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Application Notes

PT4850 Series

On/Off Output Voltage Sequencing

The power-up characteristic of the PT4850 series of

DC/DC converters meets the requirements of micro-

processor and DSP chipsets. All three outputs from the

converter are internally sequenced to power up in

unison. Figure 3 shows the waveforms from a PT4851

after power is applied to the input of the converter.

During power-up, all three output voltages rise together

until each reaches their respective output voltage. The

waveforms of Figure 3 were measured with loads of ap-

proximately 50% on each output, with an input source of

48VDC. The converter typically produces a fully regu-

lated output within 150ms.

Using the On/Off Enable Controls on the PT4850

Series of Triple Output DC/DC Converters

The PT4850 (48V input) series of 25-A, triple-output

DC/DC converters incorporate two output enable controls.

EN1 (pin 3) is the Negative Enable input, and EN2 (pin 4)

is the Positive Enable input. Both inputs are electrically

the converter. A pull-up resistor is not required, but may

be added if desired. Voltages of up to 70V can be safely

applied to the either of the Enable pins.

Automatic (UVLO) Power-Up

(pin 4) open-circuit configures the converter for auto-

matic power up. (See data sheet “Typical Application”).

The converter control circuitry incorporates an “Under

Voltage Lockout” (UVLO) function, which disables the

converter until the minimum specified input voltage is

circuitry ensures a clean transition during power-up and

power-down, allowing the converter to tolerate a slow-

rising input voltage. For most applications EN1 and

EN2, can be configured for automatic power-up.

Positive Output Enable (Negative Inhibit)

To configure the converter for a positive enable func-

system On/Off control signal to EN2 (pin 4). In this

applied to pin 4 disables the converter outputs. Figure 1

is an example of this configuration.

Negative Output Enable (Positive Inhibit)

To configure the converter for a negative enable function,

EN2 (pin 4) is left open circuit, and the system On/Off

control signal is applied to EN1 (pin 3). A low-level

DC/DC

Module

EN 1*

EN 2

–Vin

–V

IN

1 =Outputs Off

4

3

2

BSS138

DC/DC

Module

EN 1*

EN 2

–Vin

–V

IN

1 =Outputs On

4

3

2

BSS138

Figure 2; Negative Enable Configuration

Figure 1; Positive Enable Configuration

During turn-off, all outputs drop rapidly due to the

discharging effect of actively switched rectifiers. The

period. The discharge time is typically 100µs, but will

vary with the amount of external load capacitance.

pin 3 in order to enable the outputs of the converter.

An example of this configuration is detailed in Figure 2.

Note: The converter will only produce and output voltage if a

Vo1 (1V/Div)

V02 (1V/Div)

Vo3 (1V/Div)

HORIZ SCALE: 20ms/Div