Time = 2 msec / div

EN = 2 V / div

Vout1 = 1 V / div

Vout2 = 1 V / div

Time = 2 msec / div

EN = 2 V / div

Vout1 = 1 V / div

Vout2 = 1 V / div

TPS54521

SLVS981 – JUNE 2010

www.ti.com

Figure 28. Ratio-metric Startup with Vout2 Leading Vout1

Figure 29. Simultaneous Startup

Output Overvoltage Protection (OVP)

The device incorporates an output overvoltage protection (OVP) circuit to minimize output voltage overshoot. For

example, when the power supply output is overloaded, the error amplifier compares the actual output voltage to

the internal reference voltage. If the VSENSE pin voltage is lower than the internal reference voltage for a

considerable time, the output of the error amplifier demands maximum output current. Once the condition is

removed, the regulator output rises and the error amplifier output transitions to the steady state voltage. In some

applications with small output capacitance, the power supply output voltage can respond faster than the error

amplifier. This leads to the possibility of an output overshoot. The OVP feature minimizes the overshoot by

comparing the VSENSE pin voltage to the OVP threshold. If the VSENSE pin voltage is greater than the OVP

threshold the high-side MOSFET is turned off preventing current from flowing to the output and minimizing output

overshoot. When the VSENSE voltage drops lower than the OVP threshold, the high-side MOSFET is allowed to

turn on at the next clock cycle.

Overcurrent Protection

The device is protected from overcurrent conditions by cycle-by-cycle current limiting on both the high-side

MOSFET and the low-side MOSFET.

High-side MOSFET overcurrent protection

High-side MOSFET overcurrent protection is achieved by an internal current comparator that monitors the current

in the high-side MOSFET on a cycle-by-cycle basis. If this current exceeds the current limit threshold, the

high-side MOSFET is turned off for the remainder of that switching cycle.

18

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