PT6935 datasheet(11/12 Pages) TI

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Application Notes

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PT6935 Series

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 Vin (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

Max

Enable (VIH)

—

Open circuit

Disable (VIL)

–0.1V

0.4V 1

VSTBY (open circuit)

12.6V 2

15V

ISTBY (IIL)

—

–0.5mA

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.

2The 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.

Figure 1

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 VSTBY. 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

V1 (1V/Div)

V2 (1V/Div)

Vstby

(10V/Div)

HORIZ SCALE: 5ms/Div

Inhibit

COM

BSS138

PT6935

18 - 21

7 - 11

4, 5, 6

GND

2 (adj)

1 (sns)

1 (adj)

12 - 15

2(sns)

STBY

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