Single-Output Charge Pump ICs

Charge pumps (switched-capacitor) ICs provide dc-dc voltage conversion using a switch network to charge and discharge one or more capacitors. The switch network toggles between charge and discharge states of the capacitors. As shown in Fig. 1, the "flying capacitor " (C1) shuttles charge, and the "reservoir capacitor " (C2) holds charge and filters the output voltage.

The basic charge pump lacks regulation, which is generally added using either linear regulation or charge-pump modulation. Linear regulation offers the lowest output noise, and therefore provides better performance. Charge-pump modulation (which controls the switch resistance) offers more output current for a given die size (or cost), because the regulator IC need not include a series pass transistor.

A major advantage of the charge pump is elimination of the magnetic fields and EMI that comes with an inductor or transformer. There is one possible EMI source - the high charging current that flows to a "flying capacitor" when it connects to an input source or another capacitor with a different voltage.

Following are descriptions of typical single-output charge pump ICs.

1. This 100mA dc-dc converter has over 90% efficiency at full load with a typical operating current of just 160µA, making it ideally suitable for portable and system voltage conversions. Using only two low-cost identical capacitors, the charge-pump's 100mA output replaces switching regulators, eliminating inductors and their associated cost, size and EMI.

Equipped with an onboard oscillator circuit, the IC allows flexibility in selecting appropriate operating frequencies. The oscillator can operate at either low or high frequencies. When selecting the low, 5kHz operating frequency, a very low quiescent current will result. A smaller capacitor can be used at the higher, 50kHz operating frequency.

The device can operate in three modes: inverter, splitter or doubler. As an inverter, it converts the input voltage in a range of 1.5V to 5.5V to -1.5V to -5.5V output. Also, it can split or double a full load, +5.0V input voltage, of a power supply or battery, providing +2.25V in splitter mode and +9.5V in doubler mode. Because the IC does not require an external diode in voltage doubler mode, it occupies less p. c. board area in designs.

In the inverter mode, the SYNC pin may be overdriven by an external clock source. A square wave signal of 2V peak-to-peak (typical) may be applied to SYNC via a 2 to 5nF capacitor to overdrive the internal oscillator. In some applications, the 5kHz output ripple frequency may be low enough to interfere with other circuitry. Increasing the clock frequency increases the IC's quiescent current, but also allows smaller capacitance value to be used for C1 and C2.

For highest performance, capacitors with low effective series resistance (ESR) should be used. When using the inverting mode with a supply voltage less than 2V, connect the LV pin toVSS. This bypasses the internal regulator circuitry and provides best performance in low-voltage applications. When using the inverter mode with a supply voltage above 2V, LV must be left open.

Low ESR capacitors should be used at the output of the IC to minimize output ripple, output resistance and to maximize efficiency. This can be achieved using ceramic capacitors, but certain types of tantalum capacitors may be sufficient.

Although its output is not actively regulated, the IC is very insensitive to load current changes. With an input of +5V the output voltage is -5V under light load, and decreases only to 4.5V with a load of 100mA.

2. This switched capacitor charge pump steps the input voltage down to a regulated 1.5V ±4% output from a 3.1V to 5.5V input. Low operating current (35mA with no load, <1