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Application Notes continued
Using the Standby Function on 20-A
Programmable Switching Regulators
For applications requiring On/Off control of the output
voltage, the 20-A programmable regulators incorporate
a standby function. This feature may be used for power-
up/shutdown sequencing, or to change the output voltage
while input power is applied. See related note: “Pin-Coded
Output Voltage Adjustment of 20-A Programmable Regu-
lators”
The standby function is provided by the STBY* control,
pin 5. If pin 5 is left open-circuit the regulator operates
normally, providing a regulated output whenever a valid
supply voltage is applied to Vin (pins 7-11) with respect to
GND (pins 13-19). Connecting pin 5 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 5 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 input requirements.
Table 1 Standby Control Input Requirements (1, 2)
Parameter
Min
Typ
Max
Enable
2V
Open Cct. 1
Disable
–0.2V
0.8V
Istby (low)
0.5mA
Vstby (o/c)
Vin
Notes:
1. The Standby input 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 Vin. 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 Q1, or rise time of the standby input
is not critical with these regulators. Turning Q1 off slowly,
over periods up to 100ms, will not affect regulator operation.
A slow turn-off time will merely increase both the initial
delay and rise-time of the output voltage.
Figure 1
Turn-On Time:
Turning Q1 in Figure 1 off, removes the
low-voltage signal at pin 5 and enables the output.
Following a delay of approximately 10ms, the regulator
output voltage rises to full regulation within 30ms 3.
Figure 2 shows the typical output voltage waveform of a
PT7709 following the turn-off of Q1 at time t =0 secs.
The output voltage is set to 3.3V by connecting VID0
(pin 1), VID2 (pin 3), and VID3 (pin 4) to the ‘Rem
Sns (–)’, (pin 12)*. The waveform was measured with a
+5V input source voltage, and 10-A resistive load.
Figure 2
C
out
+
C
in
+
5V
COM
Inhibit
L
O
A
D
PT770x/PT761x
64
32
1
26
20-25
12
5
13-19
27
7-11
Vo
Vin
GND
Rem Sns (+)
Synch
Out
STBY
VID4 - VID0
Rem Sns (–)
V
o =3.3V
COM
Q1
BSS138
* Consult the data sheet for details on other VID codes.
0
5
10
15
20
25
30
t (milli - secs)
Vo (2V / Div)
Iin 10A / Div)
PT7708/09, PT7615