an 001 may 1997 1 a versatile step - up and step - down converter design using aic1628 the aic1628 step - up dc/dc converter can achieve very high efficiency in various applications, especially for +12v flash memory programming voltage and color lcd contrast bias control voltage applications; the efficiency of these applic ation can reach as high as 85% to 95%. the greatest concern in systems using battery power is the power conversion efficiency. high efficiency converters can not only provide good power conversi on efficiency at normal operation , but also reduce its own current consumption during idle time . in order to maintain good conversion efficiency from light to full loads, the aic1628 uses the intermittent switch ing control method named pfm (pulse - frequency modulation) rather than the conventional pwm control method. fig. 1 shows the ic? s basic function block . when the feedback voltage is great than the reference voltage, the drive output is 0v. w hen the feed back voltage is lower than the reference voltage, the oscillator starting output and send signal to the drive r . this kind of control method works similar to pwm at full load, with a stable switch waveform; while light load it uses intermittent switchin g to efficiently sustain output loading requirements. + - + - error comparator latch current limit comparator 1.22v reference voltage pfm oscillator vref vin shdn 4 3 2 1 fb output driver v in v i n 60mv dhi dlow cl 8 7 6 5 gnd fig. 1 aic1628 function block in addition, the aic1628 converter has the following features: 1. able to operate under input voltage range from 4v to 24v. 2. out put voltage can be adjusted externally. 3. it has a pfm design adjust ing switching frequency and duty cycle automatically , which makes it possible to obtain highly efficient conversion over a wide input and output voltage range. 4. build in shutdown mode control. 5. w orks in high frequency range from 90khz to 250khz, hence only requires small size inductor . 6. it has complementary push - pull output driver , is able to drive either npn transistor or mosfet . 7. low cost. some application examples are explained below. the circuit shown in fig. 2 is a power supply
an 001 2 design for color lcd brightness contrast bias control . when v in is higher than 7v, a high efficiency of 93% can be obtained at full load. the input power can come directly from a battery or the main power source without requiring secondary conversion. at the same time it also provides higher gate drive voltage to the mosfet, this result the reduc ing of mosfet?s switch on resistance , and hence reduce s the conduction loss directly . all of these factors contribute to the high conversion efficiency. if an npn is used for lower cost consideration , efficiency may be decreased about 2% to 5%. the circ uit shown in fig. 3 provides a +12v output for flash memory programming power. the efficiency is always in the range of 90% to 95% when v in is in the range from 5v to 11v and the load is larger than 60ma condition . in other applications where battery volt age is lower than 5v, the aic1628 can be configured as the circuit shown in fig. 4. note that the aic1628 itself is powered by a 5v main power assumed available. in this way, it is easier to drive a mosfet to obtain a higher efficiency. if cost re duce are required, the mosfet can be replaced by an npn transistor. efficiency, however, will also be reduced by about 2% to 5%. aic1628 cl dhi dlow fb gnd shdn vin vref +36v >50ma v out 1n5819 c5 + c6 v in +5v ~ 24v c1 120 m f + 5v *buss fuse mcr-2a **sumida cdr105 *fuse 60m w 68 m h 0.047 m f c2 r1 470k r2 16.5k 0.033 m f c3 220 m f 0.1 m f d1 2sk940 0.01 m f c4 **l1 q1 load current ( ma) efficiency (%) v in =13v 0 50 100 150 200 80 85 90 95 v in =18v v in =9v v in =5v v out =36v (a) (b) fig. 2 lcd contrast bias application (a) application circu it (b) efficiency vs. load current
an 001 3 * buss fuse mcr-2a ** s umida rch108 aic1628 cl dhi dlow fb shdn vin vref 2sk940 q1 +12v >150ma v out d1 1n5819 300 m f c3 + v in +5v ~ 11v c1 120 m f + *fuse 60m w **l1 47 m h gnd r1 100k r2 11.3k c2 0.047 m f load current (ma) efficiency (%) 0 50 100 150 200 250 300 350 85 90 95 100 v in =11v v in =9v v in =7v v in =5v efficiency vs load current (a) (b) fig. 3 flash memory programming power (a) application circuit (b) efficiency vs. load current battery powered 180 m f *l1 47 m h +2v~ 5v + c1 150 m f >50ma +12v d1 1n5819 v out *sumida rch108 + c4 aic1628 cl dhi dlow fb gnd shdn vin vref + c2 5v 0.047 m f c3 33 m f 2sk940 q1 r2 20k r1 470k load current ( ma) efficiency(%) 0 50 100 150 75 80 85 90 v in =2v efficiency vs load current v in =3v v in =4v ( a) (b) fig. 4 2 cells to 12v step - up power supply (a) application circuit (b) efficiency vs. load current fig. 5 is a circuit configuration of a high efficiency step - down dc/dc converter, providing a conversion from 5v to 3.3v at 2a output loading. a ch arge pump circuit is used to generate a 2 ?? v in voltage for the aic1628 to drive an n - mosfet. the advantages of using n - mosfet include lower cost and higher efficiency than p - channel counterpart. typically, a 50m w r ds(on) (v gs =5v) mosfet with an mpp core can deliver 2a output current and efficiency can exceed 92% at 1a loading. higher output driving capability mandates lower r ds(on) of n - mosfet and lower v f of schoktty diode.
an 001 4 d1 1n5820 d2 d3 *l1 33 m h r2 20k r1 34k c1 c4 100 m f + 1 m f 330 m f + + 2sk1463 q1 c3 c2 *mpp core c5 47 m f + 1n5819 v in +5v 0.04 m f aic1628 cl dhi dlow fb gnd shdn vin vref 1n5819 v out +3.3v 2a 95 75 80 85 90 10 100 1000 load current ( ma) efficiency(%) efficiency vs load current (a) (b) fig. 5 5v to 3.3v step - down converter (a) application circuit (b) efficiency vs. load current
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