PT7615 Datasheet, PDF (5/7 Pages) Texas Instruments – 20-A Low-Voltage Programmable Integrated Switching Regulator

Application Notes continued
PT7708/09, PT7615
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 V_in (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
Disable
Istby _(low)
Vstby (o/c)
2V
–0.2V
Open Cct. 1
0.8V
0.5mA
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 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 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
5V
Inhibit
COM
64321
VID4 - VID0
26
Rem Sns (+)
7-11
Vin
PT770x/PT761x
20-25
Vo
Synch
STBY Out
5
27
GND
13-19
Rem Sns (–)
12
+
C _in
Q1
BSS138
+
C_out
V_o =3.3V
L
O
A
D
COM
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
Vo (2V / Div)
Iin 10A / Div)
0
5
10
15
20
25
30
t (milli - secs)
* _{Consult the data sheet for details on other VID codes.}
For technical support and more information, see inside back cover or visit www.ti.com