PTB78520W Datasheet, PDF (7/15 Pages) Texas Instruments – 20-A, 18–60-V Input Auto-Track Compatible Isolated DC/DC Converter

Application Notes
PTB78520W
Not Recommended for New Designs
Output Voltage Adjustment
The ‘V_O Adjust’ control sets the output voltages to a value
higher than 1.8 V. For output voltages other than 1.8 V a
single external resistor, R_SET, must be connected directly
between the ‘V_O Adjust’ (pin 7) and ‘(–)Sense’ (pin 6) pins.
A 0.05-W rated resistor can be used. The tolerance should
be 1%, with a temperature stability of 100 ppm/°C (or
better). Place the resistor close to the converter and
connect it directly between pins 7 & 6 using dedicated
PCB traces (see typical application). Table 1-1 gives the
preferred value of the external resistor for a number of
standard voltages, along with the actual output voltage
that this resistance value provides.
For other output voltages the value of the required adjust
resistor may be calculated using the following formula.
R_SET
= 6.49 kΩ · 1.225 V – 4.42 kΩ
V_SET – 1.805 V
Table 1-1; Preferred Values of R_SET for Standard Output Voltages
V_SET (Standard)
3.6 V
3.3 V
2.5 V
2.0 V
1.8 V
R_SET (Pref’d Value)
0Ω
887 Ω
6.98 kΩ
35.7 kΩ
Open
V_SET (Actual)
3.604V
3.303 V
2.503 V
2.003 V
1.805 V
Input Current Limiting
The converter is not internally fused. For safety and
overall system protection, the maximum input current to
the converter must be limited. Active or passive current
limiting can be used. Passive current limiting can be a
fast acting fuse. A 125-V fuse, rated no more than 10 A,
is recommended. Active current limiting can be imple-
mented with a current limited “Hot-Swap” controller.
Thermal Considerations
Airflow may be necessary to ensure that the module can
supply the desired load current in environments with
elevated ambient temperatures. The required airflow
rate may be determined from the Safe Operating Area
(SOA) thermal derating chart (see converter specifica-
tions).
For technical support and further information visit http://power.ti.com