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PTB78520WAS Datasheet(PDF) 9 Page - Texas Instruments

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Part # PTB78520WAS
Description  20-A, 18??0-V Input Auto-Track Compatible Isolated DC/DC Converter
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Manufacturer  TI1 [Texas Instruments]
Direct Link  http://www.ti.com
Logo TI1 - Texas Instruments

PTB78520WAS Datasheet(HTML) 9 Page - Texas Instruments

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Application Notes
For technical support and further information visit http://power.ti.com
PTB78520W
Configuring the PTB78520 DC/DC Converter to
Power-Up Sequence with POL Modules
Overview
The PTB78520 DC/DC converter has two outputs,
‘VO Seq’ and ‘VO Bus’. ‘VO Bus’ is the main output
from the converter. ‘VO Seq’ is an output that is derived
from ‘VO Bus’ and can be sequenced with other supply
voltages during power-up. Both outputs are regulated
to the same set-point voltage, except that the rise in the
‘VO Seq’ output is controlled by a pin called ‘Track’, and
delayed during power-up events. This delay allows the
PTB78520W to both power and sequence with one or
more non-isolated, 3.3-V input, Auto-Track compatible
modules 1. In these applications, the PTB78520W incor-
porates the necessary timing to coordinate the rise of
all sequenced outputs using a common track control
signal. The hold-off delay time also complies with the
power-up requirements of the downstream non-isolated
modules, without the need for additional components.
PTB78520W Auto-Track Features
Figure 2-1 shows a block diagram of the PTB78520W
Auto-Track features. During power up, ‘VO Bus’ (pin 9)
rises promptly, whenever the converter is connected to
a valid input source and its output is enabled. ‘VO Seq’
(pin 10) is the Auto-Track compatible output that is derived
from ‘VO Bus’ but directly controlled by the voltage pre-
sented at the Track input (pin 4). The control relationship
is on a volt-for-volt basis, and is active from 0 V up to a
voltage just below the ‘VO Bus’ output. Between these two
limits, the voltage at ‘VO Seq’ will follow that at the ‘Track’
input. However, once the Track input is at the ‘VO Bus’
voltage, raising it higher has no further effect. The volt-
age at ‘VO Seq’ cannot go higher than ‘VO Bus’, and if it
is connected to ‘+Sense’ (pin 8), it will then regulate at
the set-point voltage. 2
The control relationship between ‘VO Seq’ and the Track
input is the same as other Auto-Track compatible outputs,
across all module types. By connecting the Track input of
the PTB78520W to the Track input of other Auto-Track
compatible modules, the output voltages can be made to
follow a common signal during power-up transitions. 3
Each Track input produces a suitable track control signal
from an internal R-C time constant. An input signal can
also be provided from an externally generated ramp wave-
form. 4, 5
The Track input of the PTB78520W has a pull-up resis-
tor to ‘VO Bus’, and a capacitor to ‘VO COM’. This enables
its Track input to rise automatically; once it is allowed to
do so. In sequencing applications, the non-isolated modules
are powered by the ‘VO Bus’ output. A MOSFET, internal
to the PTB78520W, holds the Track voltage (and the
‘VO Seq’ output) at ground for 20 ms after the ‘VO Bus’
output is in regulation. This gives the non-isolated mod-
ules time to initialize so that their outputs can rise with
the ‘VO Seq’ output.
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. When 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
track control voltage during power-up.
2. When ‘+Sense’ (pin 8) is connected to the ‘VO Seq’ output
(pin 10), the ‘Vo Seq’ output will be tightly regulated to
the PTB78520W’s set-point voltage. In this configuration,
the voltage at the ‘VO Bus’ output (pin 9) will be up to
100 mV higher.
3 The ‘VO Seq’ output cannot sink load current. This
constraint does not allow the PTB78520W 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 ‘VO 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 allowed to rise, using the
internal R-C time constants at the Track input of all
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-k
Ω is recommended for this purpose. This
is necessary to protect the internal transistor to the
PTB78520W converter’s Track control input. This
transistor holds the track control voltage at ground
potential for 20 ms after the ‘VO Bus’ output is in
regulation.
Figure 2-1; Block Diagram of PTB78520 Auto-Track Features
Smart Sense
RTRK
24.9 k
CTRK
1 µF
Supply
Supervisor
20 ms
Delay
Vo Bus
Vo Seq
(+)Sense
Vo COM
To
feedback
error amp’
Track
Not Recommended for New Designs


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