AN001 A Versatile Step-Up and Step-down Converter Design Using AIC1628 The AIC1628 step-up DC/DC Converter can achieve very high efficiency in VIN 8 CL VREF 2 PFM OSCILLATOR 3 1.22V REFERENCE VOLTAGE programming voltage and color LCD contrast bias control voltage applications; the efficiency of these 60mV + various applications, especially for +12V flash memory VIN CURRENT LIMIT COMPARATOR 1 SHDN application can reach as high as 85% to 95%. FB The greatest concern in systems using battery + - 4 VIN 7 LATCH DHI OUTPUT DRIVER 6 DLOW 5 ERROR COMPARATOR Fig. 1 AIC1628 Function Block power is the power conversion efficiency. High efficiency converters can not only provide good power conversion efficiency at normal operation, In addition, the AIC1628 converter has the but also reduce its own current consumption following features: during idle time. In order to maintain good 1. Able to operate under input voltage range from 4V to 24V. conversion efficiency from light to full loads, the AIC1628 uses the intermittent switching control 2. Output voltage can be adjusted externally. method 3. It has a PFM design adjusting switching named PFM (Pulse-Frequency Modulation) rather than the conventional PWM frequency and duty cycle automatically, control method. Fig. 1 shows the IC’s basic which makes it possible to obtain highly function block. When the feedback voltage is great efficient conversion over a wide input and than the reference voltage, the drive output is 0V. output voltage range. When the feedback voltage is lower than the 4. Build in shutdown mode control. reference voltage, the oscillator starting output 5. Works in high frequency range from 90KHz to and send signal to the driver. This kind of control 250KHz, hence only requires small size method works similar to PWM at full load, with a inductor. stable switch waveform; while light load it uses 6. It has complementary push-pull output driver, intermittent switching to efficiently sustain output is able to drive either NPN transistor or loading requirements. MOSFET. 7. Low cost. Some application examples are explained below. The circuit shown in Fig. 2 is a power supply May 1997 1 GND AN001 design for color LCD brightness contrast bias The circuit shown in Fig. 3 provides a +12V output control. When VIN is higher than 7V, a high for efficiency of 93% can be obtained at full load. The efficiency is always in the range of 90% to 95% input power can come directly from a battery or when VIN is in the range from 5V to 11V and the the requiring load is larger than 60mA condition. In other secondary conversion. At the same time it also applications where battery voltage is lower than provides higher gate drive voltage to the MOSFET, 5V, the AIC1628 can be configured as the circuit this result the reducing of MOSFET’s Switch ON shown in Fig. 4. Note that the AIC1628 itself is resistance, and hence reduces the conduction powered by a 5V main power assumed available. loss directly. All of these factors contribute to the In this way, it is easier to drive a MOSFET to high conversion efficiency. If an NPN is used for obtain a higher efficiency. If cost reduce are lower cost consideration, efficiency may be required, the MOSFET can be replaced by an decreased about 2% to 5%. NPN transistor. Efficiency, however, will also be main power source without flash memory programming power. The reduced by about 2% to 5%. +5V ~ 24V VIN + *Fuse 60mΩ SHDN DLOW **L1 68µH 1N5819 D1 GND Q1 2SK940 AIC1628 C5 220µF +36V VOUT + C6 >50mA Efficiency (%) 0.047µF 5V VREF DHI FB VIN=13V CL VIN C2 95 C1 120µF VIN=18V VIN=9V 90 VIN=5V 85 0.1µF VOUT=36V R1 R2 80 16.5K 0.033µF C3 470K 0.01µF C4 *Buss Fuse MCR-2A **Sumida CDR105 0 50 100 150 200 Load Current (mA) (a) (b) Fig. 2 LCD Contrast Bias Application (a) Application Circuit (b) Efficiency vs. Load Current 2 AN001 *Fuse 60mΩ Efficiency vs Load Current C1 +5V ~ 11V 120µF 100 CL AIC1628 SHDN D1 DLOW R2 R1 11.3K 100K 1N5819 VOUT GND FB VIN=11V **L1 47µH DHI 2SK940 Q1 + +12V >150mA Efficiency (%) VREF C3 VIN 300µF C2 0.047µF VIN + * Buss Fuse MCR-2A ** Sumida RCH108 95 VIN=9V VIN=7V 90 VIN=5V 85 0 50 100 150 200 250 300 350 Load Current (mA) (a) (b) Fig. 3 Flash Memory Programming Power (a) Application Circuit (b) Efficiency vs. Load Current 5V + C2 33µF + C1 VIN 90 VIN=4V 150µF CL VREF VIN=3V *L1 47µH DHI SHDN D1 DLOW 1N5819 VOUT FB GND 2SK940 Q1 AIC1628 R2 R1 20K 470K +12V >50mA + C4 180µF Efficiency(%) 0.047µF C3 Efficiency vs Load Current Battery Powered +2V~ 5V 85 VIN=2V 80 75 0 *Sumida RCH108 50 100 150 Load Current (mA) (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 counterpart. Typically, a 50mΩ RDS(ON) (VGS =5V) step-down a MOSFET with an MPP core can deliver 2A output conversion from 5V to 3.3V at 2A output loading. A current and efficiency can exceed 92% at 1A charge pump circuit is used to generate a 2 VIN loading. Higher output driving capability mandates voltage for the AIC1628 to drive an N-MOSFET. lower RDS(ON) of N-MOSFET and lower VF of The advantages of using N-MOSFET include Schoktty diode. DC/DC converter, providing lower cost and higher efficiency than P-channel 3 1N5819 1N5819 AN001 D2 D3 47µF Efficiency vs Load Current VIN +5V + 95 100µF 90 C4 VIN CL VREF 0.04µF SHDN 1µF 2SK1463 Q1 DHI 33µH DLOW FB Efficiency(%) + C5 + C2 C1 D1 GND 1N5820 AIC1628 R2 R1 20K 34K *L1 C3 330µF + VOUT +3.3V 2A 85 80 75 100 10 1000 Load Current (mA) *MPP Core (a) (b) Fig. 5 5V to 3.3V Step-Down Converter (a) Application Circuit (b) Efficiency vs. Load Current 4