PTB78520W Datasheet, PDF (8/15 Pages) Texas Instruments – 20-A, 18–60-V Input Auto-Track Compatible Isolated DC/DC Converter

Application Notes
PTB78520W
Not Recommended for New Designs
Using the Output Enable Control on the PTB78520
Auto-Track Compatible DC/DC Converter
The ‘V_O Enable’ (pin 2) control is an active low input that
allows the output voltage from the converter to be turned
on and off while it is connected to the input source. The
‘V_O Enable’ input is referenced to the –V_IN (pin 3) ¹, on
the primary side of the converter’s isolation, and has its
own internal pull up. The open-circuit voltage is approxi-
mately 20% of the applied input source voltage.
For the converter to function normally pin 2 must be
pulled low to –V_IN potential ². The converter output will
then produce a regulated voltage whenever a valid source
voltage is applied between +V_IN (pin 1) and –V_IN (pin 3) ³.
If the voltage at pin 2 is allowed to rise above V_IH(min),
(see specification table), the output from the converter
will be turned off.
Figure 1-1 is an application schematic that shows the
typical use of the Output Enable function. Note the discrete
transistor (Q₁). Either a discrete MOSFET or bipolar
transistor is recommended to control this input. Table 1-1
gives the threshold requirements.
When placed in the “Off” state the output will neither
source or sink output current. The load voltage will then
decay as the output capacitance is discharged by the load
circuit. With the output turned off, the current drawn
from the input source is typically reduced to 2 mA.
Table 1-1; Output Enable Control Requirements ¹
Parameter
Min
Typ
Max
Enable (V_IH)
2V
Disable (V_IL)
—
V_O/C [Open-Circuit]
—
I_IN [pin 1 at –V_IN]
—
—
—
—
0.8 V
—
13.5 V
—
–0.6 mA
Notes:
1. The Output Enable control uses –V_IN (pin 3) as its ground
reference. All voltages are with respect to –V_IN.
2. Use an open-collector (or open-drain) discrete transistor
to control the Output Enable input. A pull-up resistor is
not necessary. To disable the converter the control pin
should be pulled low to less than +0.8 V. If the Output
Enable feature is not used, pin 2 should be permanently
connected to –V_IN (pin 3).
3. The converter incorporates an “Under-Voltage Lockout”
(UVLO). The UVLO does not allow the converter to
power up until the input voltage is close to its minimum
specified operating voltage. This is regardless of the state
of the Output Enable control. Consult the specifications
for the UVLO thresholds.
Figure 1-1; Output Enable Operation
4
Track
PTB78520W
8
+Sense
+VIN
1
+VIN
10
VO Seq
9
VO Bus
1 = Enable
2
7
VO Enable VO Adjust
L
Q1
R2
O
R1
BSS138
887
A
10 k
5
D
–VIN
VO COM
3
6
–VIN
–Sense
Turn-On Time: In the circuit of Figure 1-1, turning Q₁
off allows the voltage at pin 2 to rise to its internal pull-
up voltage. This disables the converter output. When Q₁
is then turned on, it applies a low-level voltage to pin 2,
and enables the output of the converter. The converter
produces a regulated output voltage within 50 ms. Fig-
ure 1-2 shows the output response of a PTB78520W
after Q₁ is turned on. The turn on of Q₁ corresponds to
the drop in the Q₁ Vds waveform. Although the output
voltage rise-time is short (<10 ms), the indicated delay
time (t_d) will vary depending upon the input voltage and
the module’s internal timing. The output voltage of
the PTB78520W was set to 3.3 V. The waveforms were
measured with 24-Vdc input voltage, and a 10 A resistive
load.
Figure 1-2; Output Enable Power-Up Characteristic
V_o Bus (1 V/Div)
td
HORIZ SCALE: 5 ms/Div
Iin (1 A/Div)
Q₁Vds (5 V/Div)
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