PT7671 Datasheet, PDF (7/9 Pages) Texas Instruments – 30-A Programmable Integrated Switching Regulator

Application Notes
PT7670 Series
Using the Standby Function on the PT7670
Series of 30-A Switching Regulators
For applications requiring On/Off control of the output
voltage, the 30-A rated PT7670 series of Excalibur ISRs
incorporate an on/off “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 notes: “Pin-Coded Output Voltage Adjustment
of the PT7670 Series of 30-A ISRs.”
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
disable the regulator output ². This places the regulator
in standby mode, and reduces the input current to typically
6mA. If a ground signal is applied to pin 5 prior to power-
up, the regulator output will remain inactive 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
3-1). Table 3-1 gives the input requirements.
Table 3-1 Standby Control Input Requirements
Parameter
Min
Typ
Max
Enable
Disable
Istby (low)
Vstby (o/c)
2V
–0.2 V
Open Cct. 1
0.8 V
0.5 mA
Vin
Notes:
1. The Standby input of the PT7670 series of regulators
should 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.
Figure 3-1
2. In the standby mode the output of the regulator is tri-
state, and the output voltage falls at the rate that the
load circuit discharges the output filter capacitors.
3. When the ground signal to the Standby pin is removed,
the regulator output initiates a soft-start cycle by first
asserting a low impedance to ground. If an external
voltage is applied to the output bus, it will sink current and
possibly over-stress the part.
4. The turn-off time of Q₁, or rise time of the standby input
is not critical. Turning Q₁ off slowly will not affect regulator
operation. However, a slow turn-off time will increase
both the initial delay and rise-time of the output voltage.
Turn-On Time: Turning Q₁ in Fig. 3-1 off, removes the
low-voltage signal at pin 5 and enables the output ³. Af-
ter a delay of less than 5 ms, the output voltage rises to
full regulation within 30 ms ⁴. Fig. 3-2 shows the typical
output voltage waveform of a PT7671 following the
turn-off of Q₁ at time t =0 secs. In the circuit of Fig. 3-1,
the output voltage is set to 3.3 V. The waveform was
measured with a +5-V input source voltage, and 15-A
output load.
Figure 3-2
Vo (2V/Div)
Iin (5A/Div)
HORIZ SCALE: 5ms/Div
Vstby (5V/Div)
5V
Inhibit
COM
4321
VID3 - VID0
7–11
Vin
PT7671
STBY
5
GND
13–19
+
C _in
Q₁
BSS138
26
Senes(+)
20–25
Vo
Senes(–)
12
+
C _out
V_o =3.3 V
L
O
A
D
COM
For technical support and more information, see inside back cover or visit www.ti.com