Application Information: continued
and provided that PWRGD is low. It is also required that
V
CORE
R1
the overvoltage condition be present for at least the
PWRGD delay time for the OVP signal to be activated. The
resistor values shown in Figure 15 are for VDAC = +2.8V
(DAC = 10111). The VOVP (overvoltage trip-point) can be
set using the following equation:
15K
56K
Q3
2N3906
+5V
R2
5K
R2
R1
OVP
VOVP = VBEQ3 1 +
20K
10K
+5V
(
)
10K
Q2
2N3904
CS5166
10K
Q1
2N3906
PWRGD
Figure 15: Circuit to implement a dedicated OVP output using the
CS5166.
Power-Good Circuit
The Power-Good pin (pin 13) is an open-collector signal
consistent with TTL DC specifications. It is externally
pulled -up, and is pulled low (below 0.3V) when the regu-
lator output voltage typically exceeds ± 8.5% of the nomi-
nal output voltage. Maximum output voltage deviation
before Power-Good is pulled low is ± 12%.
Trace 4 = 5V from PC Power Supply (5V/div.)
Trace1 = Regulator Output Voltage (1V/div.)
Trace 2 = Inductor Switching Node (5V/div.)
Figure 13: OVP response to an input-to-output short circuit by immedi-
ately providing 0% duty cycle, crow-barring the input voltage to
ground.
2.825V
Trace 2 - PWRGD (2V/div)
Trace 4 - V
(1V/div)
OUT
Figure 16: PWRGD signal becomes logic high as VOUT enters -8.5% of
lower PWRGD threshold, VOUT = +2.825V (DAC = 10111).
Trace 4 = 5V from PC Power Supply (2V/div.)
Trace 1 = Regulator Output Voltage (1V/div.)
Figure 14: OVP response to an input-to-output short circuit by pulling
the input voltage to ground.
Trace 1 PWRGD (2V/div)
Trace 4 V (1V/div)
FB
Figure 17: Power-Good response to an out of regulation condition.
11
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
