ACT4088
Rev0, 13-May-08
Rectifier Diode
Shutdown Control
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).
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.
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.
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
(1Mꢀ) 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.
Selecting the Output Capacitor
A 22µF ceramic capacitor with X5R or X7R dielec-
tric provides the best results over a wide range of
applications.
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 9kꢀ/1kꢀ
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.
The output capacitor also needs to have low ESR
to keep low output voltage ripple. The output ripple
voltage is:
VIN
VRIPPLE = IOUTMAX KRIPPLE RESR
+
(2)
8 × fSW 2LCOUT
where IOUTMAX is the maximum output current,
KRIPPLE is the ripple factor (typically 20% to 30%),
RESR resistance is the ESR of the output capacitor,
fSW is the switching frequency, L is the inductor
value, and COUT is the output capacitance.
In the case of ceramic output capacitors, RESR is
very small and does not contribute to the ripple. In
the case of tantalum or electrolytic type, the ripple is
dominated by RESR multiplied by the ripple current.
In that case, the output capacitor is chosen to have
sufficiently low due to ESR, typically choose a ca-
pacitor with less than 50mꢀ ESR.
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.
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.
Innovative PowerTM
- 7 -
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