AP2181A/AP2191A

Document number: DS37618 Rev. 2 - 2

9 of 17

www.diodes.com

May 2018

© Diodes Incorporated

AP2181A/AP2191A

Application Information

Power Supply Considerations

A 0.01μF to 0.1μF 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

(10μF minimum) 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 0.01μF to 0.1μF 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 AP2181A/AP2191A senses the short circuit and immediately clamps output current to a certain safe level namely ILIMIT.

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 current limiting mode and the current is clamped at ILIMIT.

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 threshold (ITRIG) is reached or until the thermal limit of the device is exceeded. The AP2181A/AP2191A 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 and is set at ILIMIT.

Note that when the output has been shorted to GND at extremely low temperature (< -30°C), a minimum 120

μF electrolytic 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 7ms

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 7ms deglitch

timeout. The AP2181A/AP2191A 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:

PD = RDS(ON)× I

2

Finally, calculate the junction temperature:

TJ = PD x RθJA + TA

Where:

TA = Ambient temperature °C

RθJA = Thermal resistance

PD = Total power dissipation

Thermal Protection

Thermal protection prevents the IC from damage when heavy-overload or short-circuit faults are present for extended periods of time. The

AP2181A/AP2191A 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 7ms deglitch.