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MP2908A 4v-60v input, current mode, synchronous step-down controller MP2908A rev. 1.01 www.monolithicpower.com 1 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. the future of analog ic technology description the MP2908A is a high-voltage, synchronous step-down controller that directly steps down voltages from up to 60v. the MP2908A uses pwm current control architecture with accurate cycle-by-cycle current limiting and is capable of driving dual n-channel mosfets. advanced asynchronous mode (aam) enables non-synchronous operation to optimize light- load efficiency. the operating frequency of the MP2908A can be programmed by an external resistor or synchronized to an external clock for noise- sensitive applications. full protection features include precision output over-voltage protection (ovp), output over-current protection (ocp), and thermal shutdown. the MP2908A is available in tssop20-ep and qfn-20 (3mmx4mm) packages. applications ? automotive ? industrial control systems features ? wide 4v to 60v operating input range ? dual n-channel mosfet driver ? 0.8v voltage reference with 1.5% accuracy over temperature ? low dropout operation: maximum duty cycle at 99.5% ? programmable frequency range: 100khz - 1000khz ? external sync clock range: 100khz- 1000khz ? 180o out-of-phase synco ? programmable soft start ? power good output voltage monitor ? selectable cycle-by-cycle current limit ? output over-voltage protection (ovp) ? over-current protection (ocp) ? internal ldo with external power supply option ? programmable ccm, aam mode ? tssop20-ep and qfn-20 (3mmx4mm) packages all mps parts are lead-free, halogen-free, and adhere to the rohs directive. for mps green status, pleas e visit the mps website under quality assurance. ?mps? and ?the future of analog ic technology? are registered trademarks of monolithic power systems, inc. typical application
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 2 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. ordering information part number package top marking MP2908Agf * tssop-20 ep see below MP2908Agl ** qfn-20 see below * for tape & reel, add suffix ?z (e.g. MP2908Agf?z) * *for tape & reel, add suffix ?z (e.g. MP2908Agl?z) top marking (tssop-20 ep) mps: mps prefix yy: year code ww: week code MP2908A: product code of MP2908Agf lllllllll: lot number top marking (qfn-20(3mm x4 mm)) mp: mps prefix: yy: year code; w: week code: 2908a: part number; lll: lot number;
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 3 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. package reference top view top view bst en/sync tg in ccm/aam synco freq pg vcc1 vcc2 comp fb ss sgnd ilim bg pgnd sw sense+ sense- MP2908A 1 2 3 4 5 6 7 8 9 10 20 19 18 17 16 15 14 13 12 11 tssop-20 ep qfn-20 (3mmx4mm) absolute maxi mum ratings (1) input supply voltage (v in )............................. 65v bst supply voltage (v bst ).................. v in + 6.5v sw ..................................................-0.3v to 65v bst - sw .................................................... 6.5v supply voltage (vcc1) ............................... 6.5v external supply voltage (vcc2)................... 15v sense + / - ................................................. 28v differential sense (sense+ to sense-) ............ .....................................................-0.7v to +0.7v tg ...............................v sw - 0.3v to v bst + 0.3v bg ................................... -0.3v to vcc1 + 0.3v all other pins ................................-0.3v to +6.5v continuous power dissipation (t a = +25c) (2) tssop-20 ep............................................ 3.1w qfn-20 (3mmx4mm) ................................. 2.6w junction temperature ................................150 c lead temperature .....................................260 c storage temperature ................ -65 c to +175 c recommended operating conditions (3) supply voltage (v in ) ............................ 4v to 60v output voltage (v out ).................................. 24v supply voltage for (vcc2)............... 4.5v to 12v operating junction temp. (t j )....-40c to +125c thermal resistance (4) ja jc tssop-20 ep ........................ 40 ....... 8.... c/w qfn-20 (3mmx4mm).............. 48 ...... 10... c/w notes: 1) exceeding these ratings may damage the device. 2) the maximum allowable power dissipation is a function of the maximum junction temperature t j (max), the junction-to- ambient thermal resistance ja , and the ambient temperature t a . the maximum allowable continuous power dissipation at any ambient temperature is calculated by p d (max) = (t j (max)-t a )/ ja . exceeding the maximum allowable powe r dissipation produces an excessive die temperature, causing the regulator to go into thermal shutdown. internal thermal shutdown circuitry protects the device from permanent damage. 3) the device is not guaranteed to function outside of its operating conditions. 4) measured on jesd51-7, 4-layer pcb.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 4 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. electrical characteristics v in = 24v, t j = +25 c, en = 2v, v ilimit = 75mv, unless otherwise noted. parameters symbol condition min typ max units input supply v in uvlo threshold (rising) in uv _ rising 4.5 5 v v in uvlo threshold (falling) in uv _ falling 3.7 3.95 v v in uvlo hysteresis in uv _ hys 800 mv v in supply current with vcc2 bias i q _ vcc2 vcc2 = 12v, external bias 25 40 a v in supply current without vcc2 bias i q vcc2 = 0, v fb = 0.84v, v aam = 5v, sense+ = sense- = 0.3v 750 1000 a v in aam current i q _ aam v aam =0.6v, v fb =0.84v, sense+ = sense- = 0.3v 250 350 a v in shutdown current i shdn v en = 0v 0.5 1.5 a v cc regulator vcc1 regulator output voltage from v in vcc1_ vin vin > 6v 5 v vcc1 regulator load regulation from v in load = 0 to 50ma, vcc2 floating or connects to gnd 1 3 % vcc1 regulator output voltage from vcc2 vcc1_ vcc2 vcc2 > 6v 5 v vcc1 regulator load regulation from vcc2 load = 0 to 50ma, vcc2 = 12v 1 3 % vcc2 uvlo threshold (rising) vcc2_ rising 4.7 4.92 v vcc2 uvlo threshold (falling) vcc2_ falling 4.45 v vcc2 threshold hysteresis vcc2_ hys 250 mv v aam = 5v, v fb = 0.84v, vcc2 = 12v 800 a vcc2 supply current i vcc2 v aam = 0.6v, v fb = 0.84v, vcc2 = 12v 200 a feedback (fb) feedback voltage v fb 4v v in 60v 0.788 0.800 0.812 v feedback current i fb v fb = 0.8v 10 na enable (en) enable threshold (rising) v en _ rising 1.16 1.22 1.28 v enable threshold (falling) v en _ falling 1.03 1.09 1.15 v enable threshold hysteresis v en _ th 130 mv en input current i en v en = 2v 2 a enable turn-off delay t off 10 15 s
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 5 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. electrical characteristics (continued) v in = 24v, t j = +25 c, en = 2v, v ilimit = 75mv, unless otherwise noted. parameters symbol condition min typ max units oscillator and sync operating frequency f sw r freq = 65k ? 240 300 360 khz foldback operating frequency f sw _ foldback v fb =0.1v 50% f sw maximum programmable frequency f swh 1000 khz minimum programmable frequency f swl 100 khz sync/en frequency range f sync 100 1000 khz sync/en voltage rising threshold v sync _ rising 2 v sync/en voltage falling threshold v sync _ falling 0.35 v current sense current sense common mode voltage range v sense+/- 0 24 v i lim = gnd, v sense+ = 3.3v 15 25 35 mv i lim = vcc1, v sense+ = 3.3v 40 50 60 mv current limit sense voltage v ilimit i lim = float, v sense+ = 3.3v 65 75 85 mv i lim = gnd, v sense+ = 3.3v 8 i lim = vcc1, v sense+ = 3.3v 17 reverse current limit sense voltage v rev _ ilimit i lim = float, v sense+ = 3.3v 24 mv i lim = gnd, v sense+ = 3.3v 22.5 i lim = vcc1, v sense+ = 3.3v 47.5 valley current limit v val _ ilimit i lim = float, v sense+ = 3.3v 72.5 mv v sense+/-(cm) = 0v -45 a v sense+/-(cm) = 3.3v 115 a input current of sensor i sense v sense+/-(cm) > 5v 150 a soft start (ss) soft-start source current i ss ss = 0.5v 2 4 6 a error amplifier error amp transconductance (5) g m v = 5mv 500 s error amp open loop dc gain (5) a o 70 db error amp sink/source current i ea fb = 0.7/0.9v 30 a protection over-voltage threshold v ov 110% 115% 120% v fb over-voltage hysteresis v ov _ hys 10% v fb thermal shutdown (6) 170 c thermal shutdown hysteresis (6) 20 c
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 6 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. electrical characteristics (continued) v in = 24v, t j = +25 c, en = 2v, v ilimit = 75mv, unless otherwise noted. parameters symbol condition min typ max units gate driver tg pull-up resistor r tg _ pullup 2 ? tg pull-down resistor r tg _ pulldn 1 ? bg pull-up resistor r bg _ pullup 3 ? bg pull-down resistor r bg _ pulldn 1 ? dead time t dead c load = 3.3nf 60 ns tg maximum duty cycle d max v fb = 0.7v 98 99.5 % tg minimum on time (6) t on _ min _ tg 92 ns bg minimum on time t on _ min _ bg 175 250 ns power good power good low v pg _ low i load = 4ma 0.1 0.3 v v out rising 85% 90% 96.5% pg rising threshold pg vth _ rsing v out falling 101% 107% 112.5% v fb v out falling 81% 87% 92.5% pg falling threshold pg vth _ falling v out rising 105% 110% 116.5% v fb pg threshold hysteresis pg vth _ hys 3% v fb power good leakage i pg _ lk pg = 5v 2 a power good delay t pg _ delay 25 s aam/ccm aam output current i aam r freq = 65 k ? 9.2 a ccm required aam threshold voltage v ccm _ th 2.3 v notes: 5) guaranteed by design, not tested. 6) derived from bench characte rization, not tested in production.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 7 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. typical performanc e characteristics v in = 24v, v out = 5v, l = 4.7h, t a = +25c input voltage (v) load current (ma) ic thermal rise line regulation load regulation output current (a) i out (ma) efficiency vs. load current v out =5v 40 50 60 70 80 90 100 1 10 100 1000 10000 v in =12v v in =24v v in =48v 0 5 10 15 20 25 30 35 40 012345678 -0.15 -0.10 -0.05 0.00 0.05 1 10 100 1000 10000 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0.20 10 20 30 40 50 60 i out =0a i out =4a i out =8a
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 8 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. typical performanc e characteristics (continued) v in = 24v, v out = 5v, l = 4.7h, t a = +25c start-up through vin i out =0 start-up through vin i out =7a shutdown through vin i out =7a shutdown through vin i out =0 start-up through en i out =7a start-up through en i out =0 v out /ac 20mv/div. v out 2v/div. v sw 10v/div. v in 10v/div. i l 500ma/div. v out /ac 20mv/div. v sw 10v/div. i l 2a/div. v out /ac 20mv/div. v sw 10v/div. i l 2a/div. v sw 20v/div. i l 1a/div. v out 2v/div. v in 10v/div. v sw 20v/div. i l 5a/div. v out 2v/div. v in 10v/div. v sw 10v/div. i l 500ma/div. v out 2v/div. v in 10v/div. v sw 20v/div. i l 5a/div. v out 2v/div. v en 2v/div. v sw 20v/div. i l 2a/div. v out 2v/div. v en 2v/div. v sw 20v/div. i l 5a/div. steady state i out =0, aam mode steady state i out =0, forced ccm mode steady state i out =7a
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 9 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. typical performanc e characteristics (continued) v in = 24v, v out = 5v, l = 4.7h, t a = +25c scp entry i out =7a to short circuit scp steady state scp recovery short circuit to i out =7a scp recovery short circuit to i out =0 v out 2v/div. v pok 2v/div. v sw 20v/div. i l 10a/div. v out 1v/div. v ss 1v/div. v sw 20v/div. i l 5a/div. v out 2v/div. v pok 2v/div. v sw 20v/div. i l 10a/div. v out 2v/div. v pok 2v/div. v sw 20v/div. i l 10a/div. v out /ac 100mv/div. v sw 20v/div. i l 5a/div. shutdown through en i out =0 shutdown through en i out =7a scp entry i out =0 to short circuit v out 2v/div. v en 2v/div. v sw 5v/div. i l 2a/div. v out 2v/div. v en 2v/div. v sw 20v/div. i l 5a/div. v out 2v/div. v pok 2v/div. v sw 20v/div. i l 10a/div.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 10 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. pin functions tssop pin # qfn20 pin # name description 1 19 in input supply. the MP2908A operates on a 4v to 60v input range. a ceramic capacitor is needed to prevent large voltage spikes at the input. 2 20 en/sync enable input. the threshold is 1.22v with 140mv of hysteresis and is used to implement an input under-voltage lockout (uvlo) function externally. if an external sync clock is applied to en/sync, the internal clock follows the sync frequency. 3 1 vcc2 external power supply for the internal vcc1 regulator. vcc2 disables the power from v in as long as vcc2 is higher than 4.5v. do not connect a power supply greater than 12v to v cc2. connecting vcc2 to an external power supply reduces power dissipation and increases efficiency. 4 2 vcc1 internal bias supply. decouple vcc1 with a ceramic capacitor 1f or greater. the capacitance should be no more than 4.7f. 5 3 sgnd low-noise ground reference. sgnd should be connected to the v out side of the output capacitors. 6 4 ss soft-start control input. ss is used to program the soft-start period with an external capacitor between ss and sgnd. 7 5 comp comp is used to compensate the regulation control loop. connect an rc network from comp to gnd to compensate for the regulation control loop. 8 6 fb feedback. fb is the input to the error amplifier. an external resistive divider connected between the output and gnd is compared to the internal +0.8v reference to set the regulation voltage. 9 7 ccm/aam continuous conduction mode/advanced asynchronous mode. floating ccm/aam or connecting ccm/aam to vcc1 sets the part to operate in ccm. connecting an appropriate external resistor from ccm/aam to gnd (so aam is at a low level) sets the part to operate in aam. the aam voltage should be no less than 480mv. 10 8 freq connect a resistor between freq and gnd to set the switching frequency. 11 9 pg power good output. the output of pg is an open drain. 12 10 ilim sense voltage limit set. the voltage at ilim sets the nominal sense voltage at the maximum output current. there are three fixed options: float, vcc1, and gnd. 13 11 synco synco outputs an out-of-phase 180oclock when the part works in ccm for dual-channel operation. 14 12 sense- negative input for the current sense. the sensed inductor current limit threshold is determined by the status of ilim. 15 13 sense+ positive input for the current sense. the sensed inductor current limit threshold is determined by the status of ilim. 16 14 pgnd high-current ground reference for the internal low-side switch driver and the vcc1 regulator circuit. connect pgnd directly to the negative terminal of the vcc1 decoupling capacitor.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 11 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. pin functions (continued) tssop pin # qfn20 pin # name description 17 15 bg bottom gate driver output. connect bg to the gate of the synchronous n-channel mosfet. 18 16 sw switch node. sw is the reference for the v bst supply and high-current returns for the bootstrapped switch. 19 17 tg top gate drive. tg drives the gate of the top n-channel synchronous mosfet. the tg driver draws power from the bst capacitor and returns to sw, providing a true oating drive to the top n-channel mosfet. 20 18 bst bootstrap. bst is the positive power supply for the internal, floating, high- side mosfet driver. connect a bypass capacitor between bst and sw. a diode from vcc1 to bst charges the bst capacitor when the low-side switch is off.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 12 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. time sequence vin vcc1 0 0 0 0 0 en pg sw vcc1 en ss 25us 25us 0 start up ocp oc release ov 0 il 90%re f 25u s normal normal nor mal 15 us 25u s shutdown v o il=ilimit thr es hold thr es hold 90 % ref 11 0% re f 62.5% ref figure 1: time sequence
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 13 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. operation the MP2908A is a high-performance, step- down, synchronous dc/dc controller ic with a wide input voltage range. it implements current mode and switching frequency programmable control architecture to regulate the output voltage with external n-channel mosfets. the MP2908A senses the voltage at fb; the difference between the fb voltage and an internal 0.8v reference is ampli ed to generate an error voltage on comp. this is used as a threshold for the current sense comparator with a slope compensation ramp. under normal load conditions, the controller operates in full pwm mode (see figure 2). at the beginning of each oscillator cycle, the top gate driver is enabled. the top gate turns on for a period determined by the duty cycle. when the top gate turns off, the bottom gate turns on after a dead time and remains on until the next clock cycle begins. there is an optional power-save mode for light- load or no-load conditions. advanced asynchronous mode (aam) the MP2908A employs aam functionality to optimize efficiency during light-load or no-load conditions (see figure 2). it is enabled when ccm/aam is at a low level by connecting an appropriate resistor to gnd to ensure that v aam is no less than 480mv. see equation (1): v aam (mv) = i aam ( a) x r aam (k ? ) (1) where i aam is the ccm/aam output current. the ccm/aam output current (i aam ) is shown in equation (2). aam is disabled when ccm/aam is floating or connected to vcc1. i aam ( a) = 600 (mv) / r freq (k ? ) (2) if aam is enabled, the MP2908A first enters non-synchronous operation as long as the inductor current approaches zero at light-load. if the load decreases further to make the comp voltage drop below the ccm/aam voltage (v aam ), the MP2908A enters aam. in aam, the internal clock resets whenever v comp crosses over v aam ; the crossover time is the benchmark for the next clock cycle. when the load increases and the dc value of v comp is higher than v aam , the operation mode is dcm or ccm, which has a constant switching frequency. aam mode (aam=low) inductor current t t t load decreased pwm mode (aam=high) inductor current t t t load decreased figure 2: aam and pwm floating driver and bootstrap charging the floating top gate driver is powered by an external bootstrap capacitor (c bst ), which is normally refreshed when the high-side mosfet (hs-fet) turns off. this floating driver has its own uvlo protection. this uvlo?s rising threshold is 3.05v with a hysteresis of 170mv. if the bst voltage is lower than the bootstrap uvlo, the MP2908A enters constant-off-time mode to ensure that the bst cap is high enough to drive the hs-fet. vcc1 regulator and vcc2 power supply both the top and bottom mosfet drivers and most of the internal circuitries are powered by the vcc1 regulator. an internal, low, dropout linear regulator supplies vcc1 power from vin. connect a ceramic capacitor 4.7 f or smaller from vcc1 to gnd. if vcc2 is left open or connected to a voltage less than 4.5v, an internal 5v regulator supplies power to vcc1 from v in . if vcc2 is greater than 4.5v, the internal regulator that supplies power to vcc1 from vcc2 is triggered. if vcc2 is greater than 4.5v but less than 5v, the 5v regulator is in dropout, and vcc1 is approximately equal to vcc2. using the vcc2 power supply allows the vcc1 power to be derived from a high-efficiency external source, such as one of the MP2908A?s switching regulator outputs.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 14 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. error amplifier the error amplifier compares the fb voltage with the internal 0.8v reference (ref) and outputs a current proportional to the difference between the two input voltages. this output current is then used to charge or discharge the external compensation network to form the comp voltage, which is used to control the power mosfet current. adjusting the compensation network from comp to gnd optimizes the control loop for good stability or fast transient response. current limit function there are three fixed current limit options: 25mv, when ilim is connected to gnd; 50mv, when ilim is connected to vcc1; and 75mv, when ilim is floating. when the peak value of the inductor current exceeds the set current limit threshold, the output voltage begins dropping until fb is 37.5% below the reference. the MP2908A enters hiccup mode to restart the part periodically. the frequency is lowered when fb is below 0.4v. this protection mode is especially useful when the output is dead- shorted to ground. the average short-circuit current is reduced greatly to alleviate thermal issues. the MP2908A exits hiccup mode once the over-current condition is removed. low dropout operation in low dropout mode, the MP2908A is designed to operate in a hs fully on mode as long as the voltage difference across bst - sw is greater than 3.05v, improving dropout. when the voltage from bst to sw drops below 3.05v, an under-voltage lockout (uvlo) circuit turns off the high-side mosfet (hs-fet). at the same time, the low-side mosfet (ls-fet) turns on to refresh the charge on the bst capacitor. after the bst capacitor voltage is re-charged, the hs-fet turns on again to regulate the output. since the supply current sourced from the bst capacitor is low, the hs-fet can remain on for more switching cycles than are required to refresh the bst capacitor, increasing the effective duty cycle of the switching regulator. the low dropout operation makes the MP2908A suitable for automotive cold-crank. power good (pg) function the MP2908A includes an open-drain power good output that indicates whether the regulator?s output is within 10% of its nominal value. when the output voltage falls outside of this range, the pg output is pulled low. it should be connected to a voltage source no more than 5v through a resistor (e.g., 100k ? ). the pg delay time is 25s. soft start (ss) the soft start (ss) is implemented to prevent the converter output voltage from overshooting during start-up. when the chip starts, the internal circuitry generates a soft-start voltage that ramps up from 0v to 1.2v. when it is lower than the internal reference (ref), ss overrides ref, so the error amplifier uses ss as the reference. when ss is higher than ref, ref regains control. an external capacitor connected from ss to sgnd is charged from an internal 4 a current source, producing a ramped voltage. the soft- start time (t ss ) is set by the external ss capacitor and can be calculated by equation (3): () () () () ai vvnfc mst ss ref ss ss = (3) where c ss is the external ss capacitor, v ref is the internal reference voltage (0.8v), and i ss is the 4 a ss charge current. there is no internal ss capacitor. ss is reset when a fault protection other than ovp occurs. output over-voltage protection (ovp) the output over-voltage is monitored by the fb voltage. if the fb voltage is typically 10% higher than the reference, the MP2908A enters discharge mode: the hs-fet turns off, and the ls-fet turns on. the ls-fet remains on until the reverse current limit is triggered. the ls- fet then turns off, and the inductor current increases to 0. the ls-fet is turned on again after zcd is triggered. the MP2908A works in discharge mode until the over-voltage condition is cleared.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 15 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. en/sync control the MP2908A has a dedicated enable (en/sync) control. it uses a bandgap- generated precision threshold of 1.22v. by pulling it high or low, the ic can be enabled or disabled. to disable the part, en/sync must be pulled low for at least 15s. tie en/sync to vin through a resistor divider r5 and r6 to program the vin start-up threshold (see figure 3). the en/sync threshold is 1.09v (falling edge), so the v in uvlo threshold is 1.09v x (1+ r5/r6). figure 3: en resistor divider synchronize the MP2908A can be synchronized to an external clock ranging from 100khz up to 1000khz through en/sync. the internal clock rising edge is synchronized to the external clock rising edge. the pulse width (both on and off) of the external clock signal should be no less than 100ns. under-voltage lockout (uvlo) under-voltage lockout (uvlo) is implemented to protect the chip from operating at insufficient input supply voltages. the MP2908A uvlo rising threshold is about 4.5v while its falling threshold is a consistent 3.7v. thermal protection thermal protection prevents damage to the ic from excessive temperatures. the die temperature is monitored internally until the thermal limit is reached. when the silicon die temperature is higher than 170c, the entire chip shuts down. when the temperature is below its lower threshold (typically 20c), the chip is enabled again. start-up and shutdown if both vin and en/sync are higher than their respective thresholds, the chip starts up. the reference block starts first, generating stable reference voltages and currents. the internal regulator is then enabled. the regulator provides a stable supply for the remaining circuitry. three events can shut down the chip: en low, vin low, and thermal shutdown. during the shutdown procedure, the signal path is blocked first to avoid any fault triggering. the comp voltage and the internal supply rail are then pulled down. the floating driver is not subjected to this shutdown command. pre-bias start-up if ss is less than fb at start-up, the output has a pre-bias voltage and neither tg nor bg is turned on until ss is greater than fb.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 16 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. vcc regulator oscillator reference error amplifier control current limit comparator boost regulator hs driver ls driver fb pgnd sw bst vcc1 ccm/aam tg bg current sense amplifer sense+ sense- comp in freq vcc1 en/sync pg sgnd v pg vref 12x 4.5v vcc2 ilim synco ss ss vcc1 figure 4: block diagram
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 17 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. application information setting the output voltage the external resistor divider is used to set the output voltage (see figure 5). fb vout r17 r18 figure 5: external resistor divider if r17 is known, then r18 can be calculated with equation (4): 1 0.8v v r r out 17 18 ? = (4) table 1 lists the recommended feedback resistor values for common output voltages. table 1: resistor selection for common output voltages (needs callout) v out (v) r17 (k ? ) r18 (k ? ) 3.3 37.4 (1%) 12 (1%) 5 63.4 (1%) 12 (1%) 12 169 (1%) 12 (1%) setting current sensing the MP2908A has three fixed current limit options: 25mv, when ilim is connected to gnd; 50mv, when ilim is connected to vcc1; and 75mv, when ilim is floating. the current sense resistor (r sense ) monitors the inductor current. its value is chosen based on the current limit threshold. the relationship between the peak inductor current (i pk ) and r sense can be calculated with equation (5): ipk ilimit v r sense = (5) the typical values for r sense are in the range of 10m ? to 50m ? . programmable switching frequency consider different variables when choosing the switching frequency. a high frequency increases switching losses and gate charge losses while a low frequency requires more inductance and capacitance, resulting in larger real estate and higher cost. it is a trade off between power loss and passive component size. in noise-sensitive applications, the switching frequency should be out of a sensitive frequency band. the MP2908A?s frequency can be programmed from 100khz to 1000khz with a resistor from freq to sgnd (see table 2). the value of r freq for a given operating frequency can be calculated with equation (6): freq s 20000 r(k) 1 f(khz) =? (6) to get f s = 500khz, set r freq to 39k ? . table 2: frequency vs. resistor resistor (k ? ) frequency (khz) 65 300 39 500 19 1000 v cc regulator connection vcc1 can be powered from both vin and vcc2. if connecting vcc2 to an external power supply to improve the overall efficiency, vcc2 should be larger than 4.5v but smaller than 12v (see figure 6). figure 6: internal circuitry of vcc2
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 18 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. if v out is higher than 4.5v but less than 12v, vcc2 can be connected to v out directly (see figure 7). figure 7: configuration of vcc2 connecting to v out selecting the inductor an inductor with a dc current rating at least 25% higher than the maximum load current is recommended for most applications. a larger value inductor results in less ripple current and a lower output ripple voltage. however, the larger value inductor also has a larger physical size, higher series resistance, and lower saturation current. choose the inductor ripple current approximately 30% of the maximum load current. the inductance value can be then be calculated with equation (7): out in out in l s v(v-v) l v if = (7) where v out is the output voltage, v in is the input voltage, f s is the 300khz switching frequency, and l i is the peak-to-peak inductor ripple current. the maximum inductor peak current can be calculated with equation (8): l l(max) load i i=i+ 2 (8) where i load is the load current. input capacitor selection since the input capacitor absorbs the input switching current, it requires an adequate ripple current rating. the selection of the input capacitor is based mainly on its maximum ripple current capability. the rms value of the ripple current flowing through the input capacitor can be calculated with equation (9): out out rms load in in vv i=i (1- ) vv (9) the worst-case condition occurs at v in = 2v out , shown in equation (10): i rms = i load /2 (10) the input capacitor must be capable of handling this ripple current. output capacitor selection the output capacitor keeps the output voltage the output capacitor impedance should be low at the switching frequency. the output voltage ripple can be estimated with equation (11): out out out esr si n s o vv 1 v1r fl v 8fc ?? ?? =? + ?? ?? ?? ?? (11) where c o is the output capacitance value and r esr is the equivalent series resistance (esr) value of the output capacitor. for tantalum or electrolytic capacitor application, the esr dominates the impedance at the switching frequency. formula 11 can then be approximated with equation (12): out out out esr si n vv v1r fl v ?? =? ?? ?? (12) compensation components the MP2908A employs current-mode control for easy compensation and fast transient response. comp is the output of the internal error amplifier and controls system stability and transient response. a series capacitor-resistor combination sets a pole-zero combination to control the control system?s characteristics. the dc gain of the voltage feedback loop can be calculated with equation (13): out fb o cs load vdc v v agra = (13) where a o is the error-amplifier voltage gain 3000v/v, g cs is the current-sense transconductance 1/(12xr sense ) (a/v), and r load is the load resistor value.
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 19 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. comp r7 c4 c5 figure 8: compensation network the system has two important poles: one from the compensation capacitor (c4) and the output resistor of the error amplifier and the other from the output capacitor and the load resistor (see figure 8). these poles can be calculated with equation (14) and equation (15): o m p1 ac42 g f = (14) load p2 rco2 1 f = (15) where g m is the error-amplifier transconductance 500 a/v, and co is the output capacitor. the system has one important zero due to the compensation capacitor and the compensation resistor (r7), and can be calculated with equation (16): r7c42 1 f z1 = (16) the system may have another significant zero if the output capacitor has a large capacitance or a high esr value, and can be calculated with equation (17): esr esr rco2 1 f = (17) in this case, a third pole set by the compensation capacitor (c5) and the compensation resistor can compensate for the effect of the esr zero. this pole is calculated with equation (18): r7c52 1 f p3 = (18) the goal of the compensation design is to shape the converter transfer function for a desired loop gain. the system crossover frequency where the feedback loop has unity gain is important, since lower crossover frequencies result in slower line and load transient responses, and higher crossover frequencies lead to system instability. set the crossover frequency to ~0.1f sw . follow the steps below to design the compensation: 1. choose r7 to set the desired crossover frequency with equation (19): fb out cs m c v v gg fco2 r7 = (19) where f c is the desired crossover frequency. 2. choose c4 to achieve the desired phase margin. for applications with typical inductor values, set the compensation zero (f z1 ) < 0.25 x f c to provide a sufficient phase margin. c4 is then calculated with equation (20): c fr72 4 c4 > (20) 3. c5 is required if the esr zero of the output capacitor is located at <0.5f sw , or equation (21) is valid: 2 f rco2 1 sw esr < (21) if this is the case, use c5 to set the pole (f p3 ) at the location of the esr zero. determine c5 with equation (22): r7 rco c5 esr = (22)
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 20 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. typical application circuits figure 9: application circuit for 5v output vin m1 m2 100k r2 37.4k r1 16v 16v synco en/sync in 1 freq 10 en/sync 2 synco 13 ss 6 ilimit 12 comp 7 fb 8 ccm/aam 9 sense- 14 sense+ 15 pg 11 vcc2 3 bg 17 vcc1 4 sw 18 tg 19 bst 20 u1 MP2908A 16v-60v 0.1uf c6 16v 0.007 r11 169k r17 12k r18 vout 12v/7a 15uh l1 4.7uf c1b 100v 10nf c9 0 r9 0 r12 d1 1n4148ws 2.2 r10 0.47uf c1 pg 1uf c2 vcc2 0 r3 vout vout vcc1 10 r13 220pf c7 100k r4 agnd vin vin 1m r5 4.7uf c3 220uf cout1 16v cout2 vcc1 10 r16 1% 100v 0 r14 0 r15 agnd agnd agnd agnd gnd gnd 16 5 4.7uf c1a 100v 22uf 16v 220pf c4 10k r7 82pf c5 agnd 0 r19 150pf c8 agnd ns r6 agnd pgnd note: thermal pad should connect to pgnd and agnd ccm/aam 37.4k r8 agnd 1 2 3 jp1 ccm aam vcc1 1 2 3 jp2 vcc1 dfls 1150 d2 figure 10: application circuit for 12v output
MP2908A?4v to 60v synchrono us step-down controller MP2908A rev. 1.01 www.monolithicpower.com 21 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. package information tssop-20 ep note: 1) all dimensions are in millimeters . 2) package length does not include mold flash , protrusion or gate burr. 3) package width does not include interlead flash or protrusion. 4) lead coplanarity (bottom of leads after forming ) shall be 0.10 millimeters max. 5) drawing conforms to jedec mo- 153, variation act . 6) drawing is not to scale . 0.19 0.30 6.40 6.60 seating plane 0.65 bsc pin 1 id 4.30 4.50 6.20 6.60 11 0 11 20 0.80 1.05 1.20 max 0.00 0.15 top view front view side view 0.09 0.20 bottom view 2.60 3.10 3.80 4.30 recommended land pattern 5.80 typ 1.60 typ 0.40 typ 0.65 bsc 3.20 typ 4.40 typ detail a 0.45 0.75 0 o -8 o 0.25 bsc gauge plane see detail "a"
MP2908A?4v to 60v synchrono us step-down controller notice: the information in this document is subject to change wi thout notice. users should warra nt and guarantee that third party intellectual property rights are not infringed upon w hen integrating mps products into any application. mps will not assume any legal responsibility for any said applications. MP2908A rev. 1.01 www.monolithicpower.com 22 1/27/2016 mps proprietary information. patent protec ted. unauthorized photocopy and duplication prohibited. ? 2016 mps. all rights reserved. qfn-20 (3mmx4mm) side view bottom view note: 1) all dimensions are in millimeters. 2) exposed paddle size does not include mold flash. 3) lead coplanarity shall be 0.10 millimeters max. 4) jedec reference is mo-220. 5) drawing is not to scale. pin 1 id marking top view pin 1 id index area recommended land pattern pin 1 id see detail a detail a pin 1 id option a 0.30x45 typ. pin 1 id option b r0.20 typ.

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