CS5157
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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
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
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. CS5157 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 μs
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
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.
Figure 14. OVP Response to an InputtoOutput
Short Circuit by Immediately Providing 0% Duty
Cycle, CrowBarring the Input Voltage to Ground
M 10.0 μs
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.)