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Application Notes
28
PT4820 Series
On/Off Enable Turn-On Time
When the On/Off enable inputs, EN1 or EN2 are used
to enable the PT4820's output voltages, the turn-on
delay time (measured from the transition of the enable
signal to the instance the outputs begin to rise) will vary
with the input voltage and the module’s internal timing.
At an input voltage of 48V, the total turn-on time is
between 20 and 60 milliseconds. This turn-on time
reduces as the input voltage is increased. The rise time
of the output voltages is between 10 and 15 milliseconds.
Using the On/Off Enable Controls on the PT4820
Series of Triple Output DC/DC Converters
The PT4820 (48V input) series of triple-output DC/DC
converters incorporate two output enable controls. EN1
(pin 1) is the Positive Enable input, and EN2 (pin 2) is the
Negative Enable input. Both inputs are electrically refer-
enced to -Vin (pin 4) on the primary or input side of the
converter. The Enable pins are ideally controlled with an
open-collector (or open-drain) discrete transistor. A pull-
up resistor is not required. If a pull-up resistor is added,
the pull-up voltage must be limited to 15V.
Automatic (UVLO) Power-Up
Connecting EN2 (pin 2) to -Vin (pin 4) and leaving EN1
(pin 1) 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
present at ±Vin. (See data sheet Specifications). The UVLO
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-
tion, connect EN2 (pin 2) to -Vin (pin 4), and apply the
system On/Off control signal to EN1 (pin 1). In this
configuration, applying less than 0.8V (with respect to
-Vin potential) to pin 1 disables the converter outputs.
Figure 2-1 is an example of this implemention using a
buffer transistor.
DC/DC
Module
EN 1
EN 2
–Vin
–V
IN
1 =Outputs Off
1
2
4
BSS138
DC/DC
Module
EN 1
EN 2
–Vin
–V
IN
1 =Outputs On
1
2
4
BSS138
Figure 2-2; Negative Enable Configuration
Figure 2-1; Positive Enable Configuration
Negative Output Enable (Positive Inhibit)
To configure the converter for a negative enable function,
EN1 (pin 1) is left open circuit, and the system On/Off
control signal is applied to EN2 (pin 2). Applying less
than 0.8V (with respect to -Vin potential) to pin 2, enables
the converter outputs. An example using a buffer transistor
is again detailed in Figure 2-2. Note: The converter will
only produce and output voltage if a valid input voltage is
applied to ±Vin.