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Application Notes
continued
PT7761
Using the Standby Function on the PT7761
Excalibur
Integrated Switching Regulator
For applications requiring output voltage On/Off control,
the PT7761 integrated switching regulator (ISR) module
incorporates a standby function. This feature may be used
for power-up/shutdown sequencing, and to change the
output voltage while input power is applied.
See related
application notes:
“Pin-Coded Output Voltage Adjustment
of PT7761 Excalibur ISRs.”
The standby function is provided by the
STBY*
control,
pin 6. If pin 6 is left open-circuit the regulator operates
normally, providing a regulated output whenever a valid
supply voltage is applied to V
in
(pins 9–14) with respect to
GND (pins 16–22). Connecting pin 6 to ground
1
will set
the regulator output to zero volts
2
. This places the regu-
lator in standby mode, and reduces the input current to
typcially 30mA (60mA max). If a ground signal is applied
to pin 6 prior to power-up, the regulator output will be
held at zero volts during the period that input power is
applied.
The standby input must be controlled with an open-
collector (or open-drain) discrete transistor (See Figure 1).
Table 1 gives the threshold requirements.
Table 1 Standby Control Input Requirements
(1)
Parameter
Min
Enable
2V
Disable
–0.2V
I
stby
Vstby (o/c)
Typ
Max
Open Cct.
1
0.8V
0.4mA
V
in
Notes:
1. The Standby input on the PT7761 regulator must be
controlled using an open-collector (or open-drain) discrete
transistor.
Do Not
use a pull-up resistor. The control input
has an open-circuit voltage equal to V
in
. To set the
regulator output to zero, the control pin must be “pulled”
to less than 0.8Vdc with a 0.5mA sink to ground.
2. When placed in the standby mode, the regulator output
discharges the output capacitance with a low impedance to
ground.
3. The turn-off time of Q
1
, or rise time of the standby input
is not critical for the PT7761. Turning Q
1
off slowly, over
periods up to 100ms, will not affect regulator operation.
However, a slow turn-off time will increase both the initial
delay and rise-time of the output voltage.
Figure 1
Turn-On Time:
Turning Q
1
in Figure 1 off, removes the low-
voltage signal at pin 6 and enables the output. Following a
brief delay of approximately 10ms
3
, the output voltage of
the PT7761 series regulators rise to full regulation within
15ms
3
. Figure 2 shows the typical output voltage wave-
form of the PT7761 following the prompt turn-off of Q
1
at
time t =0 secs. The output voltage in Figure 1 is set to 3.3V
by connecting VID0 (pin 1), VID2 (pin 3), and VID3 (pin 4)
to the Rem Sense GND (pin 15)
*
. The waveform in Figure 2
was measured with a +5V input source voltage, and 10A resis-
tive load.
Figure 2
C
out
+
C
in
+
+5V
COM
Inhibit
L
O
A
D
+3.3V
COM
Q1
BSS138
PT7761
4
3
2
1
31
23
–
30
15
6
16
–
22
9
–
14
5
7
Sync
GND
Sns (+)
Sns (
–
)
V
OUT
V
IN
Stby
VID0 - VID4
*Consult the data sheet for details on other VID codes.
Vo (1V/Div)
Iin (5A/Div)
Vstby (5V/Div)
HORIZ SCALE: 5ms/Div