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Application Notes continued

PT7761

Using the Standby Function on the PT7761

Excalibur™ Integrated Switching Regulator

For applications requiring output voltage On/Off control,

the PT7761 integrated switching regulator (ISR) module

incorporates a standby function. This feature may be used

for power-up/shutdown sequencing, and to change the

output voltage while input power is applied. See related

application notes: “Pin-Coded Output Voltage Adjustment

of PT7761 Excalibur ISRs.”

The standby function is provided by the STBY* control,

pin 6. If pin 6 is left open-circuit the regulator operates

normally, providing a regulated output whenever a valid

supply voltage is applied to Vin (pins 9–14) with respect to

GND (pins 16–22). Connecting pin 6 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 (60mA max). If a ground signal is applied

to pin 6 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-

collector (or open-drain) discrete transistor (See Figure 1).

Table 1 gives the threshold requirements.

Table 1 Standby Control Input Requirements (1)

Parameter

Min

Typ

Max

Enable

Open Cct. 1

Disable

–0.2V

0.8V

Istby

0.4mA

Vstby (o/c)

Vin

Notes:

1. The Standby input on the PT7761 regulator 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 equal to Vin. To set the

regulator output to zero, the control pin must be “pulled”

to less than 0.8Vdc with a 0.5mA sink to ground.

2. When placed in the standby mode, the regulator output

discharges the output capacitance with a low impedance to

ground.

3. The turn-off time of Q1, or rise time of the standby input

is not critical for the PT7761. 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 1

Turn-On Time:

Turning Q1 in Figure 1 off, removes the low-

voltage signal at pin 6 and enables the output. Following a

brief delay of approximately 10ms 3, the output voltage of

the PT7761 series regulators rise to full regulation within

15ms 3. Figure 2 shows the typical output voltage wave-

form of the PT7761 following the prompt turn-off of Q1 at

time t =0 secs. The output voltage in Figure 1 is set to 3.3V

by connecting VID0 (pin 1), VID2 (pin 3), and VID3 (pin 4)

to the Rem Sense GND (pin 15)*. The waveform in Figure 2

was measured with a +5V input source voltage, and 10A resis-

tive load.

Figure 2

out

+5V

COM

Inhibit

+ 3.3V

COM

BSS138

PT7761

3 1

23 – 30

61 6 – 22

9 – 14

Sync

G N D

Sns (+)

Sns ( – )

OUT

Stby

VID0 - VID4

* Consult the data sheet for details on other VID codes.

Vo (1V/Div)

Iin (5A/Div)

Vstby (5V/Div)

HORIZ SCALE: 5ms/Div

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