Adding an extra output to a boost converter

In battery-powered portable devices, designers use step-up converter ICs. These chips usually provide an output with a fixed or adjustable voltage. Some chips have LBI/LBO (low battery input/low battery output) functions. Chip manufacturers originally intended these pins to monitor low-battery conditions and issue a warning when the battery is empty. However, this function can also be used to generate an additional voltage output.

Maxim's MAX756 step-up converter provides fixed outputs of 3.3V or 5V, with currents of 300mA and 200mA, respectively (Figure 1). The input voltage range is 0.7V to 5.5V. For low-voltage detection, the device has an on-chip circuit that includes a comparator, a voltage reference, and an open-drain MOSFET. When the voltage at the LBI input is below its 1.25V threshold, the MOSFET at the LBO output drains current to ground.


Figure 1. A second output voltage can be generated using the battery monitoring circuit in this fixed-output step-up converter (Courtesy of MAXIM).

Using these components, a second output with regulation can be created (Figure 2). R1 and R2 determine the voltage of the second output as follows: Output 2 = VREF (R1 + R2) / R2, where VREF is the reference voltage, which is 1.25V for this chip. The voltage of output 2 can be set to 1.25V~5V, as long as it is less than the voltage of output 1. Because output 2 is derived from output 1, the total output current of the two outputs should not exceed 200mA and 300mA when the voltages of output 1 and output 2 are 5V and 3.3V, respectively.



Figure 2, using the LBI and LBO pins of this boost converter IC, a low-dropout second output voltage can be generated.

The LBI/LBO function can also be used as a second boost converter (Figure 3). The CD4093 four Schmitt-triggered NAND gates, inductor, L2, R2-R4, Q1, D1, C1 and C2 form this boost converter. IC1B plus C1 and R2 form a self-excited oscillator for gated start of IC1A. For the values ​​of R2 and C1 shown in the figure, the oscillator frequency is about 17kHz. R1 pulls up the open-drain LBO output.



Figure 3 If a second output needs to be boosted, the LBI and LBO pins can be used to make another switching boost converter.

When the voltage at the LBI pin is less than 1.25V, the LBO pin is low, thus operating oscillator IC1B. IC1C and IC1D drive power MOSFET transistor Q1. When Q1 is on, it pulls current from inductor L1. When Q1 is off, this energy charges capacitor C2 through flyback diode D1. Feedback from resistor divider R3 and R4 determines the voltage at output 2 as follows: Output 2 = 1.25V × (R3 + R4) / R4. IC1 is powered from output 1.





Schematic diagram

The voltage at output 2 is a function of the output current and the input voltage (Figure 4). If the input voltage is appropriate, the output is a flat section when the IC regulation is effective (Figures 5 and 6).