ACT4050 datasheet(5/13 Pages) ACTIVE-SEMI

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Active-Semi

ACT4050

Rev 2, 01-Jul-11

- 5 -

www.active-semi.com

Innovative Power

CAPACITOR VALUE

DELAY TIME (ms)

2.2nF

1.2

3.3nF

1.9

10nF

5.5

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 × CEN (nF)

Table 1:

Enable Delay Time vs. EN Capacitor Value

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 .

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