AP2181D datasheet(9/17 Pages) DIODES

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AP2181D/AP2191D

1.5A SINGLE CHANNEL CURRENT-LIMITED POWER

SWITCH WITH OUTPUT DISCHARGE

AP2181D/2191D

Document number: DS32251 Rev. 2 - 2

9 of 17

www.diodes.com

September 2010

Application Note

The AP2181D and AP2191D are integrated high-side

power switches optimized for Universal Serial Bus (USB)

that require protection functions. The power switches are

equipped with a driver that controls the gate voltage and

incorporates slew-rate limitation. This, along with the

various protection features and special functions, makes

these power switches ideal for hot-swap or hot-plug

applications.

Protection Features:

Under-voltage Lockout (UVLO)

Under-voltage lockout function (UVLO) guarantees that

the internal power switch is initially off during start-up.

The UVLO functions only when the switch is enabled.

Even if the switch is enabled, the switch is not turned ON

until the power supply has reached at least 1.9V.

Whenever the input voltage falls below approximately

1.9V, the power switch is turned off. This facilitates the

design of hot-insertion systems where it is not possible to

turn off the power switch before input power is removed.

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.

The different overload conditions and the corresponding

response of the AP2181D/AP2191D are outlined below:

S. NO

Conditions

Explanation

Behavior of the AP2181D/AP2191D

Short circuit condition at

start-up

Output is shorted before

input voltage is applied or

before the part is enabled

The IC senses the short circuit and immediately

clamps output current to a certain safe level namely

ILIMIT.

Short-circuit or

Overcurrent condition

Short-Circuit or Overload

condition that occurs when

the part 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.

Gradual increase from

nominal operating current

to ILIMIT

Load increases gradually

until the current-limit

threshold.(ITRIG)

The current rises until ITRIG or thermal limit. 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 (< -20

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.

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

time.

The

AP2181D/AP2191D

implements

thermal

sensing

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.

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