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Application Notes
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0
5
10
15
20
25
30
35
t (milliseconds)
Vo
1
(2V/Div)
Vo
2
(2V/Div)
I
IN
(0.5A/Div)
Figure 2-3; Vo
1
, Vo
2
Power-Up Sequence
PT4660 & PT4680 Series
On/Off Output Voltage Sequencing
The output voltages from these dual-output DC/DC
converters are independantly regulated, and are inter-
nally sequenced to meet the power-up requirements of
popular microprocessor and DSP chipsets. Figure 2-3
shows the waveforms from a PT4661 after the converter
is enabled at t=0s. During power-up, the Vo
1
and Vo
2
voltage waveforms typically track within 0.4V prior to
Vo
2
reaching regulation. The waveforms were measured
with a 5-Adc resistive load at each output, and with a
48-VDC input source applied. The converter typically
produces a fully regulated output within 25ms. The
actual turn-on time will vary slightly with input voltage,
but the power-up sequence is independent of the load at
either output.
Using the On/Off Enable Controls on the PT4660
& PT4680 Series of Dual-Output Converters
The PT4660 (48V input) and PT4680 (24V input) se-
ries of dual-output DC/DC converters incorporate
both positive and negative logic output enable controls.
EN1
(pin 3) is the negative enable input, and
EN2
(pin 4)
is the positive enable input. Both inputs are TTL logic
compatible, and are electrically referenced to
-V
in
(pin 2)
on the primary (input) side of the converter. A pull-up
resistor is not required, but may be added if desired.
Adding a pull-up resistor from either
EN1
or
EN2
, up to
+V
in
, will not damage the converter.
Automatic (UVLO) Power-Up
Connecting
EN1
(pin 3) to -V
in
(pin 2) and leaving
EN2
(pin 4) open-circuit configures the converter for automatic
power up. (See data sheet “Typical Application”). The
converter control circuitry incorporates an “Under Voltage
Lockout” (UVLO) function, which disables the output
until the minimum specified input voltage is present
(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 function,
connect
EN1
(pin 3) to
-V
in
(pin 2), and apply the system
On/Off control signal to
EN2
(pin 4). In this configura-
tion, a logic ‘0’ (
-V
in
potential) applied to pin 4 disables
the converter outputs. An example of this configuration
is detailed in Figure 2-1.
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 logic ‘0’ (
-V
in
potential) must then be applied to pin 3 in order to enable
PT4660
EN 1*
EN 2
–Vin
–V
IN
1 =Outputs Off
4
3
2
BSS138
PT4660
EN 1*
EN 2
–Vin
–V
IN
4
3
2
1 =Outputs On
BSS138
Figure 2-2; Negative Enable Configuration
Figure 2-1; Positive Enable Configuration
During turn-off, both outputs drop rapidly due to the
discharging effect of actively switched rectifiers. The
voltage at Vo
1
remains higher than Vo
2
during this
period. The discharge time is typically 100μs, but will
vary with the amount of external load capacitance.
the outputs of the converter. An example of this configu-
ration is detailed in Figure 2-2.
Note: The converter will
only produce and output voltage if a valid input voltage is
applied to ±V
in
.