CS5151
Applications Information: continued
If the maximum on time is exceeded while responding to a
sudden increase in load current, a normal off time occurs to
prevent saturation of the output inductor.
Trace1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Trace 3 - Output Current (13 to 0.5 Amps) (20V/div.)
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 3 - Regulator Output Current (20V/div.)
Figure 7: CS5151 demonstration board response to a 0.5 to 13A load
pulse (output set for 2.8V).
Figure 9: CS5151 demonstration board response to 13A load turn off
(output set for 2.8V). V
2
™ control topology immediately connects
inductor to ground, providing 0% duty cycle. Regulation is achieved in
less than 10µs.
Protection and Monitoring Features
V
CC1
Monitor
To maintain predictable startup and shutdown characteris-
tics an internal V
CC1
monitor circuit is used to prevent the
part from operating below 3.75V minimum startup. The
V
CC1
monitor comparator provides hysteresis and guaran-
tees a 3.70V minimum shutdown threshold.
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 (V
FFB
< 1V), the V
FFB
low comparator sets the FAULT latch. This causes the MOS-
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 current.
If the short circuit condition persists, the regulator output
will not achieve the 1V low V
FFB
comparator threshold
before the soft start capacitor is charged to its upper 2.5V
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 (2µA/60µA = 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
Trace 1 - Regulator Output Voltage (1V/div.)
Trace 2 - Inductor Switching Node (5V/div.)
Trace 3 - Output Current (0.5 to 13 Amps) (20V/div.)
Figure 8: CS5151 demonstration board response to 13A load turn on
(output set for 2.8V). Upon completing a normal off time, the V
2
™ con-
trol loop immediately connects the inductor to the input voltage, pro-
viding 100% duty cycle. Regulation is achieved in less than 20µs.
8
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
