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ADM8828

Switched-Capacitor Voltage Inverter with Shutdown

Analog Devices 

ADM8828/ADM8829

GENERAL INFORMATION

The ADM8828/ADM8829 is a switched capacitor voltage con-

verter that can be used to invert the input supply voltage.

The voltage conversion task is achieved using a switched capaci-

tor technique using two external charge storage capacitors. An

on-chip oscillator and switching network transfers charge between

the charge storage capacitors. The basic principle behind the

voltage conversion scheme is illustrated below.

V+

S1

S2

CAP+ S3

+

C1

S4

CAP–

⌽1

،2

⌽2

OSCILLATOR

OUT = –V+

+

C2

Figure 11. Voltage Inversion Principle

An oscillator generating antiphase signals φ1 and φ2 controls

switches S1, S2 and S3, S4. During φ1, switches S1 and S2 are

closed while S3 and S4 are open, thereby charging C1 up to the

voltage at V+. During φ2, S1 and S2 open and S3 and S4 close.

The positive terminal of C1 is connected to GND via S3 during

this phase and the negative terminal of C1 connects to VOUT

via S4. The net result is voltage inversion at VOUT wrt GND.

Charge on C1 is transferred to C2 during φ2. Capacitor C2

maintains this voltage during φ1. The charge transfer efficiency

depends on the on-resistance of the switches, the frequency at

which they are being switched and also on the equivalent series

resistance (ESR) of the external capacitors. For maximum effi-

ciency, capacitors with low ESR are, therefore, recommended.

Shutdown Input

The ADM8828 contains a shutdown input that can be used to

disable the device and hence reduce the power consumption. A

logic high level on the SHDN input shuts the device down

reducing the quiescent current to 0.02 µA. During shutdown

the output voltage discharges to 0 V. Therefore, ground ref-

erenced loads are not powered during this state. When exiting

shutdown, it takes several cycles (approximately 175 µs) for the

charge pump to reach its final value. If the shutdown function is

not being used, SHDN should be hardwired to GND.

Capacitor Selection

The flying capacitor C1 can be increased to reduce the output

resistance.

The output capacitor size C2 affects the output ripple. Increas-

ing the capacitor size reduces the peak-peak ripple. The ESR

affects both the output impedance and the output ripple.

Reducing the ESR reduces the output impedance and ripple.

For convenience it is recommended that both C1 and C2 be the

same value.

The ac impedance of the ADM8828/ADM8829 may be reduced

by using a bypass capacitor on the input supply. This capacitor

should be connected between the input supply and GND. It will

provide instantaneous current surges as required. Suitable capaci-

tors of 1 µF or greater may be used.

C1 +

1␮F

+1.5V TO +5.5V

INPUT

CAP+

IN

ADM8828/

ADM8829

CAP–

SHDN

GND

OUT

SHUTDOWN

CONTROL

C2

+ 1␮F

INVERTED

NEGATIVE

OUTPUT

Figure 12. Typical Circuit Configuration

REV. A

–5–

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