PT6937A Datasheet, PDF (11/14 Pages) Texas Instruments – Internal Sequencing Short Circuit Protection 23-pin Space-Saving Package

Application Notes
PT6935 Series
Not Recommended For New Designs
Using the Standby Function on the PT6935
Series of Dual-Output Voltage Regulators
Both output voltages of the 23-pin PT6935 dual-output
converter may be disabled using the regulator’s ‘Standby’
function. This function may be used in applications 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 Vo₁ (pins
12–15) and Vo₂ (pins 18–21) whenever a valid supply volt-
age is applied to V_in (pins 4, 5, & 6) with respect to GND
(pins 7-11). If a low voltage¹ is then applied to pin-3 both
regulator outputs will be simultaneously disabled and the
input current drawn by the ISR will drop to a typical value
of 7mA. The standby control may also be used to hold-off
both regulator outputs during the period that input power is
applied.
The standby pin is ideally controlled using an open-collector
(or open-drain) discrete transistor (See Figure 1). The
open-circuit voltage is typically 12.6V. Table 1 gives the
circuit parameters for this control input.
Table 1 Standby Control Parameters ^{1, 2}
Parameter
Min
Enable (VIH)
Disable (VIL)
VSTBY (open circuit)
I_STBY (I_IL)
—
–0.1V
12.6V 2
—
Max
Open circuit
0.4V 1
15V
–0.5mA
Figure 1
V
in
Inhibit
COM
+
C₁
4, 5, 6
V
IN
22
1
V o₂(sns) V o₁(sns)
18 - 21
V o₂
PT6935
12 - 15
V o₁
STBY GND
Vo₂(adj) Vo₁(adj)
3 7 - 11
23
16
Q1
BSS138
+
C₂
+
C₃
Vo
2
Vo
1
COM
Turn-On Time: Turning Q₁ in Figure 1 off removes the low-
voltage signal at pin 3 and enables the PT6935 regulator.
Following a delay of about 15ms, Vo₁ and Vo₂ rise together
until the lower voltage, Vo₂, reaches its set output. Vo₁
continues to rise until both outputs reach full regulation
voltage. The total power-up time is less than 30ms, and is
relatively independent of load, temperature, and output
capacitance. Figure 2 shows waveforms of the output voltages,
Vo₁ and Vo₂, for a PT6937 (3.3V/1.8V). The turn-off of
Q₁ corresponds to the rise in V_STBY. The waveforms were
measured with a 5V input voltage, and with resistive loads
of 4.5A and 1.9A at the Vo₁ and Vo₂ outputs respectively.
Figure 2
Notes:
1. The standby control input is Not compatible with TTL or
other devices that incorporate a totem-pole output drive. Use
only a true open-collector device, preferably a discrete bipolar
transistor (or MOSFET). To ensure the regulator output is
disabled, the control pin must be pulled to less than 0.4Vdc
with a low-level 0.5mA sink to ground.
2 The standby control input requires no external pull-up resistor.
The open-circuit voltage of the STBY* pin is typically 12.6V.
3. When the regulator output is disabled the current drawn from
the input source is typically reduced to 7mA.
HORIZ SCALE: 5ms/Div
V1 (1V/Div)
V2 (1V/Div)
Vstby
(10V/Div)
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