Diodes AP2156 Manuel d'utilisateur télécharger pdf (Page 12)

AP2146/AP2156

Document number: DS31813 Rev. 3 - 2

12 of 17

www.diodes.com

March 2013

AP2146/

P2156

Application Information

Power Supply Considerations

A 0.01-F to 0.1-F 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-F 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-F to 0.1-F 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 V

has been applied. The AP2146/AP2156 senses the short circuit and immediately clamps output current to a certain safe level namely I

LIMIT

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

LIMIT

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

TRIG

) 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 I

LIMIT

Note that when the output has been shorted to GND at extremely low temperature (< -30

C), a minimum 120-F 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.

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

) and R

DS(ON)

, the power dissipation can be calculated by:

= R

DS(ON)

× I

Finally, calculate the junction temperature:

= P

x R



+ T

Where:

= Ambient temperature C



= Thermal resistance

= 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

AP2146/AP2156 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 7-ms deglitch.

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Diodes AP2156 Manuel d'utilisateur Page 12

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