´

D

-

Vref

R1

R2 =

Vout2 +

V

Vref

V = Vout1

Vout 2

D

-

>

´

-

´ D

R1

2800

Vout1 180

V

SS/TR

TPS54821

EN

PWRGD

SS/TR

TPS54821

EN

PWRGD

VOUT1

VOUT 2

R1

R2

R3

R4

TPS54821

SLVSB14

– OCTOBER 2011

www.ti.com

(7)

(8)

(9)

Figure 24. Ratio-metric and Simultaneous Startup Sequence

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

The device implements current mode control which uses the COMP pin voltage to control the turn off of the

high-side MOSFET and the turn on of the low-side MOSFET on a cycle by cycle basis. Each cycle the switch

current and the current reference generated by the COMP pin voltage are compared, when the peak switch

current intersects the current reference the high-side switch is turned off.

Low-side MOSFET overcurrent protection

While the low-side MOSFET is turned on its conduction current is monitored by the internal circuitry. During

normal operation the low-side MOSFET sources current to the load. At the end of every clock cycle, the low-side

MOSFET sourcing current is compared to the internally set low-side sourcing current limit. If the low-side

sourcing current is exceeded the high-side MOSFET is not turned on and the low-side MOSFET stays on for the

next cycle. The high-side MOSFET is turned on again when the low-side current is below the low-side sourcing

current limit at the start of a cycle.

16

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