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AP2146 Datasheet(PDF) 13 Page - Diodes Incorporated

Part No. AP2146
Description  0.5A DUAL CHANNEL CURRENT-LIMITED POWER SWITCH
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Maker  DIODES [Diodes Incorporated]
Homepage  http://www.diodes.com
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AP2146 Datasheet(HTML) 13 Page - Diodes Incorporated

 
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AP2146/AP2156
0.5A DUAL CHANNEL CURRENT-LIMITED POWER
SWITCH
AP2146/AP2156 Rev 1
13 of 17
APRIL 2009
www.diodes.com
© Diodes Incorporated
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 (10-
μF 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 0.01-
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 AP2146/AP2156 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 AP2146/AP2156 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
oC), 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 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 AP2146/AP2156 is designed to eliminate false over-current reporting without the need
of external components to remove unwanted pulses.


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