Application Notes
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PT3400 Series
On/Off Enable Turn-On Time
The total turn-on time of the module is the combination
of a short delay period, followed by the time it takes the
output voltage to rise to full regulation. When the con-
verter is enabled from the
EN1
or
EN2
control inputs, the
turn-on delay time (measured from the transition of the
enable signal to the instance the outputs begin to rise)
is typically 50 milliseconds. By comparison, the rise time
of the output voltage is relatively short, and is between 1
and 2 milliseconds. The rise time varies with input voltage,
output load current, output capacitance, and the
SEQ
pin
function. Figure 2-3 shows the power-up response of a
PT3401 (3.3V), following the removal of the ground
signal at EN1 in Figure 2-1.
Using the On/Off Enable Controls on the
PT3400 Series of DC/DC Converters
The PT3400 series of 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 referenced to -V
in
(pin 3), at the input or primary 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. The logic truth
table for EN1 and EN2 is given in Table 2-1, below.
Table 2-1; On/Off Enable Logic
EN1 (pin 1)
EN2 (pin 2)
Output Status
0
×
Off
1
0
On
×
1
Off
Logic ‘0’
Logic ‘1’
= –Vin (pin 3) potential
= Open Circuit
Automatic (UVLO) Power-Up
Connecting
EN2
to -V
in
and leaving
EN1
open-circuit
configures the converter for automatic power up (see data
sheet ‘Standard Application’). The converter control
circuitry incorporates an ‘under-voltage lockout’ (UVLO),
which disables the converter until a minimum input
voltage is present at ±V
in
(see data sheet specifications).
The UVLO ensures a clean transition during power up
and power down, allowing the converter to tolerate a
slowly 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 function,
connect
EN2
to
-V
in
, and apply the system On/Off control
signal to
EN1
. In this configuration, applying less than
0.8V (with respect to -V
in
) to
EN1
disables the converter
outputs. Figure 2-1 is an example of this implemention.
DC/DC
Module
EN 1
EN 2
–
Vin
–
V
IN
1 =Outputs Off
1
2
3
BSS138
DC/DC
Module
EN 1
EN 2
–
Vin
–
V
IN
1 =Outputs On
1
2
3
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
is left open circuit, and the system On/Off control
signal is applied to
EN2
. Applying less than 0.8V (with
respect to -V
in
) to
EN2
, enables the converter outputs. An
example of this configuration is provided in Figure 2-2.
Note: The converter will only produce an output voltage if a
valid input voltage is applied to ±V
in
.
Vo (2V/Div)
V
EN1
(5V/Div)
HORIZ SCALE: 5ms/DIV
Delay Time
Figure 2-3; PT3401 Enable Turn-On