PTB48520W Datasheet, PDF (11/18 Pages) Texas Instruments – 25-A, 48-V INPUT ISOLATED DC/DC CONVERTER WITH AUTO-TRACK™ SEQUENCING

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PTB48520W
SLTS233A – NOVEMBER 2004 – REVISED APRIL 2005
The Track input of the dc/dc converter includes a pullup resistor (R_TRK) to V_O Bus, and a 1-µF capacitor (C_TRK) to
V_O COM. This enables the Track input to rise automatically once the internal MOSFET turns off. In sequencing
applications, the non-isolated modules are powered by the V_O Bus output. The internal MOSFET holds the Track
voltage at ground for 20 ms after the V_O Bus output is in regulation. This gives the non-isolated modules time to
initialize so that their outputs can rise with the V_O Seq output.
V_OBus
To Feedback
Error Amplifier
Smart Sense
+Sense
V_OSeq
−
R_(TRK)
+
24.9 kW
Supply
Supervisor
20-ms
Delay
C_(TRK)
1 mF
Track
V_OCOM
Figure 8. Block Diagram of Auto-Track Features
Notes:
1. Auto-Track compatible modules incorporate a Track input that can take direct control of the output voltage
during power-up transistions. The control relationship is on a volt-for-volt basis and is active between the 0 V
and the module's set-point voltage. Once the Track input is above the set-point voltage, the module remains
at its set point. Connecting the Track input of a number of such modules together allows their outputs to
follow a common control voltage during power up.
2. When +Sense (pin 8) is connected to the V_o Seq output (pin 10), the V_o Seq output is tightly regulated to the
set-point voltage. In this configuration, the voltage at the V_O Bus output (pin 9) is up to 100 mV higher.
3. The V_O Seq output cannot sink load current. This constraint does not allow the module to coordinate a
sequenced power down.
4. The slew rate for the Track input signal must be between 0.1 V/ms and 1 V/ms. Above this range, the V_o Seq
output may no longer accurately follow the Track input voltage. A slew rate below this range may thermally
stress the converter. These slew rate limits are met whenever the Track input voltage is controlled by the
internal R-C time constant of the modules being sequenced.
5. Whenever an external voltage is used to control the Track input, the source current must be limited. A
resistance value of 2.74-kΩ is recommended for this purpose. This is necessary to protect the internal
transistor to the converter's Track control input. This transistor holds the track control voltage at ground
potential for 20 ms after the V_O Bus output is in regulation.
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