Add extra output to a boost converter

Vladimir Oleynik, Moscow, Russia; Edited by Paul Rako and Fran Granville - November 17, 2011

Designers use step-up-converter ICs in battery-powered portable equipment. These chips usually provide one output with a fixed or an adjustable voltage. Some chips contain an LBI/LBO (low-battery-in/low-battery-out) function. The chip manufacturer intends for these pins to be used for monitoring a low-battery condition and to warn gadget owners when a battery goes flat. You can instead use this function to provide an extra voltage output.

The Maxim MAX756 boost converter provides a fixed output of 3.3 or 5V at 300 and 200 mA, respectively (Figure 1). The input voltage can range from 0.7 to 5.5V. For low-battery detection, the part has on-chip circuitry comprising a comparator, a reference, and an open-drain MOSFET. When the voltage at the LBI input is lower than its threshold level of 1.25V, the MOSFET at the LBO output sinks current to ground.

Add extra output to a boost converter figure 1

You can use these components to make a second output with a regulated voltage (Figure 2). R₁ and R₂ determine the secondary output voltage according to the following equation: Output 2 =V_REF(R₁+R₂)/R₂, where V_REF is the reference voltage, which is 1.25V for this chip.

Add extra output to a boost converter figure 2

You can set Output 2 from 1.25 to 5V as long as it is less than Output 1. Because Output 2 is derived from Output 1, the total output current for both outputs should not exceed 200 and 300 mA for Output 1 and Output 2 voltages of 5 and 3.3V, respectively.

Add extra output to a boost converter figure 3

Add extra output to a boost converter figure 4

You can also use the LBI/LBO function to make a second boost converter (Figure 3). The CD4093 quad Schmitt-triggered NAND gates, inductor L₂, R₂ through R₄, Q₁, D₁, C₁, and C₂ compose this boost converter. Add C₁ and R₂ to IC_1B to make a free-running oscillator that IC_1A gates on. For the values of R₂ and C₁ in the figure, the oscillator frequency is approximately 17 kHz. R₁ pulls up the open-drain LBO output.

Add extra output to a boost converter figure 5

When the voltage at the LBI pin is lower than 1.25V, the LBO pin is low, thus allowing operation of the IC_1B oscillator. IC_1C and IC_1D 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. You apply feedback with resistor divider R₃ and R₄ to determine the Output 2 voltage, according to the following equation: Output 2=1.25V×(R₃+R₄)/R₄. IC₁ gets power from Output 1.

Add extra output to a boost converter figure 6

The voltage at Output 2 is a function of the output current and the input voltage (Figure 4). If you have adequate input voltage, the output graph shows a flat section where the IC’s regulation is effective (figures 5 and 6).