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october 2005 1 mic2198 mic2198 micrel, inc. mic2198 500khz 4mm 4mm synchronous buck controller general description the mic2198 is a high-power 500khz synchronous buck dc-to-dc controller housed in a small 4mm 4mm mlf? 12-pin package. the mic2198 operates from a wide 4.5v to 32v input and can be programmed for output voltages from 0.8v to 6v. the wide input voltage capability makes the mic2198 an ideal solution for point-of-load dc-to-dc conversion in 5v, 12v and higher systems. the 500khz switching frequency allows the use of a small inductor and small output capacitors. the current mode pwm control along with the external comp pin allows for ease of stability compensation and fast transient response across a wide range of applications. an all n-channel synchronous architecture and powerful output drivers allow up to 20a of output current capability. for higher ef?ciency at higher input voltages, refer to the 300khz mic2199. the mic2198 is available in a 12-pin 4mm 4mm mlf? package with a junction temperature range from C40c to 125c. features ? 4.5v to 32v input range ? 4mm 4mm mlf? package ? 500khz pwm operation ? >90% ef?ciency ? output voltage adjustable down to 0.8v ? 20a output current capability ? drives all n-channel mosfets ? logic controlled micropower shutdown ? cycle-by-cycle current limiting ? adjustable undervoltage lockout ? frequency foldback overcurrent protection applications ? point-of-load dc-to-dc conversion from 5v, 12v and higher supplies ? telecom equipment ? wireless modems ? servers ? base stations micrel, inc. ? 2180 fortune drive ? san jose, ca 95131 ? usa ? tel + 1 (408) 944-0800 ? fax + 1 (408 ) 474-1000 ? http://www.micrel.com typical application irf7821irf7821 1h 0.01? 3.3v/7a 10k? 6.04k? 1nf 5.5v to 24v vinen/uvlo comp csh lsd vsw vdd bst hsd gnd vout fb 2.2nf 2k? u1 mic2198bml sd103bws 0.1f 22 f 4.7f 220f v in v out 5.5vC24v to 3.3v/7a converter mlf and microleadframe are trademarks of amkor technology, inc. 50 55 60 65 70 75 80 85 90 95 100 0 2 4 6 8 i load (a) v in = 5.5v v out = 3.3v efficiency vs. load current downloaded from: http:///
mic2198 micrel, inc. mic2198 2 october 2005 pin con?guration 12 3 4 comp en/uvlo fb csh hsdvsw bst gnd vout vin 56 12 11 10 98 7 lsdvdd 12-pin 4 4 mlf (ml) pin description pin number pin name pin function 1 comp compensation (output): internal error ampli?er output. connect to capacitor or series rc network to compensate the regulator control loop. 2 en/uvlo enable/undervoltage lockout (input): low-level signal powers down the controller. input below the 2.5v threshold disables switching and functions as an accurate undervoltage lockout (uvlo). input below the threshold forces complete micropower (<0.1a) shutdown. 3 fb feedback (input): regulates fb pin to 0.8v. see applications information for resistor divider calculations. 4 csh current-sense high (input): current limit comparator non-inverting input. a built-in offset of 75mv between csh and v out pins in conjunction with the current-sense resistor set the current limit threshold level. this is also the non-inverting input to the current sense ampli?er. 5 vout current-sense low (input): output voltage feedback input and inverting input for the current limit comparator and the current sense ampli?er. 6 vin unregulated input (input): +4.5v to +32v supply input. 7 vdd 5v internal linear-regulator (output): v dd is the external mosfet gate drive supply voltage and internal supply bus for the ic. bypass to gnd with 4.7f. 8 lsd low-side drive (output): high-current driver output for low-side n-channel mosfet. voltage swing is between ground and v dd . 9 gnd ground (return). 10 bst boost (input): provides drive voltage for the high-side mosfet driver. the drive voltage is higher than the input voltage by v dd minus a diode drop. 11 vsw switch (return): high-side mosfet driver return. 12 hsd high-side drive (output): high-current driver output for high-side mosfet. this node voltage swing is between ground and v in +5v minus a diode drop. ordering information part number voltage temperature range package standard pb-free mic2198bml mic2198yml adj C40oc to +125oc 12-pin 4x4 mlf? downloaded from: http:///
october 2005 3 mic2198 mic2198 micrel, inc. absolute maximum ratings (note 1) analog supply voltage (v in ) ........................................ +34v digital supply voltage (v dd ) .......................................... +7v driver supply voltage (b st ) ...................................v in + 7v sense voltage (v out , c sh ) ...............................7v to C0.3v enable pin voltage (v en/uvlo ) ....................................... v in power dissipation (p d ) 4 4 mlf? ..................................665mw @ t a = 85c ambient storage temperature (t s ) .......... C65c to +150c esd, note 3 ................................................................ 2kv operating ratings (note 2) analog supply voltage (v in ) ......................... +4.5v to +32v output voltage range (v out ) ....................... +0.8v to +6v junction temperature (t j ) ........................ C40c to +125c package thermal resistance 4 4 mlf-12l ( ja ) ............................................... 60c/w electrical characteristics (note 4) v in = v en = 12v; t j = 25c, unless noted, bold values indicate C40c t j +125c parameter condition min typ max units feedback voltage reference (1%) 0.792 0.8 0.808 v feedback voltage reference (2%) 0.784 0.816 v feedback voltage reference 4.5v < v in < 32v, 0 < (v csh C v out ) < 60mv (3%) 0.776 0.824 v feedback bias current 10 na output voltage range 0.8 6 v output voltage line regulation v in = 4.5v to 32v, v csh C v out = 60mv 0.03 %/v output voltage load regulation 25mv < (v csh C v out ) < 60mv 0.5 % input and v dd supply quiescent current excluding external mosfet gate drive current 3.5 5 ma shutdown quiescent current v en/uvlo = 0v 0.1 5 a digital supply voltage (v dd ) i l = 0ma to 5ma 4.7 5.0 5.3 v undervoltage lockout v in upper threshold (turn-on threshold) 4.25 4.4 v v in lower threshold (turn-off threshold) 3.95 4.1 v enable/uvlo enable input threshold (rising) 0.6 1.1 1.6 v uvlo threshold (rising) 2.2 2.5 2.8 v enable input current v en/uvlo = 5v 0.1 5 a current limit current limit threshold voltage (v csh C v out ) 55 75 95 mv error ampli?er transconductance error ampli?er gm 0.2 ms oscillator section oscillator frequency 450 500 550 khz maximum duty cycle 70 76 % minimum on-time 150 200 ns frequency foldback threshold measured at v out pin 0.25 0.40 0.55 v foldback frequency 125 khz gate drivers rise/fall time c l = 3000pf 60 ns output driver impedance source 5 8.5 sink 3.5 6 driver non-overlap time 80 ns note 1. exceeding the absolute maximum rating may damage the device. note 2. the device is not guaranteed to function outside its operating rating. note 3. devices are esd protected; however, handling precautions are recommended. human body model, 1.5k in series with 100pf. note 4. speci?cation for packaged product only. downloaded from: http:///
mic2198 micrel, inc. mic2198 4 october 2005 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -40 -20 02 04 06 08 0 100120140 temperature (c) quiescent current vs. temperature pwm uvlo shutdown 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4 14 19 24 29 34 supply voltage (v) quiescent current vs. supply voltage pwm uvlo shutdown 9 0.790 0.795 0.800 0.805 0 10 15 20 25 30 35 supply voltage (v) v fb line regulation 5 0.795 0.800 0.805 0.810 0.815 0.820 -40 -20 02 04 06 08 0 100120140 temperature (c) v fb vs. temperature 4.94 4.95 4.96 4.97 4.98 4.99 5.00 5.01 5.02 0 10 15 20 25 30 load current (ma) v dd load regulation 5 4.80 4.85 4.90 4.95 5.00 5.05 5.10 5.15 5.20 -40 -20 02 04 06 08 0 100120140 temperature (c) v dd vs. temperature -10 -8 -6 -4 -2 0 2 4 6 8 10 -40 -20 02 04 06 08 0 100120140 temperature (c) oscillator frequency vs. temperature -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 0 10 15 20 25 30 35 supply voltage (v) oscillator frequency vs. supply voltage 5 typical characteristics (v in = 12v, unless otherwise noted) 64 66 68 70 72 74 76 78 -40 -20 02 04 06 08 0 100120140 temperature (c) overcurrent threshold vs. temperature v in ?=?5v v out ?=?3.3v r cs ?=?20mv 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 1 2 3 5 output current (a) current limit foldback 4 0 1.0 2.0 3.0 4.0 5.0 6.0 0 8 12 16 20 24 28 supply voltage (v) v dd line regulation 32 4 downloaded from: http:///
october 2005 5 mic2198 mic2198 micrel, inc. block diagram g m = 0.210 -3 currentlimit error v in v dd correctiveramp reset reference oscillator en/uvlo 2 710 612 11 8 9 45 3 1 comp 100k mic2198 fb r2 r1 vout csh vddbst c in vinhsd q2q1 d1 d2c bst c out v in v out l1 vswlsd pgnd c comp r comp 4.7f a v = 2 v 0.8v r1 r2 out = +1 v 6 v out(max) = .0 current r cs control pwm ?? ?? logic amp sense amp figure 1. internal block diagram functional description the mic2198 is a bicmos, switched-mode, synchronous step-down (buck) converter controller. current-mode control is used to achieve superior transient line and load regulation. an internal corrective ramp provides slope compensation for stable operation above a 50% duty cycle. the controller is optimized for high-ef?ciency, high-performance dc-dc converter applications. the mic2198 block diagram is shown above. the mic2198 controller is divided into 5 functions. ? control loop ? current limit ? reference, enable and uvlo ? mosfet gate drive ? oscillator control loop the mic2198 operates in pwm (pulse-width-modulation) mode. in pwm mode, the synchronous buck converter forces continuous current to ?ow in the inductor which also improves cross regulation of transformer coupled, multiple output con?gurations. pwm control loop the mic2198 uses current-mode control to regulate t he output voltage. this method senses the output voltage (outer loop) and the inductor current (inner loop). it uses inductor current and output voltage to determine the duty cycle of the buck converter. sampling the inductor current removes th e inductor from the control loop, which simpli?es compensation. a block diagram of the mic2198 pwm current-mode control loop is shown in figure 2 and the pwm mode voltage and current waveform is shown in figure 3. the inductor current is sensed by measuring the voltage across the resistor, r cs . downloaded from: http:///
mic2198 micrel, inc. mic2198 6 october 2005 a ramp is added to the ampli?ed current-sense signal to provide slope compensation, which is required to prevent unstable operation at duty cycles greater than 50%. a transconductance ampli?er is used for the error ampli?er, which compares an attenuated sample of the output voltage with a reference voltage. the output of the error ampli?er is current error v in v dd corrective pwm reset oscillator 710 612 11 8 9 45 3 1 comp 100k mic2198 fb r2 r1 vout csh vddbst c in vinhsd q2q1 d1 d2 c bst c out v in v out l1 vswlsd pgnd c comp r comp 4.7f s r q control logic and r cs g m = 0.210 -3 a v = 2 v 0.8v r1 r2 out = +1 reference ?? ?? pulse-width modulator comparator ramp amp sense amp figure 2. pwm operation v dd 0v 0v 0v 0v 0a v dd v in + v dd v in v sw reset v hsd v lsd i load i l1 pulse figure 3. pwm-mode timing the comp (compensation) pin, which is compared to the current-sense waveform in the pwm block. when the current signal becomes greater than the error signal, the comparator turns off the high-side drive. the comp pin (pin 1) provides access to the output of the error ampli?er and allows the use of external components to stabilize the voltage loop. downloaded from: http:///
october 2005 7 mic2198 mic2198 micrel, inc. current limit the mic2198 output current is detected by the voltage drop across the external current-sense resistor (r cs in figure 2.). the current limit threshold is 75mv25mv. the current- sense resistor must be sized using the minimum current limit threshold. the external components must be designed to withstand the maximum current limit. the current-sense resistor value is calculated by the equation below: r 55mv i cs out(max) = the maximum output current is: i 95mv r out(max) cs = the current-sense pins csh (pin 4) and v out (pin 5) are noise sensitive due to the low signal level and high input im- pedance. the pcb traces should be short and routed close to each other. a small (1nf to 0.1f) capacitor across the pins will attenuate high frequency switching noise. when the peak inductor current exceeds the current limit threshold, the current limit comparator, in figure 2, turns off the high-side mosfet for the remainder of the cycle. the output voltage drops as additional load current is pulled from the converter. when the output voltage reaches appr oximately 0.4v, the circuit enters frequency-foldback mode and the oscillator frequency will drop to 125khz while maintaining the peak inductor current equal to the nominal 75mv across the external current-sense resistor. this limits the ma ximum output power delivered to the load under a short circuit condition.reference, enable and uvlo circuits the output drivers are enabled when the following conditions are satis?ed: ? the v dd voltage (pin 7) is greater than its under- voltage threshold (typically 4.25v). ? the voltage on the enable pin is greater than the enable uvlo threshold (typically 2.5v). the internal bias circuit generates a 0.8v bandgap reference voltage for the voltage error ampli?er and a 5v v dd voltage for the gate drive circuit. the mic2198 uses fb (pin 3) for output voltage sensing. the enable pin (pin 2) has two threshold levels, allowing the mic2198 to shut down in a low current mode, or turn off output switching in uvlo mode. an enable pin voltage lower than the shutdown threshold turns off all the internal circuitry and reduces the input current to typically 0.1a. if the enable pin voltage is between the shutdown and uvlo thresholds, the internal bias, v dd , and reference voltages are turned on. the output drivers are inhibited from switching and remain in a low state. raising the enable voltage above the uvlo threshold of 2.5v enables the output drivers. either of two uvlo conditions will disable the mic2198 from switching. ? when the v dd drops below 4.1v ? when the enable pin drops below the 2.5v threshold mosfet gate drive the mic2198 high-side drive circuit is designed to switch an n-channel mosfet. referring to the block diagram in figure 2, a bootstrap circuit, consisting of d2 and c bst , sup- plies energy to the high-side drive circuit. capacitor c bst is charged while the low-side mosfet is on and the voltage on the v sw pin (pin 11) is approximately 0v. when the high-side mosfet driver is turned on, energy from c bst is used to turn the mosfet on. as the mosfet turns on, the voltage on the v sw pin increases to approximately v in . diode d2 is reversed biased and c bst ?oats high while continuing to keep the high-side mosfet on. when the low-side switch is turned back on, c bst is recharged through d2. the drive voltage is derived from the internal 5v v dd bias supply. the nominal low-side gate drive voltage is 5v and the nominal high-side gate drive voltage is approximately 4.5v due the voltage drop across d2. a ?xed 80ns delay between the high- and low-side driver transitions is used to prevent current from simultaneously ?owing unimpeded through both mosfets. oscillator the internal oscillator is free running and requires no external components. the nominal oscillator frequency is 500khz. if the output voltage is below approximately 0.4v, the oscillator operates in a frequency-foldback mode and the switching frequency is reduced to 125khz. v ss v out time v in = 7v f s = 125khz v out = 0.4v f s = 500khz v out = 3.3v figure 4. startup waveform above 0.4v, the switching frequency increases to 500khz causing the output voltage to rise a greater rate. the rise time of the output is dependent on the output capacitance, output voltage, and load current. the oscilloscope photo in figure 4 show the output voltage at startup. downloaded from: http:///
mic2198 micrel, inc. mic2198 8 october 2005 minimum pulsewidth the mic2198 has a speci?ed minimum pulsewidth. this minimum pulsewidth places a lower limit on the minimum duty cycle of the buck converter. figure 5 shows the minimum output voltage versus input supply voltage for the mic2198. for example, for v in = 15v, v out = 1.65v would be the lowest achievable voltage that conforms to the minimum-on-time. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 9.5 14.5 19.5 24.5 29.5 input voltage (v) figure 5. minimum output voltage vs. input supply voltage downloaded from: http:///
october 2005 9 mic2198 mic2198 micrel, inc. applications information the following applications information includes component selection and design guidelines. inductor selection values for inductance, peak, and rms currents are required to select the output inductor. the input and output voltages and the inductance value determine the peak to peak induc- tor ripple current. generally, higher inductance values are used with higher input voltages. larger peak to peak ripple currents will increase the power dissipation in the inductor and mosfets. larger output ripple currents will also require more output capacitance to smooth out the larger ripple cur- rent. smaller peak to peak ripple currents require a larger inductance value and therefore a larger and more expensive inductor. a good compromise between size, loss and cost is to set the inductor ripple current to be equal to 20% of the maximum output current. the inductance value is calculated by the equation below. l v out (v in(max) ? v out ) v in(max) f s 0.2 i out(max) = where: f s = switching frequency 0.2 = ratio of ac ripple current to dc output current v in(max) = maximum input voltage the peak-to-peak inductor current (ac ripple current) is: i v out (v in(max) ?v out ) v in(max) f s l pp = the peak inductor current is equal to the average o utput current plus one half of the peak to peak inductor ripple current. i i 0.5 i pk out(max) pp = + the rms inductor current is used to calculate the i 2 r losses in the inductor. i i 1 1 3 i i inductor(rms) out(max) p out(max) 2 = + ?? ? ?? ? maximizing ef?ciency requires the proper selection of core material and minimizing the winding resistance. the high frequency operation of the mic2198 requires the use of fer- rite materials for all but the most cost sensitive applications. lower cost iron powder cores may be used but the increase in core loss will reduce the ef?ciency of the power supply. this is especially noticeable at low output power. the winding resistance decreases ef?ciency at the higher output current levels. the winding resistance must be minimized although this usually comes at the expense of a larger inductor. the power dissipated in the inductor is equal to the sum of the core and copper losses. at higher output loads, the core losses are usually insigni?cant and can be ignored. at lower output currents, the core losses can be a signi?cant contributor. core loss information is usually available from the magnetics vendor. copper loss in the inductor is calculated by the equation below: p inductorcu = i inductor(rms) 2 r winding the resistance of the copper wire, r winding , increases with temperature. the value of the winding resistance us ed should be at the operating temperature. r winding(hot) = r winding(20c) 1 + 0.0042 (t hot ? t 20c ) ?? ? ?? ? where: t hot = temperature of the wire under operating load t 20c = ambient temperature r winding(20c) is room temperature winding resistance (usually speci?ed by the manufacturer) current-sense resistor selection low inductance power resistors, such as metal ?lm resistors should be used. most resistor manufacturers make low induc- tance resistors with low temperature coef?cients, designed speci?cally for current-sense applications. both resistance and power dissipation must be calculated before the resis- tor is selected. the value of r sense is chosen based on the maximum output current and the maximum threshold level. the power dissipated is based on the maximum peak output current at the minimum overcurrent threshold limit. r 55mv i sense out(max) = the maximum overcurrent threshold is: i 95mv r overcurrent(max) cs = the maximum power dissipated in the sense resistor is: p d(r sence ) = i overcurrent(max) 2 r cs mosfet selection external n-channel logic-level power mosfets must be used for the high- and low-side switches. the mosfet gate-to-source drive voltage of the mic2198 is regulated by an internal 5v v dd regulator. logic-level mosfets, whose operation is speci?ed at v gs = 4.5v must be used. it is important to note the on-resistance of a mosfet in- creases with increasing temperature. a 75c rise in junction temperature will increase the channel resistance of the mos- fet by 50% to 75% of the resistance speci?ed at 25c. this change in resistance must be accounted for when calculating mosfet power dissipation. total gate charge is the charge required to turn the mosfet on and off under speci?ed operating conditions (v ds and v gs ). the gate charge is supplied by the mic2198 gate drive circuit. at 500khz switching frequency, the gate charge can be a signi?cant source of power dissipation in the mic2198. at low output load this power dissipation is noticeable as a reduction in ef?ciency. the average current required to drive the high-side mosfet is: i q f g[high-side](avg) g s = downloaded from: http:///
mic2198 micrel, inc. mic2198 10 october 2005 where: i g[high-side](avg) = average high-side mosfet gate current. q g = total gate charge for the high-side mosfet taken from manufacturers data sheet with v gs = 5v. f s = 500khz the low-side mosfet is turned on and off at v ds = 0 because the freewheeling diode is conducting during this time. the switching losses for the low-side mosfet is usually negli- gible. also, the gate drive current for the low-side mosfet is more accurately calculated using c iss at v ds = 0 instead of gate charge. for the low-side mosfet: i c v f g[low-side](avg) iss gs s = since the current from the gate drive comes from the input voltage, the power dissipated in the mic2198 due to gate drive is: p v i i gatedrive in g[high-side](avg) g[low-side](avg) = + ( ) a convenient ?gure of merit for switching mosfets is the on-resistance times the total gate charge (r ds(on) q g ). lower numbers translate into higher ef?ciency. low gate- charge logic-level mosfets are a good choice for use with the mic2198. power dissipation in the mic2198 package limits the maximum gate drive current. parameters that are important to mosfet switch selection are: ? voltage rating ? on-resistance ? total gate charge the voltage rating of the mosfets are essentially equal to the input voltage. a safety factor of 20% should be added to the v ds(max) of the mosfets to account for voltage spikes due to circuit parasitics. the power dissipated in the switching transistor is the sum of the conduction losses during the on-time (p conduction ) and the switching losses that occur during the period of time when the mosfets turn on and off (p ac ). p p p sw conduction ac = + where: p i r conduction sw(rms) sw 2 = p p p ac ac(off) ac(on) = + r sw = on-resistance of the mosfet switch. making the assumption the turn-on and turnoff trans ition times are equal, the transition time can be approximated by: t c v c v i t iss gs oss in g = + where: c iss and c oss are measured at v ds = 0. i g = gate drive current (1a for the mic2198) the total high-side mosfet switching loss is: p ac = (v in + v d ) i pk t t f s where: t t = switching transition time (typically 20ns to 50ns). v d = freewheeling diode drop, typically 0.5v. f s it the switching frequency, nominally 500khz the low-side mosfet switching losses are negligible and can be ignored for these calculations. rms current and mosfet power dissipation calculation under normal operation, the high-side mosfets rms cur- rent is greatest when v in is low (maximum duty cycle). the low-side mosfets rms current is greatest when v in is high (minimum duty cycle). however, the maximum stress the mosfets see occurs during short circuit conditions, where the output current is equal to i overcurrent(max) . (see the sense resistor section). the calculations below are for normal operation. to calculate the stress under short circuit conditions, substitute i overcurrent(max) for i out(max) . use the formula below to calculate d under short circuit condi- tions. d 0.063 1.8 10 v shortcircuit 3 in = ? ? the rms value of the high-side switch current is: i d i i 12 sw(high side)(rms) out(max) 2 pp 2 ? = + ?? ? ?? ? i 1 d i i 12 sw(low side)(rms) out(max) 2 pp 2 ? = ? ( ) + ?? ? ?? ? where: d = duty cycle of the converter d v v out in = = ef?ciency of the converter. converter ef?ciency depends on component parameters, which have not yet been selected. for design purposes, an ef?ciency of 90% can be used for v in less than 10v and 85% can be used for v in greater than 10v. the ef?ciency can be more accurately calculated once the design is complete. if the assumed ef?ciency is grossly inaccurate, a second iteration through the design procedure can be made. downloaded from: http:///
october 2005 11 mic2198 mic2198 micrel, inc. for the high-side switch, the maximum dc power dissipa- tion is: p r i switch1(dc) ds(on)1 sw1(rms) 2 = for the low-side switch (n-channel mosfet), the dc power dissipation is: p r i switch2(dc) ds(on)2 sw 2(rms) 2 = since the ac switching losses for the low-side mosfet is near zero, the total power dissipation is: p p low-side mosfet(max) switch2(dc) = the total power dissipation for the high-side mosfet is: p p p high sidemosfet(max) switch 1(dc) ac ? = + external schottky diode an external freewheeling diode is used to keep the inductor current ?ow continuous while both mosfets are turned off. this dead time prevents current from ?owing unimpeded through both mosfets and is typically 80ns the diode conducts twice during each switching cycle. although the average current through this diode is small, the diode must be able to handle the peak current. i i 2 80ns f d(avg) out s = the reverse voltage requirement of the diode is: v v diode(rrm) in = the power dissipated by the schottky diode is: p i v diode d(avg) f = where: v f = forward voltage at the peak diode current the external schottky diode, d2, is not necessary for circuit operation since the low-side mosfet contains a parasitic body diode. the external diode will improve ef?ciency and decrease high frequency noise. if the mosfet body diode is used, it must be rated to handle the peak and average cur- rent. the body diode has a relatively slow reverse recovery time and a relatively high forward voltage drop. the power lost in the diode is proportional to the forward voltage drop of the diode. as the high-side mosfet starts to turn on, the body diode becomes a short circuit for the reverse recovery period, dissipating additional power. the diode recovery and the circuit inductance will cause ringing during the high-side mosfet turn-on. an external schottky diode conducts at a lower forw ard voltage preventing the body diode in the mosfet from turning on. the lower forward voltage drop dissipates less power than the body diode. the lack of a reverse recovery mechanism in a schottky diode causes less ringing and less power loss. depending on the circuit components and operating condi- tions, an external schottky diode will give a 1/2% to 1% improvement in ef?ciency. output capacitor selection the output capacitor values are usually determined by the capacitors esr (equivalent series resistance). voltage rating and rms current capability are two other important factors in selecting the output capacitor. recommended capacitors are tantalum, low-esr aluminum electrolytics, and os-con. the output capacitors esr is usually the main caus e of output ripple. the maximum value of esr is calculated by: r v i esr out pp ? where: v out = peak-to-peak output voltage ripple i pp = peak-to-peak inductor ripple current the total output ripple is a combination of the esr and the output capacitance. the total ripple is calculated below: ?v i (1 d) c f i r out pp out s 2 pp esr 2 = ? ?? ? ?? ? + ( ) where: d = duty cycle c out = output capacitance value f s = switching frequency the voltage rating of capacitor should be twice the output voltage for a tantalum and 20% greater for an aluminum electrolytic or os-con. the output capacitor rms current is calculated below: i i 12 c pp out(rms) = the power dissipated in the output capacitor is: p diss(c out ) = i c out(rms) 2 r esr(c out ) input capacitor selection the input capacitor should be selected for ripple c urrent rating and voltage rating. tantalum input capacitors may fail when subjected to high inrush currents, caused by turning the input supply on. tantalum input capacitor voltage rating should be at least 2 times the maximum input voltage to maximize reliability. aluminum electrolytic, os-con, and multilayer polymer ?lm capacitors can handle the higher inrush currents without voltage derating. the input voltage ripple will primarily depend on the input capacitors esr. the peak input current is equal to the peak inductor current, so: ?v i r in inductor(peak) esr(c ) in = the input capacitor must be rated for the input current ripple. the rms value of input capacitor current is determined at the maximum output current. assuming the peak-to-peak inductor ripple current is low: i i d (1 d) c (rms) out(max) in ? downloaded from: http:///
mic2198 micrel, inc. mic2198 12 october 2005 the power dissipated in the input capacitor is: p i r diss(c ) c (rms) esr(c ) in in 2 in = voltage setting components the mic2198 requires two resistors to set the output voltage as shown in figure 6. error 3 mic2198 fb v ref 0.8v r2 r1 amp figure 6. voltage-divider con?guration the output voltage is determined by the equation: v v 1 r1 r2 o ref = + ?? ? ?? ? where: v ref for the mic2198 is typically 0.8v. a typical value of r1 can be between 3k and 10k. if r1 is too large it may allow noise to be introduced into the voltage feedback loop. if r1 is too small in value it will decrease the ef?ciency of the power supply, especially at low output loads. once r1 is selected, r2 can be calculated using: r2 v r1 v v ref o ref = ? voltage divider power dissipation the reference voltage and r2 set the current through the voltage divider. i v r2 divider ref = the power dissipated by the divider resistors is: p (r1 r2) i divider divider 2 = + ef?ciency calculation and considerations ef?ciency is the ratio of output power to input power. the difference is dissipated as heat in the buck converter. under light output load, the signi?cant contributors are: ? supply current to the mic2198 ? mosfet gate-charge power (included in the ic supply current) ? core losses in the output inductor to maximize ef?ciency at light loads: ? use a low gate-charge mosfet or use the small- est mosfet, which is still adequate for maximum output current. ? use a ferrite material for the inductor core, which has less core loss than an mpp or iron power core. under heavy output loads the signi?cant contributor s to power loss are (in approximate order of magnitude): ? resistive on-time losses in the mosfets ? switching transition losses in the mosfets ? inductor resistive losses ? current-sense resistor losses ? input capacitor resistive losses (due to the capaci- tors esr) to minimize power loss under heavy loads: ? use logic-level, low on-resistance mosfets. multiplying the gate charge by the on-resistance gives a ?gure of merit, providing a good balance between low and high load ef?ciency. ? slow transition times and oscillations on the volt age and current waveforms dissipate more power during turn-on and turnoff of the mosfets. a clean layout will minimize parasitic inductance and capacitance in the gate drive and high current paths. this will allow the fastest transition times and waveforms without oscillations. low gate-charge mosfets will transition faster than those with higher gate- charge requirements. ? for the same size inductor, a lower value will have fewer turns and therefore, lower winding re- sistance. however, using too small of a value will require more output capacitors to ?lter the output ripple, which will force a smaller bandwidth, slower transient response and possible instability under certain conditions. ? lowering the current-sense resistor value will decrease the power dissipated in the resistor. however, it will also increase the overcurrent limit and will require larger mosfets and inductor components. ? use low-esr input capacitors to minimize the power dissipated in the capacitors esr. decoupling capacitor selection the 4.7f decoupling capacitor is used to minimize noise on the v dd pin. the placement of this capacitor is critical to the proper operation of the ic. it must be placed right next to the pins and routed with a wide trace. the capacitor should be a good quality tantalum. an additional 1f ceramic capacitor may be necessary when driving large mosfets with high gate capacitance. incorrect placement of the v dd decoupling capacitor will cause jitter or oscillations in the switching wave- form and large variations in the overcurrent limit. a 0.1f ceramic capacitor is required to decouple the v in . the capacitor should be placed near the ic and connected directly to between pin 6 (v in ) and pin 9 (gnd). downloaded from: http:///
october 2005 13 mic2198 mic2198 micrel, inc. pcb layout and checklist pcb layout is critical to achieve reliable, stable and ef?cient performance. a ground plane is required to control emi and minimize the inductance in power, signal and return paths. the following guidelines should be followed to insure proper operation of the circuit. ? signal and power grounds should be kept separate and connected at only one location. large currents or high di/dt signals that occur when the mosfets turn on and off must be kept away from the small signal connections. ? the connection between the current-sense resis- tor and the mic2198 current-sense inputs (pin 4 and 5) should have separate traces, through a 10 resistor on each pin. the traces should be routed as closely as possible to each other and their length should be minimized. avoid running the traces under the inductor and other switching components. the 10 resistor should be placed close as possible to pins 4 and 5 on the mic2198 and a 1nf to 0.1f capacitor placed between pins 4 and 5 will help attenuate switching noise on the current sense traces. this capacitor should be placed close to pins 4 and 5. ? when the high-side mosfet is switched on, the critical ?ow of current is from the input capacitor through the mosfet, inductor, sense resistor, output capacitor, and back to the input capacitor. these paths must be made with short, wide pieces of trace. it is good practice to locate the ground terminals of the input and output capacitors close to each. ? when the low-side mosfet is switched on, cur- rent ?ows through the inductor, sense resistor, output capacitor, and mosfet. the source of the low-side mosfet should be located close to the output capacitor. ? the freewheeling diode, d1 in figure 2, conducts current during the dead time, when both mosfets are off. the anode of the diode should be located close to the output capacitor ground terminal and the cathode should be located close to the input side of the inductor. ? the 4.7f capacitor, which connects to the v dd terminal (pin 7) must be located right at the ic. the v dd terminal is very noise sensitive and placement of this capacitor is very critical. connections must be made with wide trace. the capacitor may be located on the bottom layer of the board and con- nected to the ic with multiple vias. ? the v in bypass capacitor should be located close to the ic and connected between pins 6 and 9. connections should be made with a ground and power plane or with short, wide trace. downloaded from: http:///
mic2198 micrel, inc. mic2198 14 october 2005 package information 12-pin 4 4 mlf? (ml) micrel, inc. 2180 fortune drive san jose, ca 95131 usa tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 web http://www.micrel.com the information furnished by micrel in this datasheet is believed to be accurate and reliable. howev er, no responsibility is assumed by micrel for its use. micrel reserves the right to change circuitry and speci?cations at any time without noti?cation to t he customer. micrel products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a signi?cant injury to the user. a purchasers use or sale of micrel products for use in life support appliances, devices or systems is at purchase rs own risk and purchaser agrees to fully indemnify micrel for any damages resulting from such use or sale. ? 2003 micrel, inc. downloaded from: http:///

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