DC-DC Switch-Mode Power-Supply Controller Drives Regulated Charge Pump

摘要

Inductor-based switch-mode power supplies can take significant design effort and increase size, especially for generating negative supply currents for op amps or LCD bias. This application note describes how a switch-mode power-supply controller such as the MAX774 and a diode-capacitor network can generate modest negative supply currents without the use of inductors.

A switch-mode power-supply controller and diode-capacitor network can generate the modest negative supply currents required for op amps or for LCD bias, without the design effort and size penalty associated with inductors (Figure 1). The circuit of Figure 1 accepts inputs of 2V to 6V and produces a digitally adjustable output voltage. The diode-capacitor charge pump is driven by the switching action of DHI and DLOW, which normally drive an external MOSFET or pnp transistor in an inductor-based switch-mode power supply.

At power-up, the internal 6-bit current-output D/A converter produces a nominal V_OUT of R1 x 13.33µA. Then, by holding CTRL high and toggling ADJ, you can adjust V_OUT over a 3:1 range in 64 equal steps, according to the value of R1: R1 x 6.66µA ≤ V_OUT ≤ R1 x 20µA. If digital adjustment is not required, ground the ADJ pin and connect CTRL to V+. Or construct a similar circuit with the MAX774 DC-DC controller, which accepts inputs up to 16.5V. For positive outputs greater than V_IN, a step-up controller and modified charge-pump network (not shown) can do the job.

The maximum output current depends on V_IN, V_OUT, and the number of diode-capacitor stages, each of which consists of two diodes and two capacitors. Though only a few microamps are available at the maximum V_OUT, which equals V_IN - (0.6V x the number of stages), you can draw more current at the lower output voltages (Figure 2).

The number of diode-capacitor stages determines the maximum I_OUT for a given V_IN and V_OUT. Too few stages will not achieve the desired voltage, but too many degrades the efficiency: I_IN equals (approximately) I_OUT times the number of stages. For V_IN less than 5V, the circuit delivers less output current than is indicated by the curves of Figure 2. For example, a 4-stage circuit produces an output current of 1mA at -2V out from 2V_IN, at -7V from 3V_IN, at -11V from 4V_IN, and at -14.5V from 5V_IN. Larger pump capacitors can provide higher I_OUT.

The controller changes its behavior when V_OUT is much lower than the voltage programmed by R1. Designed for inductor-based circuits, it compensates for impending dropout by increasing the switching transistor's "on" time at the expense of "off" time. This action normally ramps up the inductor current, but it has an opposite effect for the circuit shown. Because short off times (DHI high) don't allow the capacitors to discharge fully, the available output current actually decreases instead of increasing. Thus, when V_OUT loses regulation due to overload, you must reduce the load current considerably before regulation can be regained. A reliable maximum output current is the level at which V_OUT recovers from dropout, not the higher level at which it enters dropout.

For stable operation, bypass R1 with 220pF, keep the connections between R1 and IC1 very short, and place the input bypass capacitor directly across the input pins of IC1. Output ripple is typically less than 1% for the component values shown, but ripple can be higher if the circuit includes more stages than that required by the programmed output voltage. Ripple can be lowered by increasing the output capacitance.