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Exar Corporation 48720 Kato Road, Fremont CA, 94538 • 510-668-7017 • www.exar.com

SP3220E_EB_EU_101_060311

DESCRIPTION

Charge Pump Design Guidelines

The charge pump operates with 0.1µF ca-

pacitors for 3.3V operation. For other supply

voltages, see the table for required capacitor

values. Do not use values smaller than those

listed. Increasing the capacitor values (e.g.,

by doubling in value) reduces ripple on the

transmitter outputs and may slightly reduce

power consumption. C2, C3, and C4 may be

increased without changing C1’s value.

Minimum recommended charge pump

capacitor value

Input Voltage

Vcc

Charge pump

capacitor value for

SP3220E/EB/EU

3.0V to 3.6V

C1 - C4 = 0.1µF

3.0V to 5.5V

C1 - C4 = 0.22µF

Thechargepumposcillatortypicallyoperates

at greater than 250kHz allowing the pump to

run efficiently with small 0.1μF capacitors.

Efficientoperationdependsonrapidlycharg-

ing and discharging C1 and C2, therefore

capacitors should be mounted close to the

IC and have low ESR (equivalent series

resistance).

Low cost surface mount ceramic capacitors

(such as are widely used for power-supply

decoupling) are ideal for use on the charge

pump. However the charge pumps are de-

signed to be able to function properly with a

wide range of capacitor styles and values.

If polarized capacitors are used the positive

and negative terminals should be connected

as shown in the Typical Operating Circuit.

Voltage potential across any of the capaci-

tors will never exceed 2 x VCC. Therefore

capacitors with working voltages as low as

6.3V rating may be used with a 3.0V VCC

supply. The reference terminal of the V+

capacitor may be connected either to VCC

or ground, but if connected to ground a

minimum 10V working voltage is required.

Higher working voltages and/or capacitance

values may be advised if operating at higher

VCC or to provide greater stability as the

capacitors age.

Underlightlyloadedconditionstheintelligent

pump oscillator maximizes efficiency by

running only as needed to maintain V+ and

V-. Since interface transceivers often spend

much of their time at idle this power-efficient

innovation can greatly reduce total power

consumption. This improvement is made

possiblebytheindependentphasesequence

of the Exar charge-pump design.