PTB78520W Datasheet, PDF (10/15 Pages) Texas Instruments – 20-A, 18–60-V Input Auto-Track Compatible Isolated DC/DC Converter

Application Notes
PTB78520W
Not Recommended for New Designs
Typical Power-Up Sequencing Configuration
Figure 2-2 shows how the PTB78520W (U₁) can be
configured to provide two 3.3 V sources, that allow it to
both power and sequence with one or more non-isolated
POL modules. The example shows two PTH03050W
modules (U₂ & U₃), each rated for up 6 A of output current.
Additional voltages, as well as modules with a higher
output current capability can also be specified to meet a
specific application. The number of downstream modules,
their respective output voltage and load current rating is
only limited by the amount of current available at the
‘V_O Bus’ output. This is 20 A, less the current allocated
to the load circuit via the ‘V_O Seq’ output.
The output voltage adjust range of the PTB78520W is
1.8 V to 3.6 V, which is compatible with the 3.3-V input
non-isolated POL modules. In these applications, the
PTB78520W output voltage must always be set to 3.3 V
(R₁ =887Ω). Note that this sets the output voltage of
both the ‘V_O Bus’ and ‘V_O Seq’ outputs. The 3.3-V input
non-isolated modules, U₂ and U₃, can be set to any volt-
age over the range, 0.8 V to 2.5 V. In this example they
are set to 2.5 V (R₂ =2.21 kΩ) and 1.8 V (R₃ =5.49 kΩ)
respectively. Figure 2-3 shows the power-up waveforms
from Figure 2-2 when the Track control input to all three
modules are simply connected together.
The PTB78520W provides input power to the down-
stream non-isolated modules via the ‘V_o Bus’ output.
This is the output that rises first to allow the down-
Figure 2-3; PTB78520 Power-Up Waveforms with POL Modules
Vo Bus (1 V/Div)
Vo Seq (1 V/Div)
V_POL 1 (1 V/Div)
V_POL 2 (1 V/Div)
t_d =20 ms
HORIZ SCALE: 20 ms/Div
stream modules to complete their power-up initialization.
‘V_O Seq’ (3.3 V), and the outputs V_POL1 (2.5 V) and V_POL2
(1.8 V), supply the load circuit. These three outputs
are controlled by the track control voltage, which the
PTB78520W holds at ground potential for 20 ms after
the ‘V_O Bus’ output is in regulation. When the track
control voltage is finally allowed to rise, the three outputs
rise simultaneously to their respective set-point voltages.
Figure 2-2; Power-Up Sequencing Circuit With PTB78520W & Non-Isolated POL Modules
U1
4
Track
PTB78520W
8
+Sense
+VI
10
1
VO Seq
+VI
9
VO Bus
2
VO Enable
7
VO Adjust
5
R1
VO COM
887
–VI
3
6
–V I
–Sense
U2
2
Track
3
6
VIN PTH03050W VO
Inhibit
C1
4
100 µF
GND Adjust
15
R2
2.21 k
U3
2
Track
3
6
VIN PTH03050W VO
Inhibit
C2
4
100 µF
GND Adjust
15
R3
5.49 k
Vo Seq (3.3 V)
VPOL1 (2.5 V)
C3
100 µF
VPOL2 (1.8 V)
C4
100 µF
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