TPS65281RGVR datasheet(14/31 Pages) TI

14 / 31 page

TPS65281, TPS65281-1

SLVSBH7B – JULY 2012 – REVISED DECEMBER 2012

www.ti.com

OVERVIEW

TPS65281/TPS65281-1 PMIC integrates a current-limited, power distribution switch using N-channel MOSFETs

for applications where short circuits or heavy capacitive loads will be encountered and provide a precision current

limit protection. Additional device features include over termperature protection and reverse-voltage protection.

The device incorporates an internal charge pump and gate drive circuitry necessary to drive the N-channel

MOSFET. The charge pump supplies power to the driver circuit and provide the necessary voltage to pull the

gate of the MOSFET above the source. The charge pump operates from input voltage of power switch as low as

2.5 V and requires little supply current. The driver incorporates circuitry that controls the rise and fall times of

output voltage to limit large current and voltage surges and provides built-in soft-start functionality. TPS65281-1

device limits output current to a safe level by using a constant current mode when the output load exceeds the

current limit threshold. TPS65281 device lacthes off when the load exceeds the current limit threshold. The

device asserts the nFAULT signal during over current or reverse voltage faulty condition.

TPS65281/TPS65281-1 PMIC also integrates a synchronous step-down converter with a fixed 5-V output voltage

to provide the power for power switches in the USB ports. The synchronous buck converter incorporates a

90-m

Ω high side power MOSFET and 70-mΩ low side power MOSFET to achieve high efficiency power

conversion. The converter supports an input voltage range from 4.5 V to 18 V. The converter operates in

continuous conduction mode with peak current mode control for simplified loop compensation. The switching

clock frequency can be programmed from 300 kHz to 1.4 MHz from the ROSC pin connection. The peak inductor

current limit threshold is internally set at 4 A typical. The device builds in an internal 1-ms soft-start time to

reduce inrush current during power-up.

POWER SWITCH DETAILED DESCRIPTION

Over Current Condition

The TPS65281/TPS65281-1 responds to over-current conditions on power switches by limiting the output

currents to the IOCP_SW level, which is fixed internally. The load current is less than the current-limit threshold and

the device does not limit current. During normal operation the N-channel MOSFET is fully enhanced, and

VSW_OUT = VSW_IN - (ISW_OUT x Rdson_SW). The voltage drop across the MOSFET is relatively small compared to

VSW_IN, and VSW_OUT ≈ VSW_IN. When an over current condition is detected, the device maintains a constant

output current and reduces the output voltage accordingly. 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 IOS × RLOAD, where IOS is the current-limit threshold and RLOAD is the

magnitude of the overload condition.

Three possible overload conditions can occur as summarized in Table 1.

Table 1. Possible Overload Conditions

CONDITIONS

BEHAVIORS

The output voltage is held near zero potential with respect to ground and the TPS65281 ramps output

current to IOCP_SW. The TPS65281-1 will limit the current to IOS until the overload condition is removed or

Short circuit or partial short circuit present when

the device begins to thermal cycle. The TPS65281 limits the current to IOS until the overload condition is

the device is powered up or enabled

removed or the internal deglitch time (10 ms typical) is reached and the device is turned off. The device

will remain off until power is cycled or the device enable is toggled.

The current rises until current limit. Once the threshold has been reached, the device switches into its

current limiting at IOS. The TPS65281-1 will limit the current to IOS until the overload condition is removed

Gradually increasing load (<100 A/s) from normal

or the device begins to thermal cycle. The TPS65281 limits the current to IOS until the overload condition

operating current to IOS

is removed or the internal deglitch time (10 ms typical) is reached and the device is turned off. The device

will remain off until power is cycled or the device enable is toggled.

The device responds to the over-current condition within time tIOS (see Figure 5).The current sensing

amplifier is overdriven during this time, and needs time for loop response. Once tIOS has passed, the

Short circuit, partial short circuit or fast transient

current sensing amplifier recovers and limits the current to IOS. The TPS65281-1 will limit the current to IOS

overload occurs while the device is enabled and

until the overload condition is removed or the device begins to thermal cycle. The TPS65281 limits the

powered on

current to IOS until the overload condition is removed or the internal deglitch time (10 ms typical) is

reached and the device is turned off. The device will remain off until power is cycled or the device enable

is toggled.

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