CS5157H

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10

Short Circuit Protection

A lossless hiccup mode short circuit protection feature is

provided, requiring only the Soft−Start capacitor to

the MOSFET to shut off, disconnecting the regulator from it’s

input voltage. The Soft−Start capacitor is then slowly

discharged by a 2.0

mA current source until it reaches it’s

lower 0.7 V threshold. The regulator will then attempt to

restart normally, operating in it’s extended off time mode with

a 50% duty cycle, while the Soft−Start capacitor is charged

with a 60

mA charge current.

If the short circuit condition persists, the regulator output

before the Soft−Start capacitor is charged to it’s upper 2.5 V

threshold. If this happens the cycle will repeat itself until the

short is removed. The Soft−Start charge/discharge current

ratio sets the duty cycle for the pulses (2.0

mA/60 mA = 3.3%),

while actual duty cycle is half that due to the extended off time

mode (1.65%).

This protection feature results in less stress to the

regulator components, input power supply, and PC board

traces than occurs with constant current limit protection (see

Figures 12 and 13).

If the short circuit condition is removed, output voltage

will rise above the 1.0 V level, preventing the FAULT latch

from being set, allowing normal operation to resume.

Figure 12. CS5157H Demonstration Board Hiccup

Mode Short Circuit Protection. Gate Pulses are

Delivered While the Soft−Start Capacitor Charges, and

Cease During Discharge

M 25.0 ms

Trace 3− Soft−Start Timing Capacitor (1.0 V/div.)

Trace 4− 5.0 V Supply Voltage (2.0 V/div.)

Trace 2− Inductor Switching Node (2.0 V/div.)

Figure 13. Startup with Regulator Output Shorted

M 50.0

ms

Trace 4− 5.0 V from PC Power Supply (2.0 V/div.)

Trace 2− Inductor Switching Node (2.0 V/div.)

Overvoltage Protection

Overvoltage protection (OVP) is provided as result of the

additional external components. The control loop responds

to an overvoltage condition within 100 ns, causing the top

MOSFET to shut off, disconnecting the regulator from it’s

input voltage. The bottom MOSFET is then activated,

resulting in a “crowbar” action to clamp the output voltage

and prevent damage to the load (see Figures 14 and 15 ). The

regulator will remain in this state until the overvoltage

condition ceases or the input voltage is pulled low. The

bottom FET and board trace must be properly designed to

implement the OVP function.

Figure 14. OVP Response to an Input−to−Output

Short Circuit by Immediately Providing 0% Duty

Cycle, Crow−Barring the Input Voltage to Ground

M 10.0

ms

Trace 1− Regulator Output Voltage (1.0 V/div.)

Trace 2− Inductor Switching Node (5.0 V/div.)

Trace 4− 5.0 V from PC Power Supply (5.0 V/div.)