ACT4088

Rev 2, 15-Nov-12

- 7 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

OUT

2

SW

IN

ESR

RIPPLE

OUTMAX

RIPPLE

LC

f

8

V

R

K

I

V

×

+

=

(2)

Rectifier Diode

Use a Schottky diode as the rectifier to conduct cur-

rent when the High-Side Power Switch is off. The

Schottky diode must have current rating higher than

the maximum output current and the reverse volt-

age rating higher than the maximum input voltage

(see Figure 2).

Selecting the Input Capacitor

For best performance choose a ceramic type ca-

pacitor with X5R or X7R dielectrics due to their low

ESR and small temperature coefficients. However,

low ESR tantalum or electrolytic types may also be

used, provided that the RMS ripple current rating is

higher than 50% of the output current. For most

applications, a 10µF capacitor is

sufficient. The

input capacitor should be placed close to the IN and

G pins of the IC, with shortest possible traces. In

the case of tantalum or electrolytic types, connect a

small parallel 0.1µF ceramic capacitor right next to

the IC.

Selecting the Output Capacitor

A 22µF ceramic capacitor with X5R or X7R dielec-

tric provides the best results over a wide range of

applications.

The output capacitor also needs to have low ESR

to keep low output voltage ripple. The output ripple

voltage is:

very small and does not contribute to the ripple. In

the case of tantalum or electrolytic type, the ripple is

In that case, the output capacitor is chosen to have

sufficiently low due to ESR, typically choose a ca-

pacitor with less than 50mΩ ESR.

External Bootstrap Diode

An external bootstrap diode (D2 in Figure 2) is rec-

ommended if the input voltage is less than 5.5V or if

there is a 5V system rail available. This diode helps

strengthen gate drive at lower input voltages, result-

ing in lower on-resistance and higher efficiency.

Low cost diodes, such as 1N4148 or BAT54, are

suitable for this application.

Shutdown Control

The ACT4088 enable pin provides several features

for adjusting and sequencing the power supply. An

internal 2µA current source pull-up, and a precision

1.24V comparator with hysteresis. With these com-

ponents, a user has the flexibility of using the EN

pin as:

1) A digital on/off control by pulling down the EN

current source with an external open-drain tran-

sistor. The voltage at EN is internally clamped to

6V.

2) A sequenced power supply by tying the EN pin

through a resistor to the output of another power

supply. The IC will be enabled when the voltage

at EN exceeds 1.24V, or a resistor divider can be

used to adjust the turn-on threshold.

3) An always-on converter by floating the EN pin or

pulling EN to a desired voltage with a high value

(1MΩ) external resistor. EN is internally clamped

at 6V and will dissipate power if an external re-

sistor attempts to pull EN above the 6V clamp

voltage.

4) Line UVLO. If desired, to achieve a UVLO volt-

age that is higher than the internal UVLO, an

external resistor divider from VIN to EN to GND

can be used to disable the ACT4088 until a

higher input voltage is achieved. For example, it

is not useful for a converter with 9V output to

start up with a 4.2V input voltage, as the output

cannot reach regulation. To enable the ACT4088

when the input voltage reaches 12V, a 9kΩ/1kΩ

resistor divider from IN to GND can be con-

nected to the EN pin. Both the precision 1.2V

threshold and 80mV hysteresis are multiplied by

the resistor ratio, providing a proportional 6.67%

hysteresis for any startup threshold. For the ex-

ample of a 12V enable threshold, the turn off

threshold would be 11.2V.

5) Power supply sequencing. By connecting a small

capacitor from EN to GND, the 2µA current

source and 1.24V threshold can provide a stable

and predictable delay between startup of multiple

power supplies. For example, a startup delay of

roughly 10mS is provided using 150nF, and

roughly 20mS by using 330nF. The EN current

source is active anytime an input supply is ap-

plied, so disabling the IC or resetting the delay

requires an external open-drain pull-down device

to reset the capacitor and hold the EN pin low for

shutdown.