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參數(shù)資料
| 型號(hào)： | PT7721N |
| 英文描述： | SMPS Controller |
| 中文描述： | 開關(guān)電源控制器 |
| 文件頁數(shù)： | 6/7頁 |
| 文件大小： | 142K |
| 代理商： | PT7721N |
For technical support and more information, see inside back cover or visit www.ti.com
Application Notes
continued
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
Disable (V
IL
)
–0.1V
Max
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 Q
1
, or rise time of the standby input
is not critical on the PT7720 series. Turning Q
1
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
Turn-On Time:
Turning Q
1
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
20ms
3
. Figure 3-2 shows the typical output voltage wave-
form of a PT7721, following the prompt turn-off of Q
1
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
C
out
+
C
in
+
+12V
COM
Inhibit
L
O
A
D
PT7721
6
4
3
2
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.
PT7720 Series
0
5
10
15
20
t (milli - secs)
V
o
(2V/Div)
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