . . . 23 6.1.2 output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6.1.3 start-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.1.4 transient response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 6.2 l5970ad blocks - measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.2.1 efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 6.2.2 output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.2.3 transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 7 references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 8 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
AN2559 list of figures 3/35 list of figures figure 1. the steval-psq001v1 demo board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 2. block diagram of system supply board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 figure 3. schematic of input part . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 4. location and correct polarity of the input supply connector on the board . . . . . . . . . . . . . . 6 figure 5. electrical diagram of the pm6680a section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 6. the placement of the jumpers for start-up/enable settings. . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 7. skip mode connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 figure 8. output connector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 figure 9. jumper placement for v core voltage level setting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 10. jumper placement for v i/o voltage level setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 figure 11. output voltages of l5970a parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 figure 12. schematic of the two smps?s based on the l5970ad . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 figure 13. jumper placement for enable/disable function of analog output and output3 . . . . . . . . . . 16 figure 14. schematic of the reset circuit and board placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 figure 15. pcb top layer layout and first internal layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 16. pcb second internal layer and bottom layer layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 figure 17. efficiency of the dual step-down converter at full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 figure 18. pm6680a consumption at no load condition, in the different modes . . . . . . . . . . . . . . . . . 24 figure 19. output voltage ripple in different modes of light load operation . . . . . . . . . . . . . . . . . . . . . 24 figure 20. output voltage ripple of v core at the minimum input voltage (5 v) . . . . . . . . . . . . . . . . . . 25 figure 21. output voltage ripple of v core at the maximum output voltage (36 v) . . . . . . . . . . . . . . . 25 figure 22. output voltage ripple of v i/o at the minimum input voltage (5 v) . . . . . . . . . . . . . . . . . . . . 25 figure 23. output voltage ripple of v i/o at the maximum input voltage (36 v). . . . . . . . . . . . . . . . . . . 26 figure 24. start-up without setting the sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 25. start-up with a set sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 figure 26. load transient response on v core output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 figure 27. load transient response on v i/o output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 figure 28. efficiency of output 3, by input voltage level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 figure 29. efficiency of analog output, by input voltage level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 30. analog 5 v - output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 figure 31. v sys - output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 figure 32. analog 3.3 v - output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 figure 33. v aux 2.5 v - output voltage ripple. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 figure 34. transient response of v sys based on the l5970ad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 figure 35. transient response of v aux generated by the ldo kf25 . . . . . . . . . . . . . . . . . . . . . . . . . 33
main characteristics AN2559 4/35 1 main characteristics the main characteristics of the smps are listed below: input: 5 v - 36 v dc, surge protection outputs: the performance of the 6 outputs are described in ta b l e 1 below. table 1. output voltages (positive version) label v out i out max tolerance output1 (v core ) selectable from: 0.9, 1.0, 1.2, 1.5, 1.8 or 2.5 v 4 a continuous 6 a peak 3% output2 (v i/o ) selectable from: 1.0, 1.2, 1.5, 1.8, 2.5 v or 3.3 v 2 a continuous 3 a peak 3% output3 v sys 3.3 v 0.4 a (0.8 a peak) 4% output3 v aux 2.5 v 0.4 a 2% analog 5 v 5 v 0.8 a 4% analog 3.3 v 3.3 v 0.15 a 2%
AN2559 description 5/35 2 description the system supply board described in this ap plication note is a dedicated design which illustrates a typical solution for complete syst em supply, and can also be used as a direct supply for customer solutions during the design process. figure 1. the steval-psq001v1 demo board the block diagram of the system supply board is shown in figure 2 . there are four dc-dc converters, two linear regulators and a reset circuit. these parts are split into five relatively independent units: the input part, a dual dc-dc converter based on the pm6680a and generating 2 outputs (output 1 and output 2), two single dc-dc converters based on the l5970a (output 3 and output 4) with linear regulator, and the reset circuit. figure 2. block diagram of system supply board ai12693 input 5 - 36 v input protection skip mode settings pm6680a v core vi/o e/d + start up sequence settings e/d analog l5970ad l5970ad e/d v sys + v aux fb v i/o fb v core reset signal analog 5 v analog 500 ma 3.3 v analog 150 ma output 3 v sys 3.3 v 400 ma v aux 2.5 v 400 ma output 2 v i/o 1.0 - 3.3 v 2 a output 1 v core 0.9 - 2.5 v 4 a v i/o voltage settings v core voltage settings stm6719 lk112 m33 kf25
description AN2559 6/35 2.1 input part the input part shown in figure 3 consists of the input connectors (industrial - j16 or power jack - j3), input storage capacitor (c1) and transil (d1). the input electrolytic capacitor and transil serve to reduce input voltage spikes (surge). figure 3. schematic of input part figure 4 displays the placement of the input connectors on the board. the board can be supplied either from the jack connector (j3) or the industrial removable terminal plate (j16). the polarity of the input voltag e must be correctly applied in accordance with the illustration in figure 4 . if the connection is made incorrectly, the input protection d1 shorts the input voltage. it should be pointed out that the total input current is about 4 a at maximum output power and minimum input voltage. figure 4. location and correct polarity of the input supply connector on the board vin d1 sm6t39a c1 47 f / 50 v j16 3 2 1 j3 ai12691 - + - + - + - +
AN2559 pm6680a block 7/35 3 pm6680a block figure 5. electrical diagram of the pm6680a section r9 3.3 ? q2 sts7nf60l r3210 k ? vldo q3 sts7nf60l c39 22 f / 6.3 v vldo r23 0r d9 stps1l40 r20 680 r c26 2.2 nf vldo vin d8 stps1l40 vin r24 0r r208 820 k ? vin r19 1 k ? r10 1.8 k ? l4 3.8 h / 6 a l3 5.0 h / 3 a c29 330 f / 6.3 v c28 330 f / 6.3 v s5 c22 330 f / 6.3 v s9 1 v vldo hgate2 10 phase2 11 lgate2 13 csense2 12 v5sw 17 out2 8 comp2 2 pgoo d2 27 fb2 7 shdn 5 en1 25 en2 4 vre f 32 sk ip 24 fsel 3 nc 6 hgate1 22 phase1 21 lgate1 15 csense1 20 pgnd 14 out1 29 comp1 30 pgoo d1 26 fb1 28 sgnd1 1 sgnd2 16 boot1 23 boot2 9 ldo 5 18 v cc 31 vi n 19 u5 pm6680 s6 shdn ski p mode s8 2.5 v r205 200 r s7 c30 100 nf c35 10 nf r39 3.3 k ? r40100 k ? v io r206 6.8 k ? r35 51 k ? s14 1.2 v r37 47r r102 2 k ? v core s15 1.5 v r36 51 k ? s17 2.5 v s13 1 v s16 1.8 v r101 1 k ? c31 220 nf r11 560r r31 10 k ? r34 10 k ? c27 120 pf r109 3.3 k ? r21 0r c20 1.8 nf c21 91 pf c15 470 nf 7 6 2 1 3 8 4 5 q1 sts4dnf60 r26 10r r207 9.1 k ? c14 4.7 f / 1 0 v c18 4.7 f / 50 v/x7r d7 baw56/sot r110 4.7 k ? c23 4.7f / 50 v / x7r c24 4.7f / 50 v / x7r r201 1 k ? c17 100 nf c41 4.7 f / 50 v/x7r r202 2 k ? r203 3 k ? r204 3 k ? c34 12 nf c25 100 nf 2 1 3 s4 ch1 en/sus c19 100 nf c16 3.3 f / 3 5 v r22 0r 2 1 3 s3 ch2 en/sus r25 10r r105 200 r r108 820 k ? r106 6.8 k ? s10 1.2 v s12 1.8 v s11 1.5 v r107 9.1 k ? r103 3 k ? r104 3 k ? r29 51 k ? d10 4v7 shdn r27 62 k ? vldo r38 3.3 k ? r28 10 k ? c40 22 f / 6.3 v ai14512
pm6680a block AN2559 8/35 the pm6680a block is most important part of board. it contains two dc-dc converters. each output has a selectable output voltage level. the first converter is capable of delivering up to 4 a for each voltage level, while the second converter can deliver up to 2 a on the output. both converters are controlled by the pm6680a device. the pm6680a is a dual step-down controller specifically designed to provide ex tremely high efficiency conversion, with loss- less current sensing. the constant on-time arch itecture assures fast l oad transient response and the embedded voltage feed-forward provides nearly constant switching frequency operation. an embedded integrator control loop compensates the dc voltage error due to the output ripple. the pulse skipping technique increases efficiency at very light loads. moreover, a minimum switching frequency of 33 khz is selectable to avoid audio noise issues. the pm6680a provides a selectable switching frequency, allowing either 200 / 300 khz, 300 / 400 khz or 400 / 500 khz operation of the two switching sections. the output voltages out1 and out2 can be adjusted from 0.9 v to 5 v and from 0.9 v to 3.3 v, respectively. a detailed description of this device can be found in the datasheet. figure 5 shows the full electrical diagram of the block with the pm6680a that controls the two dc-dc converters. the components around the pm6680a form several functional blocks: the power management block, v core step down block, v i/o step down block and start-up/enable control system block. 3.0.1 power management block the pm6680a has two supply voltage inputs - v cc and v in . the v cc pin should be connected to the 5 v bus (maximum input voltage is 6 v, minimum 4.5 v) and it is dedicated for the supply of the chip itself. the v in pin should be connected to the input power bus and it is used inside the chip for two reasons. the first is to supply the integrated ldo. the second is the fact that the controller must sense the converter input voltage level for proper functioning of the converter. the v cc pin is supplied from the integrated ldo (connected output of ldo and v cc ) on the reference board. the v5sw feature of the ldo is disabled. the power management block consists of components c14 - c17, c31, r9, r29, r37 and d10. the important parts of the power management block of the device are the low pass filters (r9, c16, c17 and r37, c31) applied to reduce the influence of transience on the device v cc and v in main power inputs. the resistor r29 and the diode d10 generate the shdn (shut down) signal, which is active in low level. this signal activates the pm6680a immediately after v in is connected to the input. the v ref and ldo signals start to work simultaneously with activation of the shdn pin. 3.0.2 start-up/enable block the pm6680a has several inputs and outputs dedicated to the control of each channel. each channel has an independent enable signal (en - active in high level) and "power good" signal (pgood - open collector) activated by channel in cases where the output voltage is within 10% tolerance. these control pins can be used either for simple enabling/disabling or for delaying the start-up of one channel rather than another. the jumpers s3 and s4 with resistors r28, r31, r32, r34, r35 and r36 are used for systems independently allowing either enabling or disabling of each channel or setting up a different start-up sequence of both channels. figure 6 displays the placement of jumpers s3 and s4 on the board, and the settings are shown in ta bl e 2 .
AN2559 pm6680a block 9/35 figure 6. the placement of the jumpers for start-up/enable settings the skip mode connector (shown in the schematic as s5 - s7) is dedicated for the control of skip mode. this connector setting is common for both channels. figure 7 shows the placement of the skip mode connector, while the settings are shown in ta bl e 3 . there are three possible settings. standard skip mode, no audible mode or pwm mode. in standard skip mode the converter reduces the switching frequency at light load to maintain good efficiency even in this condition. there is no lower limit for switching frequency. in no audible mode the converter reduces switching frequency at light load, but this frequency never drops below 30 khz to avoid possible audible noise caused by the mechanical table 2. start-up/enable jumper settings jumper settings function both channels are disabled. an open connector for each channel means that the channel is disabled. both channels are disabled. both channels are enabled and start at same time. v core voltage starts first, and v i/o starts second. v i/o voltage starts first, and v core starts second. vcore vi/o 1st 1st vcore vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st v core vi/o 1st 1st
pm6680a block AN2559 10/35 construction of passive components (inducto rs or ceramic capacitors). in pwm mode the converter maintains a constant switching frequency independently on the load. the fsel pin the pm6680a dedicated for operating frequency setting is connected to gnd. this means that the switching frequency of the v core branch is 200 khz and switching frequency of v i/o is 300 khz. figure 7. skip mode connector 3.0.3 step-down parts the pm6680a is a dual step-down controller and drives two step-down converters. the schematic of both channels are almost identical, with only a few small differences. since each channel is for a different output power, the main difference is in the components? values. figure 8 displays the output connector polarities of the pm6680a section. table 3. skip mode connector jumper settings jumper settings function skip mode at light load. no audible skip mode at light load (frequency never drops below 30 khz). pwm mode. constant frequency even at light or zero load. skip pw m audio skip pw m audio skip pw m audio skip pw m audio skip pw m audio skip pw m audio skip pw m audio skip pw m audio skip pw m audio
AN2559 pm6680a block 11/35 figure 8. output connector the power components of the step-down part are input capacitors (c23, c24 or c18, c41), the half bridge driver containing two n-channel mosfets (q2, q3 or q1), inductors (l4 or l3) and output capacitors (c28, c29, c40 or c22, c39). ceramic high-capacitance capacitors are us ed as input capacitors. 60 v mosfets are used for the half bridge driver. a relatively high breakdown voltage is used to guarantee operation in industrial applications. because the v i/o output is designed for lower currents (2 a), both mosfets are integrated in one so-8 package (q1 - sts4nf60). this helps to reduce the size on the pcb. two discrete mosfets (sts7nf60) are used for the v core - higher power output (4 a). schottky diodes are also used in each channel (d9 or d8). these diodes work mainly during dead time and are not mandatory for proper functioning, but their application increases efficiency. the 5 h inductor (l3) is used for the v i/o output with saturation current at 3 a. the inductor l4 has value of 3.8 h with saturation current at 6 a. a combination of tantalum low esr and ceramic type are used as output capacitors. ceramic capacitors help to reduce total output esr and reduce total output voltage ripple. the pm6680a includes a half bridge driver for each channel. the external bootstrap diode and capacitor must be applied (d7, c19 or c25) in order to drive the gates of the high side mosfets. the feedback signal is generated by the output voltage divider (r10x or r20x). the board allows the setting of different output voltages for both channels. figure 9 and figure 10 display the output voltage connector placement on the board for each channel. the jumper settings are shown in ta bl e 4 and ta b l e 5 , respectively. in classic constant on time control, the system regulates the valley value of the output voltage and not the average value. in this condition, the output voltage ripple is a source of dc static error. to compensate for this error, an integrator network is introduced in the control loop by connecting the signal output voltage to the comp1/comp2 pin through a capacitor (c20 or c26). an additional r-c network (r11 and c21 or r20 and c27) is implemented as a low pass filter to reduce noise on the input of the comp pin. since the feedback signal of the smps working in constant on time control is directly connected to the pwm comparat or, the stability of the smps is more sensitive to noise injected into the fb signal. it is possible to attenuate the affect of the noise to stabilize the smps by implementing the so called "virtual esr" network, which increases the amplitude of the feedback ripple voltage and improves signal-to-noise ratio. the virtual esr network does not increase the output ripple voltage. it is recommended to use the virtual esr network in cases where the output voltage ripple is below 30 mv. however, it is necessary to
pm6680a block AN2559 12/35 take into consideration that the influence of noise on the performance of the smps strictly depends on the pcb layout. therefore, the 30 mv is an indicative value. virtual esr networks are applied for each channel on the reference board described in this application note. the main reason for this is the fact that the smps based on the pm6680a device can generate different output voltages at a wide input voltage range. as output voltage ripple depends also on input and output voltage level, there are configurations where the virtual esr network could be mandatory. virtual esr networks consists of r40, r39, c35 or r27, r38 or c34. the esr network can be removed to observe influence of esr network to board function. to remove the virtual esr network, r40 and r27 must be removed and r39 and r38, respectively, must be shorted. figure 9. jumper placement for v core voltage level setting table 4. v core voltage level jumper settings jumper settings v core 2.5 v 1.8 v 1.5 v 1.2 v 1.0 v 0.9 v
AN2559 pm6680a block 13/35 figure 10. jumper placement for v i/o voltage level setting table 5. v i/o voltage level jumper settings jumper settings v core 3.3 v 2.5 v 1.8 v 1.5 v 1.2 v 1.0 v
pm6680a block AN2559 14/35 3.1 dc-dc converters based on the l5970ad there are two converters based on the l5970ad on the system supply board: the analog output and v sys output voltage. figure 11 shows the arrangement of output voltages on connector j18. figure 11. output voltages of l5970a parts the l5970ad is a step-down monolithic power s witching regulator with a switch current limit of 1.5 a, capable of delivering more than 1 a of dc current to the load depending on the application conditions. the output voltage can be set from 1.235 v to 35 v. the device uses an internal p-channel d-mos transistor (with a typical rds on of 200 m ? ) as a switching element to avoid the use of a bootstrap capacitor and to guarantee high efficiency. an internal oscillator fixes the swit ching frequency at 500 khz to mi nimize the size of external components. having a minimum input voltage of only 4.4 v, it is particularly suitable for 5 v buses, found in all computer-related applications. pulse-by-pulse current limiting with internal frequency modulation offers effective constant current short circuit protection. the schematic of both smps?s is displayed in figure 12 . as the schematic shows, designing with the l5970ad is very simple. it consists of a power part, feedback and enable/disable connectors. the power part contains an input capacitor (c2 or c8 - ceramic is recommended), an inductor (l1 or l2), an output capacitor (c5 or c11) and a freewheeling diode (d4 or d6). the feedback part consists of a voltage divider (r2, r3, r4 or r6, r7, r8) and a compensation rc network (r1, c3, c4 or r5, c9, c10).
AN2559 pm6680a block 15/35 figure 12. schematic of the two smps?s based on the l5970ad both converters can be switched on or off using the inhibit pin of l5970ad connected to jumpers s1 and s2. if the jumper is left open, the dc-dc converte r will not operate. thus the jumper must be shorted for the converter to operate (see figure 13 for board placement of the jumper and ta bl e 6 for the jumper settings). c9 220pf c10 22 nf r5 4.7 k ? v5a vin shdn 1 gnd 2 bypass 3 out 4 in 5 u2 lk112_3 3 v3a3 v sys v cc 8 comp 4 sync 2 v ref 6 gnd 7 inh 3 fb 5 out 1 u1 l5970ad v aux l1 33 h / 1.5 a gnda l5 10 h / 1 a r7 240 k ? r3 20 k ? d4 stps2l40 r2 120 k ? r4 5.6 k ? c5 47 f / 10 v s1 v analog en c6 100 nf vin s2 v sys en c7 10 f / 6 v c2 4.7 f / 50 v vin c8 4.7 f / 50 v vin 8 gnd 7 vout 1 gnd 6 gnd 3 gnd 2 inh 5 u4 kf25_soic8 r41 51 k ? c3 220 pf c4 22 nf r 42 51 k ? v cc 8 comp 4 sync 2 v ref 6 gnd 7 inh 3 fb 5 out 1 u3 l5970ad l2 33 h / 1.5 a d6 stps2l40 r6 18 k ? r8 10 k ? c11 100 f / 6 v r1 4.7 k ? c12 100 nf c13 10 f / 4 v ai12694
pm6680a block AN2559 16/35 figure 13. jumper placement for enable/disable function of analog output and output3 there is an ldo linear regulator (u2 and u4) on the output of each dc-dc converter. the lk112_33 is a 3.3 v linear regulator in a sot23-5 package. the kf25 is a very low dropout regulator with an output voltage of 2.5 v and output current of up to 400 ma. 3.2 reset circuit the board also features a reset circuit which su pervises the output voltages. it is based on the stm6719 series of low voltage / low supply supervisors, which are designed to monitor three system power supply voltages. two monitored supplies (v cc1 and v cc2 ) have fixed (factory trimmed) thresholds (v rst1 and v rst2 ). the third voltage is monitored using an externally adjustable rstin threshold (0.626 v internal reference). if any of the three monitored voltages drop below its factory-trimmed or adjustable thresholds, or if mr is asserted to logic low, an rst is asserted (dri ven low). once asserted, rst is maintained at low for a minimum delay period after all supplies rise above their respective thresholds and mr returns to high. this device is guaranteed to be in the correct reset output logic state when v cc1 and / or v cc2 is greater than 0.8 v. this device is available in a standard 6- pin sot23 package. table 6. jumper settings for enable/disable function of analog output and output3 jumper settings v core analog disable analog enable output3 disable output3 enable e/d e/d e/d e/d e/d e/d e/d e/d e/d e/d
AN2559 pm6680a block 17/35 figure 14 shows the schematic and placement of the reset part on the board. typically in real applications the reset circuit senses if the supply voltage drops below about 10% of nominal value. this feature cannot be implemented on the system supply board due to the fact that the output voltage is selectable, while the reset voltage is factory set. there are several types of reset circuits in the stm6719 family (see datasheet). of these, the stm6719tgwb6f was selected as optimal. the voltage thresholds of this device are 3.075 v, 1.11 v and 0.626 v. figure 14. schematic of the reset circuit and board placement r209 110 k ? v core v sys c32 1 nf c33 1 nf v io j14 reset v cc1 6 v cc2 4 mr 3 v ss 2 rstin 5 rst 1 u6 stm6719tewb6f j15 reset gnd ai12695
pcb layout AN2559 18/35 4 pcb layout the system supply board utilizes a four-layer pcb. the copper layout of each layer is shown in figure 15 and figure 16 . the top and bottom layers show also the placement of the components. to redu ce the size of board while mainta ining the ability to change some components, size 0603 was used for the majority of the passive components. all views of the pcb are from top side. figure 15. pcb top layer layout and first internal layer figure 16. pcb second internal layer and bottom layer layout
AN2559 bill of materials 19/35 5 bill of materials table 7. bill of materials item part description type size manufacturer part number 1 c1 47 f / 50 v th 6.3 x 11 e47m/50vmxa rm5 2 c2 4.7 f / 50 v smd 1812 avx 18125c475kat2a 3 c3 220 pf smd 0603 4 c4 22 nf smd 0603 5 c5 100 f / 10 v smd c avx tpsc107m010x0150 6c6 n.a. 7 c7 10 f / 6 v smd b cts 10m / 6.3 v 8 c8 4.7 f / 50 v smd 1812 avx 18125c475kat2a 9 c9 220 pf smd 0603 10 c10 22 nf smd 0603 11 c11 100 f / 10 v smd c avx tpsc107m010x0150 12 c12 100 nf smd 0805 13 c13 10 f / 6.3 v smd b cts 10 m / 6.3 v 14 c14 6.8 f / 10 v smd b cts 6 m 8 / 10 v 15 c15 470 nf smd 0805 16 c16 3.3 f / 50 v smd 1812 c1210c335k5rac 17 c17 100 nf smd 0603 18 c18 4.7 f / 50 v / x7r smd 1812 avx 18125c475kat2a 19 c19 100 nf smd 0603 20 c20 1.8 nf smd 0603 21 c21 100 pf smd 0603 22 c22 330 f / 6.3 v smd d avx tpsd337m006x0045 23 c23 4.7 f / 50 v / x7r smd 1812 avx 18125c475kat2a 24 c24 4.7 f / 50 v / x7r smd 1812 avx 18125c475kat2a 25 c25 100 nf smd 0603 26 c26 2.2 nf smd 0603 27 c27 120 pf smd 0603 28 c28 330 f / 6.3 v / 45 m ? smd d avx tpsd337m006x0045 29 c29 330 f / 6.3 v / 45 m ? smd d avx tpsd337m006x0045 30 c30 100 nf smd 0603 31 c31 220 nf smd 0805 32 c32 1 nf smd 0603
bill of materials AN2559 20/35 33 c33 1 nf smd 0603 34 c34 12 nf smd 0603 35 c35 10 nf smd 0603 36 c39 22 f / 6 . 3 v smd 1206 avx 12066d226kat2a 37 c40 100 f / 6 . 3 v smd 1210 avx 12104d107mat2a 38 c41 4 f 7 / 50 v / x7r smd 1812 avx 18125c475kat2a 39 d1 sm6t39a smd smb st sma6t39a 40 d4 stps2l40 smd smb st stps2l40 41 d6 stps2l40 smd smb st stps2l40 42 d7 baw56/sot smd sot23 43 d8 stps1l40m smd do216-aa st stps1l40m 44 d9 stps1l40m smd do216-aa st stps1l40m 45 d10 4.7 v smd sod80 46 s1 header 1 x 2 th 47 s2 header 1 x 2 th 48 s3 header 1 x 3 th 49 s4 header 1 x 3 th 50 v i/o level header 2 x 5 th 51 v core level header 2 x 5 th 52 skip header 2 x 3 th 53 j3 jack - pcb th 54 j14 header 1 x 1 th 55 j15 header 1 x 1 th 56 j16 ind. con. 2 th ph. con. mstba 2,5 / 2-g-5,08 57 j17 ind. con. 4 th ph. con. mstba 2,5 / 4-g-5,08 58 j18 ind. con. 6 th ph. con mstba 2,5 / 6-g-5,08 59 l1 33 h / 1.5 a smd coilcraft mss7341-333mlb 60 l2 33 h / 1.5 a smd coilcraft mss7341-333mlb 61 l3 5.0 h / 3 a smd coilcraft mss7341-502mlb 62 l4 3.8 h / 6 a smd coilcraft mss1038-382nlb 63 l5 1 h / 1 a smd coilcraft me3220-102mlb 64 q1 sts4dnf60 smd st sts4dnf60l 65 q2 sts7nf60l smd st sts7nf60l 66 q3 sts7nf60l smd st sts7nf60l 67 r1 4. 7 k ? smd 0603 table 7. bill of materials (continued) item part description type size manufacturer part number
AN2559 bill of materials 21/35 68 r2 36 k ? / 1% smd 0603 69 r3 200 k ? / 1% smd 0603 70 r4 10 k ? / 1% smd 0603 71 r5 4.7 k ? smd 0603 72 r6 18 k ? / 1% smd 0603 73 r7 240 k ? / 1% smd 0603 74 r8 10 k ? / 1% smd 0603 75 r9 3.3 ? smd 0805 76 r10 1.8 k ? smd 0603 77 r11 560 ? smd 0603 78 r110 4.7 k ? / 1% smd 0603 79 r19 1 k ? smd 0603 80 r20 680 ? smd 0603 81 r21 0 ? smd 0603 82 r22 0 ? smd 0603 83 r23 0 ? smd 0603 84 r24 0 ? smd 0603 85 r25 10 ? smd 0603 86 r26 10 ? smd 0603 87 r27 62 k ? smd 0603 88 r28 10 k ? smd 0603 89 r29 51 k ? smd 0603 90 r31 10 k ? smd 0603 91 r32 10 k ? smd 0603 92 r34 10 k ? smd 0603 93 r35 51 k ? smd 0603 94 r36 51 k ? smd 0603 95 r37 47 ? smd 0603 96 r38 3.3 k ? smd 0603 97 r39 3.3 k ? smd 0603 98 r40 100 k ? smd 0603 99 r41 51 k ? smd 0603 100 r42 51 k ? smd 0603 101 r101 1 k ? / 1% smd 0603 102 r102 2 k ? / 1% smd 0603 table 7. bill of materials (continued) item part description type size manufacturer part number
bill of materials AN2559 22/35 103 r103 3 k ? / 1% smd 0603 104 r104 3 k ? / 1% smd 0603 105 r105 200 ? / 1% smd 0603 106 r106 6.8 k ? / 1% smd 0603 107 r107 9.1 k ? / 1% smd 0603 108 r108 820 k ? / 1% smd 0603 109 r109 3.3 k ? / 1% smd 0603 110 r201 1 k ? / 1% smd 0603 111 r202 2 k ? / 1% smd 0603 112 r203 3 k ? / 1% smd 0603 113 r204 3 k ? / 1% smd 0603 114 r205 200 ? / 1% smd 0603 115 r206 6.8 k ? / 1% smd 0603 116 r207 9.1 k ? / 1% smd 0603 117 r208 820 k ? / 1% smd 0603 118 r209 51 k ? smd 0603 119 u1 l5970ad smd so-8 st l5970ad 120 u2 lk112_33 smd sot23-5 st lk112m33tr 121 u3 l5970ad smd so-8 st l5970ad 122 u4 kf25_soic8 smd so-8 st kf25bd-tr 123 u5 pm6680a smd vfqfpn- 32 5x5 st pm6680a 124 u6 stm6719tewb6f smd sot23-6 st stm6719tgwb6f table 7. bill of materials (continued) item part description type size manufacturer part number
AN2559 measurements 23/35 6 measurements the performance and properties of each part of the board is indicated in the measurements below. these measurements were performed for the pm6680a and l5971ad blocks independently. 6.1 pm6680a block - measurements the performance measurements of the pm6680a part focus mainly on efficiency, light load consumption, output ripple and transients. 6.1.1 efficiency and li ght load consumption modes since the device consists of three power parts (two controllers and one ldo) it makes sense to measure total efficiency. figure 17 displays how efficiency depends on input voltage level at full load output (v core 2.5 v / 4 a, v i/o 3.3 v / 2 a). figure 17. efficiency of the dual step-down converter at full load the efficiency is in the range of 83 - 91%. it should be noted that the total efficiency strictly depends on the performance of each component. the system supply board was designed to satisfy a wide input voltage range. therefore, 60 v mosfets are used on the board. if the input voltage of the end application is less (up to 30 v for instance), efficiency can be improved by using lower rds on 30 v mosfets in the same package. the expected efficiency gain is about 3 - 4%. the pm6680a can work in several modes with regard to light load. these options are mainly used for battery applications where relatively high consumption at light load can drain the battery even when no power is requested. the pm6680a allows three modes (see 3.0.2 ): pwm, no audible noise and skip. figure 18 shows the consumption of the board for different modes of the pm6680a. there is no load on the output and other parts of the smps are disabled. ai12696 efficency (%) 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 5 10 15 20 25 30 35 40 vin (v)
measurements AN2559 24/35 figure 18. pm6680a consumption at no load condition, in the different modes in analyzing the data in figure 18 , it should be noted that the consumption is slightly increased by several passive components which generate inhibit of the l5970ads. total consumption of these parts at 35 v on the input is about 1.5 ma. this is not compensated for in the chart in figure 18 . it is possible to see the effect of the different operating modes of the converter by observing the output ripple voltage waveforms in figure 19 . these measurements are made under the following conditions: v core output set to 2.5 v, no load, at 12 v on the input. figure 19. output voltage ripple in different modes of light load operation 6.1.2 output voltage ripple output voltage ripple depends on the current ripple flowing through the choke. the current ripple depends on the input and output voltage levels. therefore, it is mandatory to measure the output voltage ripple for different input and output voltage conditions. figure 20 shows the output voltage ripple of v core at the minimum input voltage (5 v), while figure 21 displays the output voltage ripple of v core at the maximum output voltage (36 v). figure 22 shows the output voltage ripple of v i/o at the minimum input voltage (5 v), and figure 20 displays the output voltage ripple of v i/o at the maximum input voltage (36 v). all of the figures represent the minimum and maximum output voltages at maximum load (0.9 v and 2.5 v at 4 a for v core, and 1 v and 3.3 v at 2 a for v i/o ). ai12697 lin (ma) 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 5 10 15 20 25 30 35 40 vin (v) pwm no audible skip
AN2559 measurements 25/35 figure 20. output voltage ripple of v core at the minimum input voltage (5 v) figure 21. output voltage ripple of v core at the maximum output voltage (36 v) figure 22. output voltage ripple of v i/o at the minimum input voltage (5 v)
measurements AN2559 26/35 figure 23. output voltage ripple of v i/o at the maximum input voltage (36 v) figure 24. start-up without setting the sequence figure 25. start-up with a set sequence
AN2559 measurements 27/35 6.1.3 start-up sequence the correct start-up sequence of the supply voltage is typically requested by the fpga device. therefore, there it is possible to set a dedicated start-up sequence on the system supply board (see 3.0.2 . figure 24 ) shows the start-up sequence waveform of v core and v i/o outputs when the jumpers described in ta bl e 2 are set in accordance with line 3 in the table. the waveforms shown in figure 25 illustrate different start-up sequences in accordance with the jumper settings displayed in ta b l e 2 , lines 4 and 5. 6.1.4 transient response transient response refers to the behavior of the output voltage when the load changes fast. this test was also performed on the outputs of the pm6680a branch. the load was changed between maximum and zero load (0 ? 2 a on v i/o output and 0 ? 4 a on the v core output). the input voltage was 12 v and output voltage was 3.3 v and 2.5 v, respectively. the repetition of load change was 500 hz. the results of the measurements are shown in figure 26 and figure 27 . the voltage spikes caused by increasing the load are quite low. it is possible to observe that the converter reacts very fast to a rising load and the undervoltage is small (left waveform in figures). if the load is decreasing fast the overvoltage spikes appear on the output (right side of picture). this effect depends partly on the reaction of the controller and partly on the parameters of the output filter. there is remaining energy stored in the inductor and if the load decreases this energy should be stored in the output capacitor. this effect can be reduced by either reducing the value of the inductor (to reduce the amount of energy stored in the inductor), or by increasing the value of the output capacitor (a higher capacitance is capable of absorbing more energy from the inductor). figure 26. load transient response on v core output
measurements AN2559 28/35 figure 27. load transient response on v i/o output
AN2559 measurements 29/35 6.2 l5970ad blocks - measurements 6.2.1 efficiency the l5970ad is a powerful converter with very good performance and efficiency. because a diode is used as a low side switch, however, the efficiency is a slightly less compared to a synchronous converter such as the pm6680a. theoretically, the efficiency declines when output voltage is decreasing and input voltage is increasing. figure 28 displays the efficiency of output 3, depending on the input voltage at full load (800 ma). figure 29 displays the same measurement for the analog output. the efficiency of the analog output is better thanks to the higher output voltage level. the efficiency of the analog output voltage was measured in a range of 7 - 35 v. it should be noted that the output voltage is 5 v, so the device does not work as a switching converter in cases where the input and output voltage are similar or lower than the required output. in this case the l5973ad works with 100% duty cycle. figure 28. efficiency of output 3, by input voltage level ai12698 efficency (%) 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 5 10 15 20 25 30 35 40 vin (v)
measurements AN2559 30/35 figure 29. efficiency of analog output, by input voltage level 6.2.2 output voltage ripple the output voltage ripple of the switching parts of the analog and v sys outputs are shown in figure 30 and figure 31 . the measurements were made for different input voltages, because the current ripple influence on the output voltage ripple depends on the input voltage level. the output voltage ripple on the 3.3 v analog output and the v aux output are displayed in figure 32 and figure 33 . as these outputs are generated by ldos, the output voltage ripple is the same (independent) for all input voltages, and is very low. therefore, only one output voltage ripple image is shown in the figures 32 and 33. all of the measurements were taken at full output load. figure 30. analog 5 v - output voltage ripple ai14500 vin (v) efficency (%) 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0 0 5 10 15 20 25 30 35 40 100.0
AN2559 measurements 31/35 figure 31. v sys - output voltage ripple figure 32. analog 3.3 v - output voltage ripple
measurements AN2559 32/35 figure 33. v aux 2.5 v - output voltage ripple
AN2559 measurements 33/35 6.2.3 transient transient responses were measured only for v aux and v sys . the transient responses are displayed in figure 34 and figure 35 . the transient waveforms of the l5970ad section show the response time. the most visible difference between the l5970ad in classic voltage mode and the pm6680a working in constant on time mode is the reaction when there is a fast load increase. whereas the pm6680a reacts asfast as possible on the rising load, the l5970ad will wait short time as th e compensation network is implemented in feedback loop (see figure 24 and figure 25 ). figure 34. transient response of v sys based on the l5970ad figure 35. transient response of v aux generated by the ldo kf25
references AN2559 34/35 7 references 1. datasheet pm6680a 2. datasheet l5970ad 3. datasheet lk112 4. datasheet kf25 5. stm6719 6. an1330 - designing with the l5970d, 1 a high efficiency dc-dc converter. 8 revision history table 8. document revision history date revision changes 25-sep-2007 1 initial release
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