115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

USB

POWER CONTROL

SWITCHES

4

FUNCTIONAL DESCRIPTION

Power switch

The power switch is an N-channel MOSFET with a maximum

on-state resistance of 140 m

Ω (V

high-side switch, the power switch prevents current flow in

either direction if disabled. The drain body diode is discon-

nected from the source when the switch is OFF allowing the

output voltage to exceed the input voltage without causing

current conduction. The power switch supplies a minimum of

500 mA per switch.

Charge pump

An internal charge pump supplies power to the driver circuit

and provides the necessary voltage to pull the gate of the

MOSFET above the source. The charge pump operates from

input voltages as low as 2.7 V. The charge pump is limited to

2.5

µA to establish a controlled turn on time of typically 1 ms.

Driver

The driver controls the gate voltage of the power switch. To

limit large current surges and reduce the associated electromag-

netic interference (EMI) produced, the driver incorporates

circuitry that controls the rise times and fall times of the output

voltage. The rise time is typically 1 ms.

ENABLE (EN or EN)

The logic enable disables the power switch and the bias for the

charge pump, driver, and other circuitry to reduce the supply

current to less than 5

µA maximum when a logic high is present

on EN (A2525) or a logic low is present (A2535). The proper

logic level restores bias to the drive and control circuits and

turns the power ON. The enable input is a compatible with both

TTL and CMOS logic levels.

Fault Flag (FLG)

This open drain output is asserted (active low) when an

overcurrent or over temperature condition is encountered. The

output will remain asserted until the overcurrent or over

temperature condition is removed.

Current sense

A sense FET monitors the current supplied to the load. The

sense FET measures current more efficiently than conventional

resistance methods. When an overload or short circuit is

encountered, the current-sense circuitry sends a control signal to

the driver. The driver in turn reduces the gate voltage and

drives the power FET into its saturation region, which switches

the output into a constant-current mode and holds the current

constant while varying the voltage on the load.

Thermal sense

An internal thermal-sense circuit shuts off the power switch

when the junction temperature rises to approximately 165

°C.

Hysteresis is built into the thermal sense circuit. After the

device has cooled approximately 10

°C, the switch turns back

ON. The switch continues to cycle off and on until the fault is

removed.

Undervoltage lockout

A voltage-sense circuit monitors the input voltage. When the

input voltage is approximately 2 V, a control signal turns OFF

the power switch.