Applications Information: continued
components, input power supply, and PC board traces
External Output Enable Circuit
than occurs with constant current limit protection (see
On/off control of the regulator can be implemented
through two additional discrete components (see Figure 12).
This circuit operates by pulling the soft start pin high, and
the VFFB pin low, emulating a short circuit condition.
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.
5V
MMUN2111T1 (SOT-23)
5
SS
CS5151
8
VFFB
IN4148
Shutdown
Input
Figure 12: Implementing shutdown with the CS5151.
Trace 4 - 5V Supply Voltage (2V/div.)
Trace 3 - Soft Start Timing Capacitor (1V/div.)
Trace 2 - Inductor Switching Node (2V/div.)
External Power Good Circuit
An optional Power Good signal can be generated through
the use of four additional external components (see Figure
15). The threshold voltage of the Power Good signal can be
adjusted per the following equation:
Figure 10: CS5151 demonstration board hiccup mode short circuit pro-
tection. Gate pulses are delivered while the soft start capacitor charges,
and cease during discharge.
(R1 + R2) × 0.65V
VPower Good
=
R2
This circuit provides an open collector output that drives
the Power Good output to ground for regulator voltages
less than VPower Good
.
5V
R3
Power Good
PN3904
10k
R1
10k
PN3904
V
OUT
CS5151
R2
6.2k
Trace 4 = 5V from PC Power Supply (2V/div.)
Trace 2 = Inductor Switching Node (2V/div.)
Figure 11: Startup with regulator output shorted.
Figure 13: Implementing Power Good with the CS5151.
Overvoltage Protection
Overvoltage protection (OVP) is provided as result of the
2
™
normal operation of the V
control topology and requires
no additional external components. The control loop
responds to an overvoltage condition within 100ns, causing
the MOSFET to shut off, disconnecting the regulator from
its input voltage.
9
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
