Datasheet

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Input Filter Capacitor (C1)

Boost Capacitor (C2)

2 3

5 4

AVIN PVIN

FB GND SW

BOOST

UDG−04132

OUT

TPS40222

Boost Diode (D2)

Output Preload Requirement

AVIN Filtering

SW Node Snubber

TPS40222

SLUS642A – OCTOBER 2005 – REVISED JANUARY 2006

Select a good quality, low ESR ceramic capacitor to bypass the input. For a conservative design, the capacitor

should have a ripple current rating equal to the load current of the converter.

The boost capacitor is sized to ensure there is enough energy available to turn on the internal MOSFET. For

most applications, use a ceramic capacitor with a value between 33 nF and 100 nF.

Figure 13. Using a Boost Diode

For some applications, the internal bootstrap diode’s voltage drop may be too high to sufficiently charge the

boost capacitor each switching cycle. For these applications, a Schottky diode, D2 shown in Figure 13 , may be

added.

One of the requirements for proper startup of the DC-to-DC converter is that the boost capacitor, C2, has

sufficient voltage across it before switching occurs. In some applications, notably those with output voltages of

3.3 V, and those with slowly rising or low input voltages, there is the need to add a small 10 mA, pre-load to the

converter to hold the SW node to GND before switching begins. Without a pre-load, the output voltage may not

reach regulation. In addition, the pre-load prevents the output from overshooting too much when the load is

stepped from a high value to zero.

Some applications may require the addition of an R-C filter on the input of AVIN to filter unwanted noise and

improve load regulation. (See Figure 14 ) Use R4=10 Ω and C5=1 µF. Connect the ground side of C5 as close as

possible to the GND pin of the device.

To attenuate excessive ringing at the SW node, an R-C network may be added across D1. (See Figure 14 )

Use R3=10 Ω and C4=680 pF as a starting point. Decrease C4 until the minimum capacitance is found for the

desired ringing attenuation.