ap2171 Diodes, Inc., ap2171 Datasheet - Page 11

• Manufacturer Part Number: ap2171
• Description: 1a Single Channel Current-limited Power Switch
• Manufacturer: Diodes, Inc.

Application Note
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. 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 over-current 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 V
circuit and immediately clamps output current to a certain safe level namely I
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 I
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 (I
the device is exceeded. The AP2161/AP2171 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 I
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 AP2161/AP2171 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 (T
calculated by:
Finally, calculate the junction temperature:
Where:
AP2161/AP2171 Rev. 5
P
T
T
R
P
J
A
D
D
θJA
= Ambient temperature °C
= P
= R
= Total power dissipation
= Thermal resistance
D
DS(ON)
x R
θJA
× I
+ T
2
A
LIMIT
.
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1A SINGLE CHANNEL CURRENT-LIMITED POWER
IN
11 of 17
has been applied. The AP2161/AP2171 senses the short
A
) and R
LIMIT
TRIG
) is reached or until the thermal limit of
.
DS(ON)
AP2161/AP2171
LIMIT
, the power dissipation can be
.
©
FEBRUARY 2009
Diodes Incorporated
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