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Application Notes
Using the Inhibit Function on the PT4470 &
PT4480 Series of Isolated DC/DC Converters
The PT4470/44480 series of DC/DC converters incor-
porate an On/Off Inhibit function. This function may be
used in applications that demand battery conservation,
power-up/shutdown sequencing, and/or to coordinate
power-up for active in-rush current control.
The On/Off feature is provided by the Inhibit control,
pin 3. The Inhibit pin of the PT4470/4480 series of
converters is an active low enable. The pin must be
either connected, or actively pulled low, to –Vin (pin 2)
to enable the converter output (see standard applica-
tion schematic). When pins 2 & 3 are connected, the
converter provides a regulated output whenever a valid
source voltage3 is applied between +Vin (pin 1), and –Vin
(pin 2). If pin 3 is disconnected, or allowed to become
high impedance, the regulator output will be disabled. 5
Table 2-1 provides details of the interface requirements
for the Inhibit pin. Figure 2-1 shows how a discrete
MOSFET (Q1) 4, may be referenced to –Vin and used to
control the input.
Table 2-1 Inhibit Control Requirements
Parameter
Min
Max
Enable (VIH)
–0.5V
0.8V
Disable (VIL)
2.5V
(Open Circuit)
Notes:
1. The Inhibit control uses –Vin (pin 2), on the primary side
of the converter, as its ground reference. All voltages
specified are with respect to –Vin.
2. The internal circuitry is simple pull-up resistor. The
open-circuit voltage may be as high as 6.5Vdc.
3. These converters incorporate an “Under-Voltage
Lockout” (UVLO) function. This function automatically
disables the converter output until there is sufficient input
voltage to produce a regulated output. Table 2 gives the
applicable UVLO thresholds.
Table 2-2 UVLO Thresholds 1
Series
UVLO Threshold
Vin Range
PT4470
17.0V Typical
18 – 36V
PT4480
34.5V Typical
36 – 75V
4. The Inhibit input must be controlled with an open-
collector (or open-drain) discrete transistor or
MOSFET.
Do not use a pull-up resistor.
5. When the converter output is disabled, the current
drawn from the input supply is typically reduced to
4mA (10mA maximum).
Figure 2-1
Turn-On Time:
With input power applied, the converter
typically produces a fully regulated output voltage within
25ms after applying a low-voltage signal to the Inhibit
control pin. The actual turn-on time will vary with
the input voltage, output load, and the total amount of
capacitance connected to the output. Using the circuit of
Figure 2-1, Figure 2-2 shows the typical output voltage
and input current waveforms of a PT4472 after Q1 is
turned on. The turn on of Q1 correlates to the fall in
Vinh. The output voltage was set to 2.5V. The waveform
was measured with a 48-Vdc input voltage, and 15-A load
current.
Figure 2-2
PT4470, PT4480 Series
6. Keep the on/off transition to less than 1ms. This
prevents erratic operation of the ISR, whereby the
output voltage may drift un-regulated between 0V
and the rated output during power-up.
Vout (1V/Div)
Iin (2A/Div)
Vinh (5V/Div)
HORIZ SCALE: 5ms/Div
PT4472
+V
sns
+V
out
-V
out
-V
sns
7
8-13
14-19
20
21
25
22
24
23
VID0 - VID4
+V
in
– V
in
1
2
3
Inhibit
C
1
+
PROGRAMMING PINS
+2.5V
+V
IN
–V
IN
0V
1 =Enable
Q
1
BSS138