PTB78560A Datasheet, PDF (18/29 Pages) Texas Instruments – 30-W, 24-V/48-V INPUT DC/DC CONVERTERS WITH AUTO-TRACK SEQUENCING

PTB78560A, PTB78560B
PTB78560C
SLTS249 – JUNE 2005
To Internal
Error
Amplifier
Smart Sense
V_O Bus
+ Sense
V_O Seq
R_TRK
24.9 kΩ
www.ti.com
Supply
Supervisor
20 ms
Delay
Track
C_TRK
1 µF
V_O COM
Note: Shaded functions are available only with the PTB78560B (3.3−V output)
Figure 26. 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 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 is connected to the V_OSeq output of the PTB78560B, the V_OSeq output is tightly regulated to
the set-point voltage. In this configuration, the voltage at the V_OBus output is up to 100 mV higher.
3. The V_OSeq output on the PTB78560B 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_OSeq
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 automatically met whenever the Track voltage is controlled by
the internal R-C time constant of the modules being sequenced.
5. If an external voltage is used to control the Track terminal, 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. This transistor holds the track control voltage at ground potential for at least 20 ms after the
V_OBus output is in regulation.
Power-Up Sequencing With A V_O SEQ Output (PTB78560B)
Figure 28 shows the PTB78560B converter (U1) providing two 3.3-V sources. This allows it to both power and
sequence with one or more downstream nonisolated modules. The example shows two 3.3-V input PTH04000W
modules (U2 and U3), each rated for up to 3 A of output current. The selection and current rating of the
nonisolated modules depends on the requirements of a specific application. The number of modules, their
respective output voltage, and load current rating combine with the load required at the V_OSeq output. The total
must be supplied by the PTB78560B, and cannot exceed that available at the V_OBus output.
The output voltage adjust range of the PTB78560B is 1.8 V to 3.6 V. In these applications, the output voltage
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