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power management 1 www.semtech.com SC4501 2amp, 2mhz step-up switching regulator with soft-start description features applications u flat screen lcd bias supplies u tft bias supplies u xdsl power supplies u medical equipment u digital video cameras u portables devices u white led power supplies the SC4501 is a high-frequency current-mode step-up switching regulator with an integrated 2a power transis- tor. its high switching frequency (programmable up to 2mhz) allows the use of tiny surface-mount external pas- sive components. programmable soft-start eliminates high inrush current during start-up. the internal switch is rated at 32v making the converter suitable for high voltage ap- plications such as boost, sepic and flyback. the operating frequency of the SC4501 can be set with an external resistor. the ability to set the operating frequency gives the SC4501 design flexibilities. a dedicated comp pin allows optimization of the loop response. the SC4501 is available in thermally enhanced 8-pin msop and 10-pin mlpd packages. typical application circuit u low saturation voltage switch: 220mv at 2a u constant switching frequency current-mode control u programmable switching frequency up to 2mhz u soft-start function u input voltage range from 1.4v to 16v u output voltage up to 32v u low shutdown current u adjustable undervoltage lockout threshold u small low-profile thermally enhanced packages revision: october 25, 2005 c1 2.2 m f 5v vin vout SC4501 2 3 1 5 6 4 gnd shdn ss in sw fb rosc comp 8 7 on off c3 10bq015 r4 d1 l1 c2 10 m f 47 nf r2 20k r1 174k 12v, 0.7a c4 r3 c6 figure 1(a). 5v to 12v boost converter. all capacitors are ceramic. msop-8 pinout f / mhz r 3 / k w r 4 / k w c 4 / pf c 6 / pf l 1 / m h 0.7 22.1 22.1 2200 - 10.5 (falco d08019) 1.35 30.9 9.31 820 - 4.7 (falco d08017) 2 63.4 4.75 470 22 3.3 (coilcraft do1813p) figure 1(b). efficiencies of 5v to 12v boost converters at 700khz, 1.4mhz and 2mhz. efficiency 50 55 60 65 70 75 80 85 90 95 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 load current (a) efficiency (%) v out = 12v 4.7 m h, 1.4mhz 10.5 m h, 700khz 3.3 m h, 2mhz v in = 5v
2 ? 2005 semtech corp. www.semtech.com power management sc450 1 absolute maximum rating electrical characteristics exceeding the specifications below may result in permanent damage to the device, or device malfunction. operation outside of the parameters specified in the electrical characteristics section is not implied. parameter test conditions min typ max unit undervoltage lockout threshold 1.3 1.4 v maximum operating voltage 16 v feedback voltage t a = 25c 1.224 1.242 1.260 v -40c < t a < 85c 1.217 1.267 v feedback voltage line regulation 1.5v < v in < 16v 0.01 % fb pin bias current 40 80 na error amplifier transconductance 60 mw -1 error amplifier open-loop gain 49 db comp source current v fb = 1.1v 5 m a comp sink current v fb = 1.4v 5 m a v in quiescent supply current v shdn = 1.5v, v comp = 0 ( not switching ) 1.1 1.6 ma v in supply current in shutdown v shdn = 0 10 18 m a switching frequency 1.3 1.5 1.7 mhz maximum duty cycle 85 90 % minimum duty cycle 0 % switch current limit 2 2.8 a switch saturation voltage i sw = 2a 220 350 mv unless specified: v in = 2v, shdn = 1.5v, r osc = 7.68k w , -40c < t a = t j < 85c parameter symbol typ units supply voltage v in -0.3 to 18 v sw voltage v sw -0.3 to 32 v fb voltages v fb -0.3 to 2.5 v shdn voltage v shdn -0.3 to v in + 1 v operating temperature range t a -40 to +85 c thermal resistance junction to ambient (msop-8) q ja 40 c/w thermal resistance junction to ambient (mlpd-10) q ja 40 maximum junction temperature t j 160 c storage temperature range t stg -65 to +150 c lead temperature (soldering)10 sec t lead 260 c esd rating (human body model) esd 2000 v
3 ? 2005 semtech corp. www.semtech.com power management sc450 1 pin configurations ordering information electrical characteristics (cont.) top view (10 pin - mlpd, 3 x 3mm) device (1)(2) package temp. range( t a ) SC4501mltrt mlpd-10 -40 to 85c SC4501msetrt (3) msop-8-edp SC4501evb evaluation board notes: (1) only available in tape and reel packaging. a reel contains 3000 devices for mlp package and 2500 devices for msop. (2) lead free product. this product is fully weee and rohs compliant. (3) contact factory for availability. 1 2 3 4 ss comp top view (8 pin msop - edp) 5 6 7 8 rosc fb in shdn sw gnd unless specified: v in = 2v, shdn = 1.5v, r osc = 7.68k w , -40c < t a = t j < 85c parameter test conditions min typ max unit switch leakage current v sw = 5v 0.01 1 m a shutdown threshold voltage 1.02 1.1 1.18 v shutdown pin current v shdn = 1.2v -4.6 m a v shdn = 0 0 0.1 m a soft-start charging current v ss = 0.3v 1.5 m a thermal shutdown temperature 160 c thermal shutdown hysteresis 10 c
4 ? 2005 semtech corp. www.semtech.com power management sc450 1 block diagram shdn comp rosc 7 reg_good ea - fb 2 ss 8 reference voltage 3 in 6 1 cmp - + supply internal shutdown thermal reg 1.1v 1.242v + 4.6 m a enable enable + r q s pwm clk - + + clk + - isen ilim + - i - limit s gnd 4 sw 5 r oscillator slope comp 1.5 m a reg sense figure 2. SC4501 (msop-8) block diagram. block diagram pin descriptions (msop-8) pin pin name pin function 1 comp the output of the internal transconductance error amplifier. this pin is used for loop compensation. 2 fb the inverting input of the error amplifier. tie to an external resistive divider to set the output voltage. 3 shdn shutdown pin. the accurate 1.1v shutdown threshold and the 4.6ua shutdown pin current hysteresis allow the user to set the undervoltage lockout threshold and hysteresis for the switching regulator. pulling this pin below 0.1v causes the converter to shut down to low quiescent current. tie this pin to in if the uvlo and the shutdown features are not used. this pin should not be left floating. 4 gnd ground. tie to the ground plane. 5 sw collector of the internal power transistor. connect to the boost inductor and the rectifying diode. 6 in power supply pin. bypassed with capacitors close to the pin. 7 rosc a resistor from this pin to the ground sets the switching frequency. 8 ss soft-start pin. a capacitor from this pin to the ground lengthens the start-up time and reduces start- up current. exposed pad the exposed pad must be soldered to the ground plane on the pcb for good thermal conduction.
5 ? 2005 semtech corp. www.semtech.com power management sc450 1 block diagram shdn comp rosc 9 reg_good ea - fb 2 ss 10 reference voltage 3 in 8 1 cmp - + supply internal shutdown thermal reg 1.1v 1.242v + 4.6 m a enable enable + r q s pwm clk - + + clk + - isen ilim + - i - limit s gnd 4 sw 6 oscillator slope comp 1.5 m a reg r sense gnd 5 sw 7 figure 3. SC4501 (mlpd-10) block diagram. pin descriptions (mlpd - 10) pin pin name pin function 1 comp the output of the internal transconductance error amplifier. this pin is used for loop compensation. 2 fb the inverting input of the error amplifier. tie to an external resistive divider to set the output voltage. 3 shdn shutdown pin. the accurate 1.1v shutdown threshold and the 4.6ua shutdown pin current hysteresis allow the user to set the undervoltage lockout threshold and hysteresis for the switching regulator. pulling this pin below 0.1v causes the converter to shut down to low quiescent current. tie this pin to in if the uvlo and the shutdown features are not used. this pin should not be left floating. 4,5 gnd ground. tie both pins to the ground plane. pins 4 and 5 are not internally connected. 6,7 sw collector of the internal power transistor. connect to the boost inductor and the rectifying diode. 8 in power supply pin. bypassed with capacitors close to the pin. 9 rosc a resistor from this pin to the ground sets the switching frequency. 10 ss soft-start pin. a capacitor from this pin to the ground lengthens the start-up time and reduces start- up current. exposed pad the exposed pad must be soldered to the ground plane on the pcb for good thermal conduction.
6 ? 2005 semtech corp. www.semtech.com power management sc450 1 typical characteristics feedback voltage vs temperature 1.15 1.2 1.25 1.3 -50 -25 0 25 50 75 100 125 temperature (oc) feedback voltage (v) switch current limit vs temperature 2 2.2 2.4 2.6 2.8 3 -50 -25 0 25 50 75 100 temperature (oc) current limit (a) r osc vs switching frequency 1 10 100 0.0 0.5 1.0 1.5 2.0 2.5 3.0 frequency (mhz) r osc ( k w ) v in = 2v 25oc switching frequency vs temperature 1.3 1.4 1.5 1.6 1.7 -50 -25 0 25 50 75 100 125 temperature (oc) frequency ( mhz ) v in = 12v v in = 2v r osc = 7.68k w v in quiescent current vs temperature 0.8 0.9 1 1.1 1.2 1.3 -50 -25 0 25 50 75 100 125 temperature (oc) v in current ( m a) v in = 16v v in = 2v not switching v in current in shutdown vs input voltage 0 10 20 30 40 50 0 5 10 15 20 input voltage (v) v in current ( m a) 125oc -40oc v shdn = 0 shutdown threshold vs temperature 1.00 1.05 1.10 1.15 1.20 -50 -25 0 25 50 75 100 125 temperature (oc) shutdown threshold (v) v in = 2v minimum v in vs temperature 1 1.1 1.2 1.3 1.4 1.5 -50 -25 0 25 50 75 100 125 temperature (oc) input voltage (v) switch saturation voltage vs switch current 0 100 200 300 400 0 0.5 1 1.5 2 2.5 3 switch current (a) v cesat (mv) 85oc 25oc -40oc
7 ? 2005 semtech corp. www.semtech.com power management sc450 1 typical characteristics shutdown pin current vs temperature -6 -5 -4 -3 -50 -25 0 25 50 75 100 125 temperature (oc) current ( m a) v in = 2v v in = 12v v shdn = 1.25v transconductance vs temperature 30 40 50 60 70 80 -50 -25 0 25 50 75 100 125 temperature (oc) transconductance ( mw -1 ) v in = 2v soft-start charging current vs temperature 1 1.2 1.4 1.6 1.8 2 -50 -25 0 25 50 75 100 125 temperature (oc) current ( m a) v ss = 0.3v v in current vs shdn pin voltage 0 0.02 0.04 0.06 0.08 0.1 0 0.2 0.4 0.6 0.8 1 1.2 shdn voltage (v) v in current ( m a) 25oc v in = 2v v in current vs shdn pin voltage 0 0.2 0.4 0.6 0.8 1 1.2 0 0.5 1 1.5 shdn voltage (v) v in current ( m a) 125oc -40oc v in = 2v -40oc 125oc
8 ? 2005 semtech corp. www.semtech.com power management sc450 1 operation the SC4501 is a programmable constant-frequency peak current-mode step-up switching regulator with an integrated 2a power transistor. referring to the block diagrams in figures 2 and 3, the power transistor is switched on at the trailing edge of the clock. switch current is sensed with an integrated sense resistor. the sensed current is summed with the slope-compensating ramp before compared to the output of the error amplifier ea. the pwm comparator trip point determines the switch turn-on pulse width. the current-limit comparator ilim turns off the power switch when the switch current exceeds the 2.8a current-limit threshold. ilim therefore provides cycle-by-cycle current limit. current-limit is not affected by slope compensation because the current comparator ilim is not in the pwm signal path. current-mode switching regulators utilize a dual-loop feedback control system. in the SC4501 the amplifier output comp controls the peak inductor current. this is the inner current loop. the double reactive poles of the output lc filter are reduced to a single real pole by the inner current loop, easing loop compensation. fast transient response can be obtained with a simple type-2 compensation network. in the outer loop, the error amplifier regulates the output voltage. the switching frequency of the SC4501 can be programmed up to 2mhz with an external resistor from the r osc pin to the ground. for converters requiring extreme duty cycles, the operating frequency can be lowered to maintain the necessary minimum on time or the minimum off time. the SC4501 requires a minimum input of 1.4v to operate. a voltage higher than 1.1v at the shutdown pin enables the internal linear regulator reg in the SC4501. after v reg becomes valid, the soft-start capacitor is charged with a 1.5 m a current source. a pnp transistor clamps the output of the error amplifier as the soft-start capacitor voltage rises. since the comp voltage controls the peak inductor current, the inductor current is ramped gradually during soft-start, preventing high input start-up current. under fault conditions (v in <1.4v or over temperature) or when the shutdown pin is pulled below 1.1v, the soft-start capacitor is discharged to ground. pulling the shutdown pin below 0.1v reduces the total supply current to 10 m a. setting the output voltage an external resistive divider r 1 and r 2 with its center tap tied to the fb pin (figure 4) sets the output voltage. ? ? ? ? - = 1 v 242 . 1 v r r out 2 1 (1) vout r2 r1 40na fb 2 SC4501 the input bias current of the error amplifier will introduce an error of: ( ) % v 242 . 1 100 r // r na 40 v v 2 1 out out = d (2) the percentage error of a v out = 5v converter with r 1 = 100k w and r 2 = 301k w is ( ) % 24 . 0 v 242 . 1 100 k 301 // k 100 na 40 v v out out = = d operating frequency and efficiency switching frequency of SC4501 is set with an external resistor from the r osc pin to the ground. a graph showing the relationship between r osc and switching frequency is given in the ?typical characteristics?. high frequency operation reduces the size of passive components but switching losses are higher. the efficiencies of 5v to 12v converters operating at 700khz, 1.35mhz and 2mhz are shown in figure 1(b). the peak efficiency of the SC4501 appears to decrease 0.5% for every 100khz increase in frequency. application information figure 4. the output voltage is set with a resistive divider
9 ? 2005 semtech corp. www.semtech.com power management sc450 1 duty cycle the duty cycle d of a boost converter is: d out cesat d out in v v v 1 v v v 1 d + - + - = (3) where v cesat is the switch saturation voltage and v d is voltage drop across the rectifying diode. maximum output current in a boost switching regulator the inductor is connected to the input. the dc inductor current is the input current. when the power switch is turned on, the inductor current flows into the switch. when the power switch is off, the inductor current flows through the rectifying diode to the output. the output current is the average diode current. the diode current waveform is trapezoidal with pulse width (1 ? d)t (figure 5). the output current available from a boost converter therefore depends on the converter operating duty cycle. the power switch current in the SC4501 is internally limited to 2a. this is also the maximum inductor or the input current. by estimating the conduction losses in both the switch and the diode, an expression of the maximum available output current of a boost converter can be derived: ( ) ? ? - - - - = in cesat d d out in lim outmax v v v d v 45 d 1 v v i i (4) where i lim is the switch current limit. it is worth noting that i outmax is directly proportional to the ratio out in v v . equation (4) over-estimates the maximum output current at high frequencies (>1mhz) since switching losses are neglected in its derivation. nevertheless it is a useful first-order approximation. using v cesat = 0.3v, v d = 0.5v and i lim = 2a in (3) and (4), the maximum output currents for three v in and v out combinations are shown in table 1. v in ( v ) v out ( v ) d i outmax ( a ) 2.5 12 0.820 0.35 3.3 5 0.423 1.14 5 12 0.615 0.76 considerations for high frequency operation the operating duty cycle of a boost converter decreases as v in approaches v out . the pwm modulating ramp in a current-mode switching regulator is the sensed current ramp of the control switch. this current ramp is absent unless the switch is turned on. the intersection of this ramp with the output of the voltage feedback error amplifier determines the switch pulse width. the propagation delay time required to immediately turn off the switch after it is turned on is the minimum switch on time. regulator closed-loop measurement shows that the SC4501 has a minimum on time of about 150ns at room temperature. the power switch in the SC4501 is either not turned on at all or for at least 150ns. if the required switch on time is shorter than the minimum on time, the regulator will either skip cycles or it will start to jitter. example: determine the maximum operating frequency of a li-ion cell to 5v converter using the SC4501. assuming that v d =0.5v, v cesat =0.3v and v in =2.6 - 4.2v, the minimum duty ratio can be found using (3). 25 . 0 5 . 0 5 3 . 0 1 5 . 0 5 2 . 4 1 d min = + - + - = application information figure 5. current waveforms in a boost regulator inductor current switch current diode current on on off on on off on off i out i in (1 - d)t dt table 1. calculated maximum output current [ equation (4)]
10 ? 2005 semtech corp. www.semtech.com power management sc450 1 application information the absolute maximum operating frequency of the converter is therefore mhz 67 . 1 ns 150 25 . 0 ns 150 d min = = . the actual operating frequency needs to be lower to allow for modulating headroom. the power transistor in the SC4501 is turned off every switching period for an interval determined by the discharge time of the oscillator ramp and the propagation delay of the power switch. this minimum off time limits the maximum duty cycle of the regulator at a given switching frequency. a boost converter with high in out v v ratio requires long switch on time and high duty cycle. if the required duty cycle is higher than the attainable maximum, then the converter will operate in dropout. (dropout is a condition in which the regulator cannot attain its set output voltage below current limit.) the minimum off times of closed-loop boost converters set to various output voltages were measured by lowering their input voltages until dropout occurs. it was found that the minimum off time of the SC4501 ranged from 80 to 110ns at room temperature. beware of dropout when operating at very low input voltages (1.5-2v) and with off times approaching 110ns. shorten the pcb trace between the power source and the device input pin, as line drop may be a significant percentage of the input voltage. a regulator in dropout may appear as if it is in current limit. the cycle-by-cycle current limit of the SC4501 is duty-cycle and input voltage invariant and is typically 2.8a. if the switch current limit is not at least 2a, then the converter is likely in dropout. the switching frequency should then be lowered to improve controllability. both the minimum on time and the minimum off time reduce control range of the pwm regulator. bench measurement showed that reduced modulating range started to be a problem at frequencies over 2mhz. although the oscillator is capable of running well above 2mhz, controllability limits the maximum operating frequency. inductor selection the inductor ripple current d i l of a boost converter operating in continuous-conduction mode is ( ) fl v v d i cesat in l - = d (5) where f is the switching frequency and l is the inductance. substituting (3) into (5) and neglecting v cesat , ? ? ? ? ? + - = d d out in in l v v v 1 fl v i (6) in current-mode control, the slope of the modulating (sensed switch current) ramp should be steep enough to lessen jittery tendency but not so steep that large flux swing decreases efficiency. inductor ripple current d i l between 25-40% of the peak inductor current limit is a good compromise. inductors so chosen are optimized in size and dcr. setting d i l = 0.3?(2) = 0.6a, v d =0.5v in (6), ? ? ? ? ? + - = ? ? ? ? ? + - d = 5 . 0 v v 1 f 6 . 0 v v v v 1 i f v l out in in d out in l in (7) where l is in m h and f is in mhz. equation (6) shows that for a given v out , d i l is the highest when ( ) 2 v v v d out in + = . if v in varies over a wide range, then choose l based on the nominal input voltage. the saturation current of the inductor should be 20-30% higher than the peak current limit (2.8a). low-cost powder iron cores are not suitable for high-frequency switching power supplies due to their high core losses. inductors with ferrite cores should be used. input capacitor the input current in a boost converter is the inductor current, which is continuous with low rms current ripples. a 2.2-4.7 f ceramic input capacitor is adequate for most applications. output capacitor both ceramic and low esr tantalum capacitors can be used as output filtering capacitors. multi-layer ceramic capacitors, due to their extremely low esr (<5m w ), are the best choice. use ceramic capacitors with stable temperature and voltage characteristics. one may be tempted to use z5u and y5v ceramic capacitors for output filtering because of their high capacitance and
11 ? 2005 semtech corp. www.semtech.com power management sc450 1 application information small sizes. however these types of capacitors have high temperature and high voltage coefficients. for example, the capacitance of a z5u capacitor can drop below 60% of its room temperature value at ?25 c and 90 c. x5r ceramic capacitors, which have stable temperature and voltage coefficients, are the preferred type. the diode current waveform in figure 5 is discontinuous with high ripple-content. in a buck converter the inductor ripple current d i l determines the output ripple voltage. the output ripple voltage of a boost regulator is however much higher and is determined by the absolute inductor current. decreasing the inductor ripple current does not appreciably reduce the output ripple voltage. the current flowing in the output filter capacitor is the difference between the diode current and the output current. this capacitor current has a rms value of: 1 v v i in out out - (8) if a tantalum capacitor is used, then its ripple current rating in addition to its esr will need to be considered. when the switch is turned on, the output capacitor supplies the load current i out (figure 5). the output ripple voltage due to charging and discharging of the output capacitor is therefore: out out out c dt i v = d (9) for most applications, a 10-22f ceramic capacitor is sufficient for output filtering. it is worth noting that the output ripple voltage due to discharging of a 10f ceramic capacitor (9) is higher than that due to its esr. rectifying diode for high efficiency, schottky barrier diodes should be used as rectifying diodes for the SC4501. these diodes should have a rms current rating of at least 1a and a reverse blocking voltage of at least a few volts higher than the output voltage. for switching regulators operating at low duty cycles (i.e. low output voltage to input voltage conversion ratios), it is beneficial to use rectifying diodes with somewhat higher rms current ratings (thus lower forward voltages). this is because the diode conduction interval is much longer than that of the transistor. converter efficiency will be improved if the voltage drop across the diode is lower. the rectifying diodes should be placed close to the sw pins of the SC4501 to minimize ringing due to trace inductance. surface-mount equivalents of 1n5817, 1n5818, mbrm120 (on semi) and 10bq015 (irf) are all suitable. soft-start soft-start prevents a dc-dc converter from drawing excessive current (equal to the switch current limit) from the power source during start up. if the soft-start time is made sufficiently long, then the output will enter regulation without overshoot. an external capacitor from the ss pin to the ground and an internal 1.5a charging current source set the soft-start time. the soft-start voltage ramp at the ss pin clamps the error amplifier output. during regulator start-up, comp voltage follows the ss voltage. the converter starts to switch when its comp voltage exceeds 0.7v . the peak inductor current is gradually increased until the converter output comes into regulation. if the shutdown pin is forced below 1.1v or if fault is detected, then the soft-start capacitor will be discharged to ground immediately. the ss pin can be left open if soft-start is not required. shutdown the input voltage and shutdown pin voltage must be greater than 1.4v and 1.1v respectively to enable the SC4501. forcing the shutdown pin below 1.1v stops switching. pulling this pin below 0.1v completely shuts off the SC4501. the total v in current decreases to 10a at 2v. figure 6 shows several ways of interfacing the control logic to the shutdown pin. beware that the shutdown pin is a high impedance pin. it should always be driven from a low- impedance source or tied to a resistive divider. floating the shutdown pin will result in undefined voltage. in figure 6(c) the shutdown pin is driven from a logic gate whose v oh is higher than the supply voltage of the SC4501. the diode clamps the maximum shutdown pin voltage to one diode voltage above the input power supply.
12 ? 2005 semtech corp. www.semtech.com power management sc450 1 application information programming undervoltage lockout the SC4501 has an internal v in undervoltage lockout (uvlo) threshold of 1.4v. the transition from idle to switching is abrupt but there is no hysteresis. if the input voltage ramp rate is slow and the input bypass is limited, then sudden turn on of the power transistor will cause a dip in the line voltage. switching will stop if v in falls below the internal uvlo threshold. the resulting output voltage rise may be non-monotonic. the 1.1v disable threshold of the SC4501 can be used in conjunction with a resistive voltage divider to raise the uvlo threshold and to add an uvlo hysteresis. figure 7 shows the scheme. both v h and v l (the desired upper and the lower uvlo threshold voltages) are determined by the 1.1v threshold crossings, v h and v l are therefore: ( ) 3 hys h hys h l 4 3 h r i v v v v v 1 . 1 r r 1 v - = - = ? ? ? ? ? + = (10) re-arranging, hys hys 3 i v r = (11) 1 1 . 1 v r r h 3 4 - = (12) figure 6. methods of driving the shutdown pin (c) 1n4148 SC4501 shdn in v in (a) SC4501 shdn in (d) SC4501 shdn in (b) SC4501 shdn in (a) directly driven from a logic gate (b) driven from an open-drain n-channel mosfet or an open-collector npn transistor (v ol < 0.1v) (c) driven from a logic gate with v oh > v in (d) combining shutdown with programmed uvlo (see section below).
13 ? 2005 semtech corp. www.semtech.com power management sc450 1 with v 4 . 1 v l > . example: increase the turn on voltage of a v in = 3.3v boost converter from 1.4v to 2.75v. using v h = 2.75v and r 4 = 100k w in (12), w = k 150 r 3 . the resulting uvlo hysteresis is: v 69 . 0 k 150 a 6 . 4 r i v 3 hys hys = w m = = . SC4501 r4 r3 shdn 3 + - in 6/8 1.1v 4.6 m a i hys comparator y switch closed when y = ?1? figure 7. programmable hysteretic uvlo circuit the turn off voltage is: v 4 . 1 v 06 . 2 69 . 0 75 . 2 v v v hys h l > = - = - = . frequency compensation figure 8 shows the equivalent circuit of a boost converter using the SC4501. the output filter capacitor and the load form an output pole at frequency: 2 out 2 out out 2 p c r 2 c v i 2 - = - = w (13) where c 2 is the output capacitor and out out out i v r = is the equivalent load resistance. the zero formed by c 2 and its equivalent series resistance (esr) is neglected due to low esr of the ceramic output capacitor. there is also a right half plane (rhp) zero at angular frequency: ( ) l d 1 r 2 out 2 z - = w (14) w z2 decreases with increasing duty cycle d and increasing i out . using the 5v to 12v boost regulator (1.35mhz) in figure 1(a) as an example, w = 3 8 . 6 a 74 . 0 v 5 r out power stage reference voltage 1.242v gm - + r3 c4 c6 ro r2 comp r1 fb c5 esr c2 r out v out v in i out figure 8. simplified block diagram of a boost converter application information
14 ? 2005 semtech corp. www.semtech.com power management sc450 1 62 . 0 5 . 0 12 3 . 0 1 5 . 0 12 5 1 d = + - + - = therefore ( ) ( ) khz 68 . 4 krads 4 . 29 f 10 8 . 6 2 1 2 p = = m w w - and ( ) khz 3 . 33 krads 209 h 7 . 4 62 . 0 1 8 . 6 1 2 2 z = = m - w 3 w - the spacing between p 2 and z 2 is the closest when the converter is delivering the maximum output current from the lowest v in . this represents the worst-case compensation condition. ignoring c 5 and c 6 for the moment, c 4 forms a low frequency pole with the equivalent output resistance r o of the error amplifier: w = w m = = - m 7 . 4 60 db 49 ce tan c transcondu gain loop open amplifier r 1 o hz 41 rads 260 pf 820 m 7 . 4 1 c r 1 1 4 o 1 p - = - = w - = - = w - c 4 and r 3 also forms a zero with angular frequency: khz 3 . 6 krads 5 . 39 pf 820 k 9 . 30 1 c r 1 1 4 3 1 z - = - = w - = - = w - the poles p 1 , p 2 and the rhp zero z 2 all increase phase shift in the loop response. for stable operation, the overall loop gain should cross 0db with -20db/decade slope. due to the presence of the rhp zero, the 0db crossover frequency should not be higher than 3 z 2 . placing z 1 near p 2 nulls its effect and maximizes loop bandwidth. thus ) max ( out 2 out 4 3 i 2 c v c r ? (15) r 3 determines the mid-band loop gain of the converter. increasing r 3 increases the mid-band gain and the crossover application information figure 9. suggested pcb layout for the SC4501. notice that there is no via directly under the device. all vias are 12mil in diameter. u1 vout shdn r1 c5 c1 l1 d1 vin u1 c3 r4 c4 r3 c6 r2 gnd c2 u1 vout shdn shdn r1 c5 c1 l1 d1 vin u1 c3 r4 c4 r3 c6 r2 gnd c2
15 ? 2005 semtech corp. www.semtech.com power management sc450 1 frequency. however it reduces the phase margin. the values of r 3 and c 4 can be determined empirically by observing the inductor current and the output voltage during load transient. compensation is optimized when the largest r 3 and the smallest c 4 without producing ringing or excessive overshoot in its inductor current and output voltage are found. figures 10(b), 11(c), 12(b) and 12(c) show load transient responses of empirically optimized dc-dc converters. in a battery-operated system, compensating for the minimum v in and the maximum load step will ensure stable operation over the entire input voltage range. c 5 adds a feedforward zero to the loop response. in some cases it improves the transient speed of the converter. c 6 rolls off the gain at high frequency. this helps to stabilize the loop. c 5 and c 6 are often not needed. board layout considerations in a step-up switching regulator, the output filter capacitor, the main power switch and the rectifying diode carry switched currents with high di/dt. for jitter-free operation, application information the size of the loop formed by these components should be minimized. since the power switch is integrated inside the SC4501, grounding the output filter capacitor next to the SC4501 ground pin minimizes size of the high di/dt current loop. the input bypass capacitors should also be placed close to the input pins. shortening the trace at the sw node reduces the parasitic trace inductance. this not only reduces emi but also decreases the sizes of the switching voltage spikes and glitches. figure 9 shows how various external components are placed around the SC4501. the frequency-setting resistor should be placed near the rosc pin with a short ground trace on the pc board. these precautions reduce switching noise pickup at the rosc pin. to achieve a junction to ambient thermal resistance ( q ja ) of 40c/w, the exposed pad of the SC4501 should be properly soldered to a large ground plane. use only 12mil diameter vias in the ground plane if necessary. avoid using larger vias under the device. molten solder may seep through large vias during reflow, resulting in poor adhesion, poor thermal conductivity and low reliability. typical application circuits upper trace : output voltage, ac coupled, 1v/div lower trace : inductor current, 0.5a/div 40 m s/div figure 10(b). load transient response of the circuit in figure 10(a). i load is switched between 0.1a and 0.4a at 1a/ m s. c1 2.2 m f 3.3v vin vout SC4501 2 3 1 5 6 4 gnd shdn ss in sw fb rosc comp 8 7 on off c3 c4 10bq015 c2 r2 r1 r3 r4 d1 l1 3.3 m h 10 m f 47 nf 1.5 nf 20k 22.1k 174k 12v, 0.4a 9.31k figure 10(a). 1.35 mhz all ceramic capacitor 3.3v to 12v boost l1: cooper-bussmann sd25-3r3 converter. pinout shown is for msop-8
16 ? 2005 semtech corp. www.semtech.com power management sc450 1 typical application circuits figure 11(b). efficiency of the single li-ion cell to 5v boost converter in figure 11(a). efficiency 50 55 60 65 70 75 80 85 90 95 0.001 0.010 0.100 1.000 load current (a) efficiency (%) v out = 5v v in = 2.6v 1.2mhz v in = 3.6v v in = 4.2v 5v, 0.8a c1 2.2 m f vout on off c3 c4 10bq015 r3 d1 l1 1.8 m h c2 10 m f 47 nf 1n f r2 100k 17.4k SC4501 2 3 1 5 6 4 gnd shdn ss in sw fb rosc comp 8 7 2.6 - 4.2v 1 - cell li - ion r1 301k r4 10.7k figure 11(a). 1.2 mhz all ceramic capacitor single li-ion cell l1: sumida cr43 to 5v boost converter. lower trace : inductor current, 0.5a/div upper trace : output voltage, ac coupled, 0.5v/div 40 m s/div figure 11(c). load transient response of the circuit in figure 11(a). i load is switched between 0.2a and 0.7a at 1a/ m s. v in =2.6v
17 ? 2005 semtech corp. www.semtech.com power management sc450 1 typical application circuits figure 12(b). load transient response of the circuit in figure upper trace : output voltage, ac coupled, 0.2v/div 40 m s/div 12(a). i load is switched between 50ma and 350ma at 1a/ m s. lower trace : input inductor current, 0.2a/div v in =3.6v 3.6 - 6v 4 - cell vout c1 2.2 m f SC4501 2 3 1 5 6 4 gnd shdn ss in sw fb rosc comp 8 7 on off c3 c4 r2 r1 r3 r4 10bq015 d1 c2 10 m f 47 nf 560p f 20k 20k 60.4k 5v 7.68k c5 47p f l1 4.9 m h c6 2.2 m f l2 4.9 m h figure 12(a). 1.5 mhz all ceramic capacitor 4-cell to 5v sepic converter. pinout shown is for msop-8. l1 and l2: coiltronics ctx5-1 lower trace : input inductor current, 0.2a/div figure 12(c). load transient response of the circuit in figure upper trace : output voltage, ac coupled, 0.2v/div 40 m s/div 12(a). i load is switched between 80ma and 600ma at 1a/ m s. v in =6v
18 ? 2005 semtech corp. www.semtech.com power management sc450 1 typical application circuits figure 13(c). tft power supply start-up transient as the shdn pin is stepped from 0 to 2v. 2ms/div ch2 ch1 ch3 ch4 ch1 : out1 voltage, 5v/div ch2 : out2 voltage, 10v/div ch3 : out3 voltage, 5v/div ch4 : shdn voltage, 2v/div 3.3v c1 2.2 m f 23v (10ma) out1 SC4501 2 3 1 5 6 4 gnd shdn ss in sw fb rosc comp 8 7 c3 c4 r3 r4 10bq015 d1 10 m f 47 nf 820p f 40.2k r2 r1 c2 49.9k 274k 8v (0.55a) 7.68k c9 0.1 m f l1 2.2 m h d6 d7 c10 1 m f d2 0.1 m f c5 d3 0.1 m f c6 d4 0.1 m f c7 d5 1 m f c8 out2 out3 - 8v (10ma) r6 r5 100k 150k figure 13(a). 1.5mhz triple-output tft power supply. d2 - d7 : bat54s l1 : cooper-bussmann sd25-2r2 ch1 : out1 voltage, 5v/div ch4 : input voltage, 2v/div 4ms/div ch2 : out2 voltage, 10v/div ch3 : out3 voltage, 5v/div ch2 ch1 ch3 ch4 figure 13(b). tft power supply v in start-up transient.
19 ? 2005 semtech corp. www.semtech.com power management sc450 1 typical application circuits c1 2.2 m f SC4501 2 3 1 5 6 4 gnd shdn ss in sw fb rosc comp 8 7 r4 10bq015 d1 c3 10 nf 4.7 m f c2 r2 43.2k r1 698 8.06k c4 10n f l1 2.2 m h r5 10k 0.1 w r6 2.6 - 4.2v 1 - cell li - ion + - + 1/2 lm358 0.2a (torch) mmbt3904t sumida cr43 on off d2 lxcl - pwf1 q1 c5 0.1 m f r6 17.4k flash torch - 3.4v to 3.8v + mmbf2201nt1 m1 0.7a (flash) figure 14(a). 1.4mhz luxeon tm flash white led driver for camera phones figure 14(b) and 14(c). photo flash led current is switched between torch mode (0.2a) and flash mode (0.7a). ch1 : torch/flash control voltage, 5v/div ch4 : inductor current, 1a/div ch2 : fb pin voltage, 1v/div ch3 : led current, 0.5a/div higher led current (>0.7a) in flash mode is possible with fresh battery. 4ms/div ch1 ch2 ch3 ch4 v in = 2.6v (b) ch1 ch2 ch3 ch4 v in = 4.2v 4ms/div (c)
20 ? 2005 semtech corp. www.semtech.com power management sc450 1 outline drawing - msop-8l-edp land pattern - msop-8l-edp 1.73 2.03 .068 .080 f f detail .193 bsc .026 bsc aaa c seating ccc c 2x n/2 tips indicator pin 1 8 bbb c a-b d see detail 0.25 a plane gage .005 1 2 n .114 .114 .118 .118 .009 - 8 01 c (l1) l a 0.13 3.00 3.00 4.90 bsc 0.65 bsc .122 .122 2.90 2.90 .015 0.22 3.10 3.10 0.38 - 0 .010 .004 - .016 .003 .024 (.037) - .000 .030 - - - - 0.25 0.10 8 0 e/2 2x e1 e a2 a bottom view exposed pad a1 d bxn dim ccc a1 e bbb aaa 01 l1 n l d e1 e a2 b c a millimeters nom dimensions inches min nom max min max f .076 1.93 - 8 0.60 (0.95) .032 .009 0.40 0.08 .043 .006 .037 0.75 0.00 - 0.80 0.23 - 0.95 1.10 0.15 - - - 3. dimensions "e1" and "d" do not include mold flash, protrusions or gate burrs. -b- controlling dimensions are in millimeters (angles in degrees). datums and to be determined at datum plane notes: 1. 2. -a- -h- side view a b c e h e/2 d plane reference jedec std mo-187, variation aa-t. 4. this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. millimeters dimensions dim inches x z y c p g f .081 2.08 (c) f f g z p x .224 .063 .016 .026 (.161) .098 (4.10) 5.70 1.60 0.40 0.65 2.50
21 ? 2005 semtech corp. www.semtech.com power management sc450 1 semtech corporation power management products division 200 flynn road, camarillo, ca 93012 phone: (805)498-2111 fax (805)498-3804 contact information land pattern - mlpd-10, 3 x 3mm min aaa bbb b e l n d c e a1 a2 a dim millimeters nom dimensions max nom inches min max .114 .118 .122 2.90 3.00 3.10 - - - - (laser mark) indicator pin 1 1 n 2 notes: controlling dimensions are in millimeters (angles in degrees). coplanarity applies to the exposed pad as well as terminals. 2. 1. .003 .007 .042 10 .009 .048 - .000 .031 (.008) 0.08 0.23 10 .011 .052 0.18 1.06 .039 - .002 - 0.00 0.80 1.31 0.30 1.21 - 0.05 1.00 (0.20) .004 0.10 0.50 bsc .020 bsc 0.30 .012 .020 .016 0.40 0.50 a aaa c a2 seating plane a1 a bxn bbb c a b b e c c d lxn e e .074 .079 .083 1.87 2.02 2.12 .087 .055 2.20 1.40 .150 .020 .012 .037 3.80 0.30 0.95 0.50 (.112) .075 1.90 (2.85) k h x this land pattern is for reference purposes only. consult your manufacturing group to ensure your company's manufacturing guidelines are met. notes: 1. inches dimensions g k h x y p z c dim millimeters y z g (c) p outline drawing - mlpd-10, 3 x 3mm

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