link to page 10 link to page 10 link to page 10 link to page 10 link to page 10 MAX1795/MAX1796/ Low-Supply Current, Step-Up DC-DC Converters MAX1797 with True Shutdown path to quickly dissipate this energy, minimizing the ringing VIN at LX. Damping LX ringing does not reduce VOUT ripple, but does reduce EMI (Figure 3, Figure 4, and Figure 5). Setting the Output Voltage BATT VOUT can be set to 3.3V or 5.0V by connecting the FB R1 22µH 200Ω pin to GND (5V) or OUT (3.3V). To adjust the output MAX1795 voltage, connect a resistive voltage-divider from OUT to MAX1796 FB to GND (Figure 6). Choose a value less than 250kΩ MAX1797 DAMPING SWITCH for R2. LX VOUT VIN OUT 47µF 47µF Figure 3. Simplified Diagram of Inductor Damping Switch BATT LX R3 OUT OUTPUT LBI SHDN 2V TO 5.5V 1M 47µF R4 MAX1795 R1 MAX1796MAX1797 LBO LOW-BATTERY OUTPUT FB GND VLX R2 1V/div Figure 6. Setting an Adjustable Output Use the following equation to calculate R1: 2µs/div R1 = R2 [(VOUT/VFB) - 1] where V Figure 4. LX Ringing for Conventional Step-Up Converter FB = +1.245V, and VOUT can range from +2V to (without Damping Switch) +5.5V. Low-Battery Detection The MAX1795/MAX1796/MAX1797 each contain an on- chip comparator for low-battery detection. If the voltage at LBI is above 0.85V, LBO (an open-drain output) sinks current to GND. If the voltage at LBI is below 0.85V, LBO goes high impedance. The LBI/LBO function remains active even when the part is in shutdown. VLX 1V/div Connect a resistive voltage-divider to LBI from BATT to GND. The low-battery monitor threshold is set by two resistors, R3 and R4 (Figure 6). Since the LBI bias current is typically 2nA, large resistor values (R4 up to 250kΩ) can be used to minimize loading of the input supply. 2µs/div Calculate R3 using the following equation: R3 = R4[(VTRIP/0.85V) - 1] Figure 5. LX Waveform with Damping Switch www.maximintegrated.com Maxim Integrated │ 10
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