ZXCP330 View Datasheet(PDF) - Zetex => Diodes

Part Name

Description

Manufacturer

ZXCP330

SWITCHED CAPACITOR DC-DC CONVERTER

Zetex => Diodes 

ZXCP330

Capacitor selection

Layout considerations

An absolute minimum value of 220nF for the flying

capacitor is needed to obtain maximum output current

and 330nF is probably safer with tolerance in mind.

However much lower capacitors can be used if the

device is being used at light loads. An output capacitor

of 10␮F is recommended and this should be as low an

ESR as possible as the output consists of large current

spikes, so ceramics are preferred. Because the flying

capacitor charges from the input via a switch, inrush

current is also large and a low ESR capacitor should be

used for input decoupling.

Careful layout of the ZXCP330 application circuit is

essential for correct operation because of its high

switching frequency and transient currents. For

optimum converter performance use a ground plane

and keep PCB tracks to all capacitors short to minimise

output voltage ripple and maintain regulation under all

conditions.

The recommended layout configuration is shown

below:

The device senses the output voltage in order to

regulate at 3.3V and any inductance in series with

either end of the output capacitor will cause ringing

which will not be damped as the output capacitor has

low ESR. This is very undesirable from a regulation or

ripple aspect, therefore short connections must be

used between device pins and all capacitors (see

Layout Considerations later) to keep the waveforms as

clean as possible.

The dielectric of the ceramic capacitor is an important

consideration for the ZXCP330 operation over

temperature. Zetex recommends minimum dielectric

specification of X7R for the flying capacitor and X5R for

the input and output capacitors. Capacitors used with a

lower specification dielectric can cause excessive

noise and output voltage ripple and can also

compromise output current over temperature. For

example a ceramic capacitor with an X7R dielectric will

lose 20% of its capacitance over a -40ЊC to 85ЊC

temperature range, whereas a capacitor with a Y5V

dielectric loses 80% of its capacitance at -40ЊC and 75%

at 85ЊC.

Image enlarged to show detail.

Actual size (8mmx5.3mm)

Thermal management

At high input voltages and load currents the ZXCP330

power dissipation is high. As mentioned previously,

the ZXCP330 will shutdown when the junction

temperature of the device reaches 150ЊC. To reduce the

junction temperature of the device a good thermal

connection to the PCB is necessary. This can be

achieved by connecting the GND pin of the ZXCP330 to

a solid ground plane running on the second layer of a 2

layer PCB, adding extra heatsinking.

PROVISIONAL ISSUE A - DECEMBER 2001

6

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