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LR8501 ? ? ? introduction: ? fea tures: the lr85 01 is a 1.5mhz constant frequency, slope compensated current mode pwm synchronous step-down converter. it is ideal for powering portable equipment which runs from a single cell lithium-ion battery. 100% duty cycle provides low dropout operation, extending battery life in portable systems. in power saving mode, 40 a quiescent current is very suitable for dsp/mcu in standby operation; and in active mode, low output ripple voltage is good enough for noise sensitive applications . the two modes can be automatically switched according to the load current. z high efficiency : up to 96% z output current: 600ma (typ.) z 1.5mhz constant switching frequency z no schottky diode required z input voltage: 1.8v to 5.5v z 0.6v reference allows low output voltage z low dropout: 100% duty cycle z low quiescent current: 40 a z shutdown current: <1 a z current mode operation for excellent line and load transient response z built-in thermal protection z package: sot-23-5 ? applica tions z cellular and smart phones z personal information appliances z wireless and dsl modems z digital still and video cameras z microprocessors core supplies z portable consumer equipments ? pin configuration: order informa tion: design ator symbol description a standard ? integer output voltage e.g.1.8v= :1, :8 adj= :, : m/mr package sot-23- 5 1.5mhz 600ma sy nchronous step-down converter with low quiescent current LR8501 series leshan radio comp any, ltd. 1/11 ver1.0
t abel1. pin description pin number pin name function m mr 1 3 ce chip enable pin 2 2 v ss gr ound 3 5 sw external inductor connection pin 4 1 v in power input 5 4 v out /fb output pin/feedback(adj version) ? block diagram ? absolut e maximum ratings p arameter symbol ratings units input v oltage v in v ss -0.3~v ss +6.5 v ce,sw ,fb/v out v oltage v ss -0.3~v in +0.3 v peak sw sink and sour ce current i sw max 1500 ma power dissipation sot-23-5 pd 400 mw operatin g temperature t opr -40~+ 85 junction t emperature t j 125 storage t emperature t stg -40~+ 125 soldering t emperature & time t solder 260 , 10s leshan radio comp any, ltd. 2/11
? electrical characteristics LR8501 series (v in =ce=3.6v, ta=25 , test circuit figure1, unless othe rwise specified) parameter symbol conditions min typ max units output voltage v out(f) (1) i out =100ma v out 0.97 v out v out 1.03 v feedback voltage v fb t a =25 0.5880 0.600 0.6120 v 0 ? t a ? 85 0.5865 0.600 0.6135 -40 ? t a ? 85 0.5850 0.600 0.6150 input voltage v in 1.8 5.5 v supply current1 ? active mode ? i ss1 v fb =0.50v 270 400 a supply current2 ? power saving mode ? i ss2 v fb =0.63v 40 50 a shutdown current i shdn v ce =v ss 0.1 1.0 a feedback current i fb v fb =0.65v 30 na maximum output current i out 600 ma v fb line regulation ? ? ? leshan radio company, ltd. 3/11
? typical application circuits c1 4.7 figure1 basic application circuit ? typical performance characteristics (test figure1 above unless otherwise specified) leshan radio company, ltd. 4/11
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? operation main control loop the LR8501 uses a constant frequency, current mode step-down architecture. both the main (p-channel mosfet) and synchronous (n-channel mosfet) switches are internal. during normal operation, the internal top power mosfet is turned on each cycle when the oscillator sets the rs la tch, and turned off when the current comparator, i comp , resets the rs latch. the peak inductor current at which i comp resets the rs latch, is controll ed by the output of error amplifier ea. when the load current increases, it causes a slight decrease in the feedback voltage, fb, relative to the 0.6v reference, which in turn, causes the ea amplifier?s output voltage to increase until the average inductor current matches the new load current. while the top mosfet is off, the bottom mosfet is turned on until either the inductor curr ent starts to reverse, as indicated by the curr ent reversal comparator i rcmp , or the beginning of the next clock cycle. maximum load current the LR8501 will operate with input voltage as low as 1.8v, however, the maximum load current decreases at lower input due to large ir drop on the main switch and synchronous rectifier. the slope compensation signal reduces the peak inductor current as a func tion of the duty cycle to prevent sub-harmonic osc illations at duty cycles greater than 50%.conversely the current limit increase as the dut y cycle decreases. discontinuous mode operation at light loads, the inductor current may reach zero reverse on each pulse. the bottom mosfet is turned off by the curr ent reversal comparator, i rcmp , and the switch voltage will ring. this is discontinuous mode operat ion, and is normal behavior for the switching regulator. at very light loads, the LR8501 will automatically skip pulses in discontinuous mode operati on to maintain output regulation. slope compensation slope compensation provides stability in constant frequency archit ecture by preventing sub-harmonic oscillations at high duty cycles. it is accomplished internally by adding a compensating ramp to t he inductor current signal at duty cycles in excess of 50%. this slope compensated current mode pwm control provides stable switching and cycle-by-cycle current limit for excellent load and line response. power saving mode operation at very light loads, the chip automatically enters power saving mode. in power saving mode at light load, a control circuit puts most of the circuit into sleep in order to reduce quiescent current and improve efficienc y at light load. when the output voltage drops to certain threshold, the control circuit turns ba ck on the oscillator and the pwm control loop, boosti ng output backup. when an upper threshold is reac hed, the control circuit again puts most of circuit into sleep, reducing quiescent current. during power saving mode operation, the converte r positions the output voltage slightly higher than the nominal output voltage during pwm operation, allowing additional headroom for voltage drop during a load transient from light to heavy load. while the power saving mode improves light load ef ficiency, however, with the turning on and off, the noise or ripple voltage is leshan radio company, ltd. 7/11
larger than that in the active mode. dropout operation as the input supply vo ltage decreases to a value approaching the out put voltage, the duty cycle increases toward the maximum on-time. further reduction of the supply voltage forces the main switch to remain on for more than one cycle until reaches 100% duty cycl e. the output voltage will then be determined by the input voltage minus the voltage drop across the p-channel mosfet and the inductor. an important detail to re member is that at low inputs supply voltages, the r ds(on) of the p-channel switch increases . therefore, the user should calculate the powe r dissipation when the LR8501 is used at 100% duty cycle with low input voltage. ? application information the basic LR8501 applic ation circuits are shown in figure 1.external component selection is driven by the load requirement and begins with the selection of l followed by c in and c out . setting the output voltage figure1 shows the basic application circuit with LR8501 adjustable output version. the external resistor sets the output voltage according to the following equation: v out 0.6v 1 r2 r1 c in required i rms i omax v out v in v out / v in ?i l v out v in v out v in lf leshan radio company, ltd. 8/11
style inductor to use often depends more on the price vs. size requirements and any radiated field/emi requirements than on what the LR8501 requires to operate. table 3 shows some typical surface mount inductors that work well in LR8501 applications. table 3.representative su rface mount inductors part number value ( h) max dcr (m ) max dc current (a) size w h l h h (mm 3 ) sumida cdrh 3d16 2.2 3.3 4.7 75 110 162 1.20 1.10 0.90 3.8 h 3.8 h 1.8 sumida cr43 2.2 3.3 4.7 71.2 86.2 108.7 1.75 1.44 1.15 4.5 h 4.0 h 3.5 sumida cdrh 4d18 2.2 3.3 4.7 75 110 162 1.32 1.04 0.84 4.7 h 4.7 h 2.0 output capacitor selection the selection of c out is driven by the required effective series resistance (esr). typically, once the esr requirement for c out has been met, the rms current rating generally far exceeds the i ripple requirement. the output ripple ? v out is determined by: ?v out ?i l esr 1 8fc out leshan radio company, ltd. 9/11
figure 2b LR8501-adj layout leshan radio company, ltd. 10/11
? z packa g sot23-5 p g ing info p ackage o u rmatio n u tline dime n n n sions leshan radio company, ltd. 11/11

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