Diodes AP2552/ AP2553/ AP2552A/ AP2553A User Manual download pdf (Page 11)

AP2552/AP2553/AP2552A/AP2553A

Document number: DS35404 Rev. 8 - 2

11 of 16

www.diodes.com

February 2014

P2552/ AP2553/ AP2552A/ AP2553A

Application Note (cont.)

Reverse-Current and Reverse-Voltage Protection

The USB specification does not allow an output device to source current back into the USB port. In a normal MOSFET switch, current will flow in

reverse direction (from the output side to the input side) when the output side voltage is higher than the input side. A reverse-current limit

(ROCP)

feature is implemented in the AP2552/53 AND AP2552A/53A to limit such back currents. The ROCP circuit is activated when the output

voltage is higher than the input voltage. After the reverse current circuit has tripped (reached the reverse current trip threshold), the current is

clamped at this IROCP level.

In addition to ROCP, reverse over-voltage protection (

ROVP) is also implemented. The ROVP circuit is activated by the reverse voltage

comparator trip point

ie; the difference between the output voltage and the input voltage.

For AP2552/53, once ROVP is activated, FAULT assertion occurs at a de-glitch time of 4ms. Recovery from ROVP is automatic when the fault is

removed. FAULT de-assertion de-glitch time is same as the de-assertion time.

Reverse-Current and Reverse-Voltage Protection (cont.)

For AP2552A/53A, once ROVP is activated and when the condition exists for more than 5ms (TYP), output device is disabled and shutdown.

This is called the "Time from Reverse-Voltage Condition to MOSFET Turn Off”. FAULT assertion occurs at a de-glitch time of 4ms after ROVP is

reached. Recovery from this fault is achieved by recycling power or toggling EN. FAULT de-assertion de-glitch time is same as the de-assertion

time.

Special Functions:

Discharge Function

When enable is de-asserted, or when the input voltage is under UVLO level, the discharge function is active. The output capacitor is discharged

through an internal NMOS that has a discharge resistance of 100. 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.

FAULT Response

The FAULT open-drain output goes active low for any of following faults: Current limit threshold, short- circuit current limit, reverse-voltage

condition or thermal shutdown. The time from when a fault condition is encountered to when the FAULT output goes low is 7ms (TYP). The

FAULT output remains low until over-current, short-circuit current limit 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 FAULT due to the 7ms

deglitch timeout. This 7-ms timeout is also applicable for over-current recovery and over-temperature recovery. The AP2552/53 and

AP2552-2/53A are designed to eliminate erroneous over-current reporting without the need for external components, such as an RC delay

network.

For the AP2552/53 and AP2552A/53A when the reverse voltage condition is triggered, FAULT output goes low after 4ms (TYP). This 4ms (TYP)

timeout is also applicable for the recovery from reverse voltage fault.

When the ILIM pin is shorted to IN or GND, current-limit threshold and short-circuit current limit will be clamped at typically 75mA.When the ILIM

pin is shorted to IN or GND the AP2552/53 and AP2552A/53A FAULT pin will not assert during current limiting conditions; The FAULT pin will

assert during short circuit conditions.

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. This limits the input

voltage drop during line transients. Placing a high-value electrolytic capacitor on the input (10F minimum) and output pin(120µF) is

recommended when the output load is heavy. This precaution also reduces power-supply transients that may cause ringing on the input.

Additionally, bypassing thedeviceoutput with a 0.1F to 4.7F 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 parasiticinductance.

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 Θ

+ T

Where:

= Ambient temperature °C



= Thermal resistance

= Total power dissipation

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