AP2145/AP2155

0.5A SINGLE CHANNEL CURRENT-LIMITED POWER

SWITCH WITH OUTPUT DISCHARGE

AP2145/AP2155

Document number: DS32031 Rev. 2 - 2

9 of 13

www.diodes.com

September 2010

© Diodes Incorporated

Application Note

Power Supply Considerations

A 0.1-

μF to 1-μF X7R or X5R ceramic bypass capacitor

between

IN

and

GND,

close

to

the

device,

is

recommended. Placing a high-value electrolytic capacitor

on the input and output pin(s) is recommended when the

output load is heavy. This precaution reduces power-

supply transients that may cause ringing on the input.

Additionally, bypassing the output with a 0.01-

μF to 0.1-μF

ceramic capacitor improves the immunity of the device to

short-circuit transients.

Over-current and Short Circuit Protection

An internal sensing FET is employed to check for over-

current conditions. Unlike current-sense resistors, sense

FETs do not increase the series resistance of the current

path. When an overcurrent condition is detected, the

device maintains a constant output current and reduces

the output voltage accordingly. Complete shutdown occurs

only if the fault stays long enough to activate thermal

limiting.

Three possible overload conditions can occur. In the first

condition, the output has been shorted to GND before the

device is enabled or before VIN has been applied. The

AP2145/AP2155 senses the short circuit and immediately

In the second condition, an output short or an overload

occurs while the device is enabled. At the instance the

overload occurs, higher current may flow for a very short

period of time before the current limit function can react.

After the current limit function has tripped (reached the

over-current trip threshold), the device switches into

In the third condition, the load has been gradually

increased beyond the recommended operating current.

The current is permitted to rise until the current-limit

device is exceeded. The AP2145/AP2155 is capable of

delivering current up to the current-limit threshold without

damaging the device. Once the threshold has been

reached, the device switches into its current limiting mode

To protect against short circuit to GND at extremely low

temperature (< -30

capacitor on the output pin is recommended.

A correct

capacitor

type

with

capacitor

voltage

rating

and

temperature characteristics must be properly chosen so

that capacitance value does not drop too low at the

extremely low temperature operation. A recommended

capacitor should have temperature characteristics of less

than 10% variation of capacitance change when operated

at extremely low temp. Our recommended aluminum

electrolytic capacitor type is Panasonic FC series.

FLG Response

When an over-current or over-temperature shutdown

condition is encountered, the FLG open-drain output

goes active low after a nominal 7-ms deglitch timeout.

The FLG output remains low until both over-current and

over-temperature conditions are removed. Connecting a

heavy capacitive load to the output of the device can

cause a momentary over-current condition, which does

not trigger the FLG due to the 7-ms deglitch timeout. The

AP2145/AP2155 is designed to eliminate false over-

current

reporting

without

the

need

of

external

components to remove unwanted pulses.

Power Dissipation and Junction Temperature

The low on-resistance of the internal MOSFET allows the

small surface-mount packages to pass large current.

Using the maximum operating ambient temperature (TA)

and RDS(ON), the power dissipation can be calculated

by:

2

Finally, calculate the junction temperature:

Where:

Thermal Protection

Thermal protection prevents the IC from damage when

heavy-overload or short-circuit faults are present for

extended

periods

of

time.

The

AP2145/AP2155

implements a thermal sensing to monitor the operating

junction temperature of the power distribution switch.

Once the die temperature rises to approximately 140°C

due to excessive power dissipation in an over-current or

short-circuit condition the internal thermal sense circuitry

turns the power switch off, thus preventing the power

switch from damage. Hysteresis is built into the thermal

sense

circuit

allowing

the

device

to

cool

down

approximately 25°C before the switch turns back on. The

switch continues to cycle in this manner until the load

fault or input power is removed. The FLG open-drain

output is asserted when an over-temperature shutdown

or over-current occurs with 7-ms deglitch.