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A pplication Notes
For technical suppor t and mor e infor mation, see inside back cover or visit www.ti.com/power tr ends
PT6920/PT6930 Series
Using the Standby Function on the PT6920 and
PT6930 Dual Output Voltage Conver ter s
Both output voltages of the 23-pin PT6920/6930 dual
output converter may be disabled using the regulator’s
standby function. This function may be used in applica-
tions that require power-up/shutdown sequencing, or
wherever there is a requirement to control the output
voltage On/Off status with external circuitry.
The standby function is provided by the STBY* control,
pin 3. If pin 3 is left open-circuit the regulator operates
normally, and provides a regulated output at both V
1out
(pins 12–15) and V
2out (pins
18–21) whenever a valid
supply voltage is applied to V
in (pins 4, 5, & 6) with re-
spect to GND (pins 7-11). If a low voltage2 is then
applied to pin-3 both regulator outputs will be simulta-
neously disabled and the input current drawn by the ISR
will typcially drop to less than 30mA (50mA max). The
standby control may also be used to hold-off both regula-
tor outputs during the period that input power is applied.
The standby pin is ideally controlled using an open-col-
lector (or open-drain) discrete transistor (See Figure 1).
It may also be driven directly from a dedicated TTL3
compatible gate. Table 1 provides details of the threshold
requirements.
Table 1 Inhibit Contr ol Thr esholds 2,3
Par ameter
Min
Max
Enable (VIH)
1.8V
Vin
Disable (VIL)
–0.1V
0.8V
Notes:
1. The Standby/Inhibit control logic is similar for all
Power Trends’ modules, but the flexibility and
threshold tolerances will be different. For specific
information on this function for other regulator
models, consult the applicable application note.
2. The Standby control pin is ideally controlled using an
open-collector (or open-drain) discrete transistor and
requires no external pull-up resistor. To disable the
regulator output, the control pin must be pulled to
less than 0.8Vdc with a low-level 0.5mA sink to
ground.
3. The Standby input on the PT6920/6930 series may
be driven by a differential output device, making it
directly compatible with TTL logic. The control
input has an internal pull-up to the input voltage V
in.
A voltage of 1.8V or greater ensures that the
regulator is enabled. Do not use devices that can drive
the Standby control input above 5.5V or V
in.
Figur e 1
Tur n-On Time:
Turning Q
1 in Figure 1 off removes the low-
voltage signal at pin 3 and enables both outputs from the
PT6920/6930 regulator. Following a delay of about 5–10ms,
V
1out and V2out rise together until the lower voltage, V2out,
reaches its set output. V
1out then continues to rise until both
outputs reach full regulation voltage. The total power-up time
is less than 15ms, and is relatively independant of load, tem-
perature, and output capacitance. Figure 2 shows waveforms of
the input current I
in, and output voltages V1out and V2out, for
a PT6921 (3.3V/2.5V). The turn-off of Q
1 corresponds to
t =0 secs. The waveforms were measured with a 5Vdc input
voltage, and with resistive loads of 5.5A and 2.2A at the V
1out
and V
2out outputs respectively.
Figur e 2
C
1
+
C
2
+
Inhibit
V
in
V
2 out
COM
Q1
BSS138
+5V V
in
PT6921
18 - 21
23
7 - 11
4,5,6
Vin
V2
out
GND
Vo
2(adj)
V
1(sns)
1
16
V0
1(adj)
12 - 15
V1
out
V
2(sns)
22
STBY
3
V
1 out
C
3
+
0246
8
10
12
14
t (milli - secs)
V1 (2V/Div)
V2 (2V/Div)
Iin (4A/Div)