PT7761-5V Datasheet, PDF (6/8 Pages) Texas Instruments – 40-A Programmable Integrated Switching Regulator Module

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
Typ
Max
Enable
Disable
Istby
Vstby (o/c)
2V
–0.2V
Open Cct. 1
0.8V
0.4mA
Vin
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 Q₁, or rise time of the standby input
is not critical for the PT7761. Turning Q₁ 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
+5V
Inhibit
COM
54321
VID0 - VID4
31
Sns (+)
9–14
V _IN
PT7761
23–30
V_OUT
Stby Sync GND Sns (–)
67
16–22 15
+
C _in
Q1
BSS138
+
C _out
+3.3V
L
O
A
D
COM
Turn-On Time: Turning Q₁ 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 Q₁ 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
Vo (1V/Div)
Iin (5A/Div)
Vstby (5V/Div)
HORIZ SCALE: 5ms/Div
* _{Consult the data sheet for details on other VID codes.}
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