AP2151D datasheet(11/17 Pages) DIODES

Part #	AP2151D

Description	0.5A SINGLE CHANNEL CURRENT-LIMITED POWER SWITCH WITH OUTPUT DISCHARGE

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Manufacturer	DIODES [Diodes Incorporated]

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AP2141D/AP2151D

0.5A SINGLE CHANNEL CURRENT-LIMITED POWER

SWITCH WITH OUTPUT DISCHARGE

AP2141D/2151D

Document number: DS32242 Rev. 2 - 2

11 of 17

www.diodes.com

September 2010

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

AP2141D/AP2151D

implements 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. The built-in

thermal hysteresis feature 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 100

Ω.

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. The 7-ms timeout is

also applicable for Over-current recovery and Thermal

recovery.

The

AP2141D/AP2151D

are

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 the output from

going negative during turn-off due to inductive parasitics.

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 (TA)

and RDS(ON), the power dissipation can be calculated by:

PD = RDS(ON)× I

2

The junction temperature can be calculated by:

TJ = PD x RθJA + TA

Where:

TA= Ambient temperature °C

RθJA = Thermal resistance

PD = Total power dissipation

Generic Hot-Plug Applications

In many applications it may be necessary to remove

modules or PC boards while the main unit is still

operating. These are considered hot-plug applications.

Such implementations require the control of current

surges as seen by the main power supply and the card

being inserted. The most effective way to control these

surges is to limit and slowly ramp up the current and

voltage being applied to the card, similar to the way in

which a power supply normally turns on. Due to the

controlled rise and fall times of the AP2141D/AP2151D,

these devices can be used to provide a softer start-up to

devices being hot-plugged into a powered system. The

UVLO feature of the AP2141D/AP2151D also ensures

that the switch is off after the card has been removed,

and that the switch is off during the next insertion.

By

placing

the

AP2141D/AP2151D

between

the

VCC input and the rest of the circuitry, the input power

reaches these devices first after insertion. The typical rise

time of the switch is approximately 1ms, providing a slow

voltage

ramp

at

the

output

of

the

device.

This

implementation controls the system surge current and

provides a hot-plugging mechanism for any device.

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