TPS2456RHHT datasheet(13/31 Pages) TI

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FT x

SENSEx

100 mV

SEN SEx

MON x

SETx

LIMIT x

MON x

SEN SEx

0.675 V

LIMITx

MONx

SENSEx

SETx

0.675 V

TPS2456

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SLVSA78A – MARCH 2010 – REVISED MARCH 2010

When the TPS2456 protects a supply output in configurations that allow the loads to hotplug, pay special

attention to coordinating load surges (due to input capacitance) and the fast trip threshold. The fast trip threshold

may need to be set 2–5 times higher than the current limit to accommodate this. Care must also be taken if the

INx voltages can have fast rising transients. The resulting charge current to capacitors on OUTx can potentially

exceed the fast trip threshold.

FAST TRIP AND CURRENT LIMITING

Figure 17 shows a simplified block diagram of the fast trip and current limit circuitry. Each channel requires an

external N-channel protection MOSFET and three external resistors. These resistors allow the user to

independently set the fast trip threshold and the current limit threshold, as described below.

The fast trip function is designed to protect the channel against short-circuit events. If the voltage across RSENSEx

exceeds 100 mV, the TPS2456 immediately turns off the protection MOSFET, M1x. The nominal fast trip limit IFTx

is defined in Equation 1.

(1)

The current limit circuit regulates VGAT1x to control the channel current from exceeding ILIMITx. The current limit

circuitry includes two amplifiers, A1 and A2, as shown in Figure 17 and Figure 4. Amplifier A1 forces the voltage

across external resistor RSETx to equal the voltage across external resistor RSENSEx. The current that flows

through RSETx also flows through external resistor RMONx, generating a voltage on the MONx pin per Equation 2.

(2)

Amplifier A2 implements a slow-reacting current limit. As long as VMONx is less than 0.675 V, GAT1x operates

normally. When VMONx exceeds 0.675 V, amplifier A2 causes a small current to be drawn from GAT1x. The

gate-to-source voltage of M1x drops until load current is reduced and the two inputs of amplifier A2 balance. The

current flowing through the channel then equals ILIMITx per Equation 3.

(3)

(4)

The recommended value of RMONx is 6.81 kΩ. This resistor should be greater than 675 Ω to prevent excessive

currents from flowing through the internal circuitry.

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