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CS5158

CPU 5−Bit Synchronous Buck Controller

ON Semiconductor 

CS5158

Short Circuit Protection

A lossless hiccup mode short circuit protection feature is

provided, requiring only the Soft Start capacitor to

implement. If a short circuit condition occurs (VFFB < 1.0 V),

the VFFB low comparator sets the FAULT latch. This causes

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 μA 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 μA charge current.

If the short circuit condition persists, the regulator output

will not achieve the 1.0 V low VFFB comparator threshold

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 μA/60 μA =

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.

M 50.0 μs

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

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

Figure 13. Startup with Regulator Output Shorted

Overvoltage Protection

Overvoltage protection (OVP) is provided as result of the

normal operation of the V2 control topology and requires no

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.

M 25.0 ms

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

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

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

Figure 12. CS5158 Demonstration Board Hiccup

Mode Short Circuit Protection. Gate Pulses are

Delivered While the Soft Start Capacitor Charges,

and Cease During Discharge

M 10.0 μs

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

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

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

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

Short Circuit by Immediately Providing 0% Duty

Cycle, Crow−Barring the Input Voltage to Ground

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