TPS65281RGVT datasheet(17/31 Pages) TI

17 / 31 page

TPS65281, TPS65281-1

www.ti.com

SLVSBH7B – JULY 2012 – REVISED DECEMBER 2012

Constant-Current vs. Latch-Off Operation and Impact on Output Voltage

Both the constant-current device (TPS65281-1) and latch-off device (TPS65281) operate identically during

normal operation, i.e. the load current is less than the current-limit threshold and the devices are not limiting

current. During normal operation the N-channel MOSFET is fully enhanced, and VSW_OUT = VSWIN - (ISW_OUT x

rDS(on)). The voltage drop across the MOSFET is relatively small compared to VSW_IN, and VSW_OUT ≈ VSW_IN.

Both the constant-current device (TPS65281-1) and latch-off device (TPS65281) operate identically during the

initial onset of an over-current event. Both devices limit current to the programmed current-limit threshold set by

RLIM by operating the N-channel MOSFET in the linear mode. During current-limit operation, the N-channel

MOSFET is no longer fully-enhanced and the resistance of the device increases. This allows the device to

effectively regulate the current to the current-limit threshold. The effect of increasing the resistance of the

MOSFET is that the voltage drop across the device is no longer negligible (VSW_IN ≠ VSW_OUT), and VSW_OUT

decreases. The amount that VSW_OUT decreases is proportional to the magnitude of the overload condition. The

expected VSW_OUT can be calculated by IOCP_SW × RLOAD, where IOCP_SW is the current-limit threshold and RLOAD

is the magnitude of the overload condition.

For example, if IOCP_SW is programmed to 1 A and a 1-Ω overload condition is applied, the resulting VOUT is 1 V.

While both the constant-current device (TPS65281-1) and latch-off device (TPS65281) operate identically during

the initial onset of an overcurrent event, they behave differently if the overcurrent event lasts longer than the

internal delay deglitch circuit (10 ms typical). The constant-current device (TPS65281-1) asserts the FAULT flag

after the deglitch period and continues to regulate the current to the current-limit threshold indefinitely. In

practical circuits, the power dissipation in the package will increase the die temperature above the

overtemperature shutdown threshold (145°C typical), and the device will turn off until the die temperature

decreases by the hysteresis of the thermal shutdown circuit (10°C typical). The device will turn on and continue

to thermal cycle until the overload condition is removed. The constant-current devices resume normal operation

once the overload condition is removed. The latch-off device (TPS65281) asserts the FAULT flag after the

deglitch period and immediately turns off the device. The device remains off regardless of whether the overload

condition is removed from the output. The latch-off device remains off and do not resume normal operation until

the surrounding system either toggles the enable or cycles power to the device.

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Product Folder Links: TPS65281 TPS65281-1

TPS65281RGVT Datasheet(PDF) 17 Page - Texas Instruments