Multiple-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, "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.

Multiple output controller ICs consist of two or more charge pump converters in a single package. They can be controllers that employs external power switches or regulators with an internal power switch. One possibility is a 5V output and a 3.3V output for processor and logic applications.

Among the features found in typical multiple output charge pump controller ICs are:

1. This switched capacitor step-down dc-dc converter produces two adjustable regulated outputs from a single 2.7V to 5.5V input. The part uses switched capacitor fractional conversion to achieve a typical efficiency increase of 50% over that of a linear regulator. No inductors are required.

The IC has two switched capacitor charge pumps to step down VIN to two regulated output voltages. The two charge pumps operate 180° out of phase to reduce input ripple. Regulation is achieved by sensing each output voltage through an external resistor divider and modulating the charge pump output current based on the error signal. A two-phase non-overlapping clock activates the two charge pumps running them out of phase from each other.

A constant frequency, spread spectrum architecture provides a very low noise regulated output as well as low noise at the input. The spread spectrum oscillator utilizes random switching frequencies between 1MHz and 1.6MHz, and sets the rate of charging and discharging of the flying capacitors. This architecture achieves extremely low output noise and input noise is significantly reduced compared to conventional charge pumps.

Low operating currents (60mA with both outputs enabled, 35mA with one output enabled) and low external parts count make the IC ideally suited for space-constrained battery-powered applications. The part also has Burst Mode® operation to provide high efficiency at low output currents, as well as ultra-low current shutdown.

In shutdown mode all circuitry is turned off and the IC draws only leakage current from the VIN supply. Furthermore, OUT1 and OUT2 are disconnected from VIN. Shutdown current = 0.01mA, typ.

The EN1 and EN2 pins are CMOS inputs with threshold voltages of approximately 0.8V to allow regulator control with low voltage logic levels. The IC is in shutdown when a logic low is applied to both enable pins. Since the mode pins are high impedance CMOS inputs, they should never be allowed to float. Always drive the enable pins with valid logic levels.