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Application Notes

PT4840 Series

On/Off Output Voltage Sequencing

series of DC/DC converters are internally sequenced to

meet the power-up requirements of popular micropro-

cessor and DSP chipsets. Figure 3 shows the waveforms

from a PT4841 after power is applied to the input of the

were measured with resistive loads of 2A, 3A, and 3A, at

Vo1, Vo2, and Vo3 respectively. The input source volt-

age was 48-VDC. The converter typically produces a

fully regulated output within 25ms.

Using the On/Off Enable Controls on the PT4840

Series of Triple Output DC/DC Converters

The PT4840 (48V input) series of 65-W, 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 (5V/Div)

Vo2 (1V/Div)

Vo3 (1V/Div)

HORIZ SCALE: 2ms/Div