Application Information: continued
threshold. If this happens the cycle will repeat itself until
the short is removed. The Soft Start charge/discharge cur-
rent ratio sets the duty cycle for the pulses (2µA/60µA =
3.3%), while actual duty cycle is half that due to the extend-
ed 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 10 and 11).
If the short circuit condition is removed, output voltage
will rise above the 1V level, preventing the FAULT latch
from being set, allowing normal operation to resume.
Trace 1 - GATE(H) (10V/div)
Trace 2 - Inductor Switching Node (5V/div)
Trace 3 -Load Current (5A/div)
Trace 4 - V
(100mV/div)
OUT
Figure 8: Output Voltage Transient Response to a 14A load step, VOUT
+2.84V(DAC = 10111).
=
Trace 4 - 5V Supply Voltage (2V/div.)
Trace 3 - Soft Start Timing Capacitor (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
Figure 10: Demonstration board hiccup mode short circuit protection.
Gate pulses are delivered while the Soft Start capacitor charges, and
cease during discharge.
Trace 1 - GATE(H) (10V/div)
Trace 2 - Inductor Switching Node (5V/div)
Trace 3 -Load Current (5A/div)
Trace 4 - V
(100mV/div)
OUT
Figure 9: Output Voltage Transient Response to a 14A load turn-off,
OUT = +2.84V (DAC = 10111).
V
Protection and Monitoring Features
Short Circuit Protection
A lossless hiccup mode short circuit protection feature is
provided, requiring only the Soft Start capacitor to imple-
ment. If a short circuit condition occurs the VFB low com-
parator sets the FAULT latch. This causes the top FET to
shut off, disconnecting the regulator from its input voltage.
The Soft Start capacitor is then slowly discharged by a 2µA
current source until it reaches its lower 0.7V threshold. The
regulator will then attempt to restart normally, operating
in its extended off time mode with a 50% duty cycle, while
the Soft Start capacitor is charged with a 60µA charge cur-
rent.
Trace 4 = 5V from PC Power Supply (2V/div.)
Trace 2 = Inductor Switching Node (2V/div.)
Figure 11: Demonstration board Start up with regulator output shorted
to ground.
If the short circuit condition persists, the regulator output
will not achieve the 1V low VFB comparator threshold
before the Soft Start capacitor is charged to its upper 2.5V
10
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
