high efficiency : up to 96% output current: 600ma typ 1.5mhz constant sw itching frequency no schottky diode required input voltage: 2.5v to 5.5v output voltage:1.2v,1 .5v,1.8v & adjustable low dropout: 100% duty cycle low quiescent current: 300 a shutdown current: <1 a current mode operation for excellent line and load transient response built-in thermal protection short circuit protection package: sot23-5 ? application 1.5mhz 600ma synchronous step-down converter LR8506 cellular and smart phones personal information appliances wireless and dsl modems portable instruments digital still and video cameras microprocessors core supplies ? feature ? introduction the LR8506 is a 1.5mhz constant frequency, slope compensated current mode pwm synchronous step-down converter. high switching frequency allows the use of small surface mount inductors and capacit ors. the internal synchronous switch increases efficiency and eliminates the need for an external schottky diode. 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. low output voltages are easily supported with the 0.6v feedback reference voltage. ? block diagram ? order information LR8506 leshan radio company, ltd. 1/10
? pin configuration ? absolute maximum ratings parameter symbol rating unit input voltage v in -0.3~6 v ce,sw,fb/v out voltage -0.3~v in +0.3 v peak sw sink and source current i swmax 1500 ma power dissipation sot23 pd 400 mw operating temperature t opr -40~+85 junction temperature t j 125 storage temperature t stg -65~+150 soldering temperature & time t solder 300 , 10s ? typical application circuit figure 1.1 LR8506-15 application circuit figure 1.2 LR8506-ad j application circuit figure 1 basic application circuit LR8506 LR8506 leshan radio company, ltd. 2/10
? electrical characteristics (vin=ce=3.6v, t a =25 , test circuit figure1, unless otherwise specifi ed) parameter symbo l conditions min. typ. max. units output voltage LR8506-12 v out i out =100ma 1.176 1.200 1.224 v LR8506-15 1.470 1.500 1.530 LR8506-18 1.764 1.800 1.836 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 2.5 5.5 v supply current i ss v fb =0.5v 300 400 a standby current i standby v ce =gnd 0.1 1 a feedback current i fb v fb =0.65v 30 na maximum output current i out 600 ma v fb line regulation ? v fb v in = 2.5v~5.5v 0.10 0.40 %/v output voltage line regulation ? v out v in = 2.5v~5.5v i out =10ma 0.10 0.40 %/v output voltage load regulation ? v load i out =1ma~600ma 0.001 %/ma oscillator frequency f osc v fb =0.6v or v out =100% 1.2 1.5 1.8 mhz peak inductor current i pk v in =3v,v fb =0.5v or vout=90% 1 a r ds(on) of p-ch fet r pfet i sw = 100ma 0.35 0.5 r ds(on) of n-ch fet r nfet i sw = -100ma 0.25 0.45 sw leakage i lsw ce=0,v sw =0 or 5v, v in =5v 0.01 1 a ce "high" voltage (1) v ce ?h? 1.5 v in v ce "low" voltage (2) v ce ?l? 0.3 v ce leakage current i ce 0.1 1 a note : (1) high voltage forcing ce above 1.5v enables the part. (2) low voltage forcing ce below 0.3v shuts down the device. in shutdown, all functions are disabled drawing <1 a supply current. do not leave ce floating. leshan radio company, ltd. 3/10
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ty p (te s p ical p e s t figure 1 e rfor m above u n m ance c n less oth e c harac e rwise sp e teristi e cified) cs leshan radio company, ltd. 6/10
main control loop the LR8506 uses a const ant 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 latch, and turned off when the current comparator, i comp , resets the rs latch. the peak induc tor current at which i comp resets the rs latch, is controlled 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 current starts to reverse, as indicated by the current reversal comparator i rcmp , or the beginning of the next clock cycle. the ovdet comparator cont rols output transient overshoots by turning the main switch off and keeping it off until the fault is removed. pulse skipping mode at light loads, the inductor current may reach zero reverse on each pulse. the bottom mosfet is turned off by the current reversal comparator, i rcmp , and the switch voltage will ring. this is discontinuous mode operation, and is normal behavior for the switching regulator. at very light loads, the lr 8506 will automatically skip pulses in pulse skipping mode operation to maintain output regulation. slope compensation slope compensation provides stability in constant frequency architecture by preventing subharmonic o scillations at high duty cycles. it is accomplished internally by adding a compensating ramp to the 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. dropout operation as the input suppl y voltage decreases to a value approaching the output 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 cycle. the output voltage will then be det ermined by the input voltage minus the voltage drop across the p-channel mosfet and the inductor. an important detail to remember is that at low inputs supply voltages, the r ds(on) of the p-channel switch increases. therefore, the user should ca lculate the power dissipation when the lr 8506 is used at 100% duty cycle with low input voltage. maximum load current the LR8506 will operate with input voltage as low as 2.5v, 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 function of the duty cycle to prevent sub-harmonic oscillations at duty cycles greater than 50%.conversely the current limit increase as the duty cycle decreases. ? operation leshan radio company, ltd. 7/10
the basic LR8506 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 . v ut 0.6v 1 r2 r1 setting the output voltage figure 1.2 shows t he basic application circuit with LR8506 adju stable output version. the external resistor sets the output voltage according to the following equation: table 1.resistor select for output voltage setting v out r1 r2 1.2v 316k 316k 1.5v 316k 470k 1.8v 316k 634k 2.5v 316k 1m ? application information ?i l v ut v n v ut v n lf inductor selection for most applications, the value of the inductor will fall in the range of 1 h to 4.7 h. its value is chosen based on the desired ripple current. large value inductor lower ripple current and small value inductor result in higher ripple currents. higher v in or v out also increases the ripple curr ent as shown in the following equation: a reasonable starting point for setting ripple current is i l =240ma (40% of 600ma). the dc current rating of the i nductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. different core materials and shapes will change the size/current and price/current relationship of an inductor. the choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/emi requi rements than on what the LR8506 requires to operate. table 2 shows some typical surface mount inductors that work well in LR8506 applications. table 2.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 c n required i ms i max v ut v n v ut / v n input capacitor selection in continuous mode, t he source current of the top mosfet is a square wave of duty cycle v out /v in . to prevent large voltage transients, a low esr input capacitor sized for the maximum rms current must be used. the maximum rms capacitor current is given by: this formula has a maximum at v in = 2v out , where i rms = i out /2. this simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. ceramic capacitors with x5r or x7r dielectrics are highly recommended because of their low esr and small temperature coefficients. a 4.7 f ceramic capacitor for most application is sufficient. leshan radio company, ltd. 8/10
pcb layout guidance when laying out the printed circuit board, the following suggestions should be taken to ensure proper operation of the LR8506. these items are also illustrated graphically in figure 2 and figure 3. 1. the power traces, in cluding the gnd trace, the sw trace and the v in trace should be kept short, direct and wide to allow large current flow. put enough multiply-layer pads when they need to change the trace layer. 2. keep the switching node, sw, away from the sensitive fb node. 3. the fb pin should di rectly connect to the feedback resistors. the resistive divider r1/r2 must be connected between the (+) plate of c out and ground. 4. connect the (+) plate of c in to the v in pin as closely as possible. 5. keep the (-) plate of c in and c out as close as possible. ?v ut ?i l esr 1 8fc ut 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: where f = operating frequency, c out = output capacitance and i l = ripple current in the inductor. for a fixed output voltage, the output ripple is highest at maximum input voltage since i l increase with input voltage. ceramic capacitors with x5r or x7r dielectrics are recommended due to their low esr and high ripple current. LR8506 LR8506 LR8506 LR8506 LR8506 LR8506 leshan radio company, ltd. 9/10
? p s o p acka g ot -23-5 p a g ing in f a ckage o u f ormat i u tline dim i on ensions leshan radio company, ltd. 10/10
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