Diodes AP2181D/AP2191D User Manual
Page 10
AP2181D/AP2191D
Document number: DS32251 Rev. 3 - 2
10 of 18
March 2013
© Diodes Incorporated
AP2181D/AP2191D
Application Note
(cont.)
Special Functions:
Discharge Function
When enable is de-asserted, the discharge function is active. The output capacitor is discharged through an internal NMOS that has a discharge
resistance of 100
Ω. Hence, the output voltage drops down to zero. The time taken for discharge is dependent on the RC time constant of the
resistance and the output capacitor.
FLG Response
The FLG open-drain output goes active low for any of the two conditions: Over-Current or Over-Temperature. The time from when a fault
condition is encountered to when the FLG output goes low is 7-ms (TYP). 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. This 7-ms timeout is also applicable for Over-current recovery and
Thermal recovery. The AP2181D/AP2191D is designed to eliminate erroneous Over-current reporting without the need for external components,
such as an RC delay network.
Power Supply Considerations
A 0.01-
μF to 0.1-μF X7R or X5R ceramic bypass capacitor between IN and GND, close to the device, is recommended. This limits the input
voltage drop during line transients. Placing a high-value electrolytic capacitor on the input (10-
μF minimum) and output pin(s) is recommended
when the output load is heavy. This precaution also 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. This capacitor also prevents
output from going negative during turn-off due to inductive parasitics.
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 (T
A
) and R
DS(ON)
, the power dissipation can be calculated by:
P
D
= R
DS(ON)
× I
2
The junction temperature can be calculated by:
T
J
= P
D
x R
θJA
+ T
A
Where:
T
A
= Ambient temperature °C
R
θJA
= Thermal resistance
P
D
= Total power dissipation
Generic Hot-Plug Applications
In many applications it may be necessary to remove modules or PC boards while the main unit is still operating. These are considered hot-plug
applications. Such implementations require the control of current surges as seen by the main power supply and the card being inserted. The most
effective way to control these surges is to limit and slowly ramp up the current and voltage being applied to the card, similar to the way in which a
power supply normally turns on. Due to the controlled rise and fall times of the AP2181D/AP2191D, these devices can be used to provide a softer
start-up to devices being hot-plugged into a powered system. The UVLO feature of the AP2181D/AP2191D also ensures that the switch is off after
the card has been removed, and that the switch is off during the next insertion.
By placing the AP2181D/AP2191D between the V
CC
input and the rest of the circuitry, the input power reaches these devices first after insertion.
The typical rise time of the switch is approximately 1ms, providing a slow voltage ramp at the output of the device. This implementation controls
the system surge current and provides a hot-plugging mechanism for any device.