Application Notes

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PT6940 Series

Operating Features of the PT6940 Series

of Dual-Output Voltage Regulators

Over-Current Protection

The PT6940 series of regulators incorporate independent

periodic shutdown of both outputs. Applying a load current,

in excess of the current limit threshold to either output,

results in the shutdown of both voltages after a short

period; typically 15ms. Following shutdown the module

periodically attempts to recover by executing a soft start

power-up at intervals of approximately 100ms. If the over-

current fault persists, each attempted restart will result in a

corresponding over-current trip and shutdown. During

the 15ms period prior to each successive shutdown, the

output with the load fault may not reach full regulation.

Figure 2

Power-Up Voltage Sequencing

The output voltages from the PT6940 series regulators are

independently regulated, and internally sequenced to meet

the power-up requirements of popular microprocessors

and DSP chipsets. Figure 1 shows the output voltage wave-

forms of a PT6942 (3.3V/1.8V) after either input power is

applied, or the regulator is enabled. In this example turning

and enables the regulator. Following a delay of about 3–5ms,

until both outputs reach full regulation. The total power-

up time is less than 15ms, and is relatively independent of

load, temperature, and output capacitance. The turn-off of

measured with a 5V input voltage, and with resistive loads

Figure 1

C

IN

+

Co

2

+

Inhibit

V

in

Vo

2

COM

COM

Q1

BSS138

Vo

1

Co

1

+

PT6940

13–16

4–6

23–25

26

3

7–12

17–22

Vin

V

V

Vo

1

Vo

2

V

V

27

2

STBY*

1

V1 (1V/Div)

V2 (1V/Div)

Vstby (5V/Div)

HORIZ SCALE (2ms/Div)

Notes:

1.The standby control input is

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 Do not use an an external pull-up resistor. The control pin

has its own internal pull-up. Adding an external pull-up

could disable the over-current protection. The open-circuit

Standby Control

The output voltages from the PT6940 may be disabled

using the regulator’s Standby control. The standby function

is provided by the “STBY*” control (pin 1). If pin 1 is left

open-circuit the regulator operates normally, and provides

(pins 13–16) with respect to GND (pins 7-12 & 17–22).

taneously disables both regulated outputs. This places the

regulator in standby mode, and reduces the input current

drawn by the ISR to typically 10mA. The Standby control

may also be used to maintain both regulator outputs at zero

volts 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 2). The