Active-Semi
addition to the open-drain or open-collector logic
device. When the logic pull-down is disabled, the
voltage at EN will ramp up at a rate determined by
the 2µA EN pull-up current and the capacitor.
Once the voltage at EN exceeds the 1.1V
threshold, the device will be enabled. For the
case of using multiple ACT4050, time-based
output sequencing can be generated by placing
different capacitors at each ACT4050 EN pin.
The start up time delay can be calculated as a
simple function of the EN capacitor using the
equation:
T (ms) = 0.55 × C
EN
(nF)
Table 1:
Enable Delay Time vs. EN Capacitor Value
CAPACITOR VALUE
2.2nF
3.3nF
10nF
DELAY TIME (ms)
1.2
1.9
5.5
ACT4050
Rev 2, 01-Jul-11
4) Fourth, by using the 1.1V precision comparator in
the EN circuitry, "power-OK" type output
sequencing can be generated. By connecting the
EN pin of one ACT4050 to the output of another
device, the ACT4050 will only start up once the
second device's output has exceeded the 1.1V
level. A resistor divider can be used to adjust the
ACT4050 startup to any point on the second
device's output range.
5) Finally, the EN comparator can be used for "Line
UVLO" to prevent the ACT4050 from starting up
before the input voltage is high enough to
support the output. By using a resistor divider
from VIN to GND (center tap = 1.1V EN
threshold), the device can be enabled and
disabled based on the voltage at VIN. Since the
internal UVLO voltage is 4.2V, Line UVLO is
recommended for outputs above this 4.2V level
to ensure clean startup. For the example of a 5V
output, it is desirable to prevent IC startup until
VIN has exceeded the 5V level. To start the IC at
6V input, we place a 10kΩ/47kΩ resistor divider
from VIN to EN to GND, which enables the IC at
VIN greater than 6.3V and disables the IC when
VIN decreases below 5.2V.
Thermal Shutdown
The ACT4050 automatically turns off when its junction
temperature exceeds 160°C and automatically turns
on again when the junction temperature falls below
140°C .
Innovative Power
TM
-5-
www.active-semi.com
Copyright © 2011 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
