DESIGN IDEAS L Step-Up/Step-Down Charge Pump DC/DC Converter Provides up to 150mA in a Tiny 2mm × 2mm DFN Package by Julian Zhu Introduction A wide variety of handheld and portable applications are powered by Li-Ion batteries or AA cells. The wide input voltage range of a single Li-Ion battery (2.7V–4.2V) or 2 AA cells (1.8V–3.0V) requires a DC/DC converter that can step-up or step-down the input voltage to provide a fixed output voltage such as 3.3V or 2.5V. The new LTC3240 step-up/step-down DC/DC converter is ideally suited for such applications and can provide up to 150mA in a tiny 6-lead 2mm × 2mm DFN package. For input voltages greater than the regulated output voltage the LTC3240 operates as a low dropout regulator. When the input voltage decreases to within about 150mV of the regulated output voltage, the LTC3240 automatically switches to step-up Figure 1. LTC3240-3.3 step-up/stepdown converter capable of delivering current up to 150mA mode. In step-up mode, the LTC3240 operates as a constant frequency (1.2MHz) voltage doubling charge pump. The LTC3240 requires only three tiny external ceramic capacitors for an ultra small application footprint as shown in Figure 1. 1µF Li-Ion OR 3-CELL NiMH C+ VOUT C– VIN 2.7V TO 4.5V 1µF LTC3240-3.3 4.7µF GND OFF ON 3.3V IOUT = 150mA SHDN Figure 2. The regulated 3.3V from battery voltage 100 IOUT = 30mA 80 3.40 70 3.35 3.30 3.25 40 10 0 2.2 2.7 3.2 3.7 4.2 4.7 INPUT VOLTAGE (V) 5.2 5.7 Figure 3. Output voltage vs input voltage (full range) Linear Technology Magazine • June 2006 VOUT 20mV/DIV DOUBLER TO LDO MODE (VIN RISING) 30 20 A typical application circuit for LTC3240-3.3 is shown in Figure 2. The input can be a single Li-Ion battery or three AA cells. Figure 3 shows the output voltage variation for the entire input voltage range at a load current of 30mA. A new or recharged battery starts out at its highest terminal voltage. As the battery discharges, its terminal voltage continues to drop until the next recharge. The LTC3240 optimizes the output efficiency by continuing to operate in the step-down (LDO mode) for most of the battery life. When the battery voltage gets low enough it automatically switches into chargepump mode to squeeze out maximum energy from the battery before its next ILOAD = 1mA 50 3.20 Application for Li-Ion or Three AA Battery Input to 3.3V Out ILOAD = 40mA 60 3.15 3.10 1.7 LDO TO DOUBLER MODE (VIN FALLING) 90 3.45 EFFICIENCY (%) OUTPUT VOLTAGE (V) 3.50 The LTC3240 features low no load operating current (65µA typical) and ultra low shutdown current (<1µA). Built-in soft-start circuitry prevents excessive inrush current during start-up. The thermal-shutdown and current-limit circuitry allow the parts to survive a continuous output short-circuit. 1.8 2.3 2.8 3.3 3.8 4.3 4.8 SUPPLY VOLTAGE (V) 5.3 5.8 Figure 4. Efficiency vs input voltage (LTC3240-3.3) VIN = 2.5V ILOAD = 150mA 2µs/DIV Figure 5. Output voltage ripple (LTC3240-3.3) 43 L DESIGN IDEAS LT3486, continued from page 11 as 70V while both providing both overvoltage protection and remaining below the 42V maximum switch voltage. The charge pump Schottky diodes and capacitors double the effective output voltage for a given duty cycle while the LT3486 LED driver continues to regulate the 100mA constant LED current. The LEDs in Figure 7 have higher forward voltage than those in Figure 1 at 100mA, ripple voltage (<1% for 3.3V output) at 150mA (See Figure 5). To extend battery life at light loads, in charge pump mode, the part operates in high efficiency Burst Mode operation. In this case, the LTC3240 delivers a minimum amount of charge for a few cycles, and then enters a low current state until the output drops low enough to require another burst of charge. Conclusion resulting in a total string voltage as high as 40V. If more LEDs are needed, the string voltage can be stacked up to 70V before hitting the overvoltage protection level, but the peak switch current limit cannot be exceeded. As the string voltage and LED current goes up, the minimum input voltage also rises. Figure 8 shows the typical peak switch current limit dropping as duty cycle increases. In addition to the peak inductor current, the voltage doubler also adds additional charge pump capacitor current. ZLLS400 ZLLS400 COUT1B 2.2µF ZLLS400 25V PVIN 8V TO 18V CPV(IN) 4.7µF 25V 2.2µF 25V ZLLS400 10µH A915AY-100M COUT1A 2.2µF 25V 100mA 10 WHITE LEDs >34V CTRL1 BRIGHTNESS ADJUST SHDN VIN OVP2 CTRL1 CTRL2 FB1 VC1 GND RT PWM1 2.8k PWM INPUT 4700pF 100Hz 5V 1000:1 DIM RATIO ZLLS400 COUT2A 2.2µF 35V REF 0.1µF CTRL2 BRIGHTNESS ADJUST FB2 63.4k The LT3486 is a dual 1.3A LED string driver with 1000:1 PWM dimming capability. Its 3% LED current accuracy, low sense voltage, low shutdown current, overvoltage protection and wide input voltage range make it ideal for high power LCD panels in a variety of applications including automotive displays and notebook computers. The simple 5:1 analog dimming ratio and more precise 1000:1 PWM dimming ratio provide the displays with enough brightness control for daylight and nighttime use while retaining their color characteristics across brightness levels. L 1800 FDN5630 PWM2 VC2 Conclusion 100mA 10 WHITE LEDs >34V SW2 OVP1 PWM1 100k 2.2µF 25V CV(IN) 4.7µF 25V SHDN LT3486EFE FDN5630 RSENSE1 2Ω 1% 10µH A915AY-100M VIN 5V SW1 COUT2B 2.2µF 25V ZLLS400 The LTC3240 step-up/step-down charge pump DC/DC converter provides fixed regulated output voltage with currents up to 150mA from a wide input voltage range in a small 6-lead 2mm × 2mm DFN package. It is ideally suited for efficient DC/DC conversion in space-constrained applications such as battery-powered handheld electronics. L PEAK SWITCH CURRENT LIMIT (mA) recharge cycle. Figure 4 shows the efficiency of LTC3240-3.3 as a function of input voltage. In step-up (charge pump) mode, the LTC3240 uses a unique architecture to optimize the charge transferred to the output in each clock cycle, thus minimizing the output ripple. The part only needs a 4.7µF, 0603 size ceramic capacitor to obtain a 32mV maximum PWM2 100k RSENSE2 2Ω 1% 2.8k 4700pF PWM INPUT 100Hz 5V 1000:1 DIM RATIO Figure 7. LED driver uses 8V–18V input to drive two strings of ten 100mA LEDs (40V max per string) with 1000:1 PWM dimming VIN = 3.6V 1700 1600 1500 1400 1300 1200 20 30 40 50 60 70 80 DUTY CYCLE (%) 90 100 Figure 8. Typical peak switch current limit drops as duty cycle increases above 50% For more information on parts featured in this issue, go to http://www.linear.com 44 Linear Technology Magazine • June 2006