ap2171dfm Diodes, Inc., ap2171dfm Datasheet - Page 11

• Manufacturer Part Number: ap2171dfm
• Description: 1a Single Channel Current-limited Power Switch With Output Discharge
• Manufacturer: Diodes, Inc.

AP2161D/AP2171D
1A SINGLE CHANNEL CURRENT-LIMITED POWER
SWITCH WITH OUTPUT DISCHARGE
Application Note
(Continued)
Thermal Protection
Thermal protection prevents the IC from damage when the die temperature exceeds safe margins. This mainly
occurs when heavy-overload or short-circuit faults are present for extended periods of time. The
AP2161D/AP2171D implements a thermal sensing to monitor the operating junction temperature of the power
distribution switch. Once the die temperature rises to approximately 140°C, the Thermal protection feature gets
activated as follows: The internal thermal sense circuitry turns the power switch off and the FLG output is asserted
thus preventing the power switch from damage. Hysteresis in the thermal sense circuit allows the device to cool
down to approximately 25°C before the output is turned back on. This built-in thermal hysteresis feature is an
excellent feature, as it avoids undesirable oscillations of the thermal protection circuit. The switch continues to
cycle in this manner until the load fault is removed, resulting in a pulsed output. The FLG open-drain output is
asserted when an over-current occurs with 7-ms deglitch.
Reverse Current Protection
In a normal MOSFET switch, current can flow in reverse direction (from the output side to the input side) when the
output side voltage is higher than the input side, even when the switch is turned off. A reverse-current blocking
feature is implemented in the AP21x1 series to prevent such back currents. This circuit is activated by the
difference between the output voltage and the input voltage. When the switch is disabled, this feature blocks
reverse current flow from the output back to the input.
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 100Ω. 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 AP2161D/AP2171D is designed to eliminate erroneous Over-current reporting without the need for
external components, such as an RC delay network.
Applications Information:
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.
AP2161D/2171D
11 of 18
June 2010
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© Diodes Incorporated
Document number: DS32250 Rev. 1 - 2