Application Notes

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PTH05000W Startup with Output Pre-Bias

A pre-bias startup condition occurs as a result of an external

voltage being present at the output of the power module

prior to its output voltage rising. This often occurs in

complex digital systems when current from another power

source is backfed through a dual-supply logic component

such as an FPGA or ASIC. Another path might be via

clamp diodes (to a higher supply voltage) as part of a

sequential power-up arrangement.

An output prebias can cause problems with power modules

that incorporate synchronous rectifiers. This is because

under most operating conditions, they can sink as well as

source ouput current. Although the PTH05000W (5-V

input) power module can sink current under normal op-

shows an application schematic that demonstrates this

capability. Figure 3-2 shows the waveforms of the circuit

after input power is applied. Note that the module’s out-

is sourced. This occurs when the output voltage is raised

above that which is backfed from the 5-V input supply, via

Notes

1. Start up includes both the application of a valid input

source voltage, or the removal of a ground signal from

the Inhibit* control (pin 3) with a valid input source

applied. The output of the regulator is effectively off

(tri-state), during the period that the Inhibit* control is

held low.

Figure 3-2; Start-up with Output Pre-Bias

Figure 3-1; Schematic Demonstrating Startup with Output Pre-Bias

2. To ensure that the regulator does not sink current, the

input voltage must always be greater or equal to the

output voltage throughout the power-up and power-down

sequence.

3. If during power up, the backfeeding source is greater

than the module’s set-point voltage, the module’s

output voltage will remain higher than its set point.

The output will remain out of regulation until the back-

feeding source is either reduced in voltage or removed.

PTH05000W

Vin (1 V/Div)

Vo (1 V/Div)

Io (2 A/Div)

HORIZ SCALE: 5 ms/Div

PTH05000W

V

1

4

5

2

3

C

IN

330 µF

(Required)

C

OUT

100 µF

(Optional)

Inhibit

GND

V

2k32

0.1 W, 1 %

V

GND

Inhibit

V

IN

V

O

L

O

A

D

GND

D

MUR460

0.55

Ω

Resistive