PT7720_13 Datasheet, PDF (6/8 Pages) Texas Instruments – High Efficiency

Application Notes continued
PT7720 Series
Using the Standby Function on the PT7720
“Big Hammer II” Programmable ISRs
For applications requiring output voltage On/Off control,
the PT7720 “Big Hammer” ISRs incorporate a standby
function. This feature may be used for power-up/shut-
down sequencing, and wherever there is a requirement
for the output status of the module to be controlled by
external circuitry.
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 (50mA 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-col-
lector (or open-drain) discrete transistor (See Figure 3-1).
Table 3-1 gives the threshold requirements.
Table 3-1 Inhibit Control Threshold ¹
Parameter
Min
Max
Disable (V_IL)
–0.1V
0.3V
Notes:
1. The Standby input on the PT7720 regulator series 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 of about 1.5Vdc. To set the
regulator output to zero, the control pin must be “pulled”
to less than 0.3Vdc with a low-level 0.1mA sink to ground.
2. When placed in the standby mode, the regulator output
discharges the output capacitance with a low impedance to
ground. If an external voltage is applied to the output, it
will sink current and possibly over-stress the part.
3. The turn-off time of Q₁, or rise time of the standby input
is not critical on the PT7720 series. 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 3-1
+12V
Inhibit
COM
64321
VID4 - VID0
26
Rem Sns (+)
7-11
Vin
PT7721
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 3-1 off, removes the
low-voltage signal at pin 5 and enables the output. Fol-
lowing a brief delay of 5-15ms, the output voltage of the
PT7720 series regulators rise to full regulation within
20ms³. Figure 3-2 shows the typical output voltage wave-
form of a PT7721, following the prompt turn-off of Q₁ at
time t =0 secs. The output voltage in Figure 3-1 is set to
3.3V by connecting VID0 (pin 1), VID2 (pin 3), and
VID3 (pin 4) to the Sense Gnd (pin 12)^*. The wave-
form in Figure 3-2 was measured with a 12-V input
source voltage, and 15-A resistive load.
Figure 3-2
Vo (2V/Div)
0
5
10
15
20
t (milli - secs)
* _{Consult the data sheet for details on other VID codes.}
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