PTB78520W Datasheet, PDF (9/15 Pages) Texas Instruments – 20-A, 18–60-V Input Auto-Track Compatible Isolated DC/DC Converter

Application Notes
PTB78520W
Not Recommended for New Designs
Configuring the PTB78520 DC/DC Converter to
Power-Up Sequence with POL Modules
Overview
The PTB78520 DC/DC converter has two outputs,
‘V_O Seq’ and ‘V_O Bus’. ‘V_O Bus’ is the main output
from the converter. ‘V_O Seq’ is an output that is derived
from ‘V_O 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
‘V_O 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, ‘V_O Bus’ (pin 9)
rises promptly, whenever the converter is connected to
a valid input source and its output is enabled. ‘V_O Seq’
(pin 10) is the Auto-Track compatible output that is derived
from ‘V_O 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 ‘V_O Bus’ output. Between these two
limits, the voltage at ‘V_O Seq’ will follow that at the ‘Track’
input. However, once the Track input is at the ‘V_O Bus’
voltage, raising it higher has no further effect. The volt-
age at ‘V_O Seq’ cannot go higher than ‘V_O Bus’, and if it
is connected to ‘+Sense’ (pin 8), it will then regulate at
the set-point voltage. ²
The control relationship between ‘V_O 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 ‘V_O Bus’, and a capacitor to ‘V_O 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 ‘V_O Bus’ output. A MOSFET, internal
to the PTB78520W, holds the Track voltage (and the
‘V_O Seq’ output) at ground for 20 ms after the ‘V_O Bus’
output is in regulation. This gives the non-isolated mod-
ules time to initialize so that their outputs can rise with
the ‘V_O Seq’ output.
Figure 2-1; Block Diagram of PTB78520 Auto-Track Features
To
feedback
error amp’
Smart Sense
V_o Bus
(+)Sense
V_o Seq
Supply
Supervisor
20 ms
Delay
RTRK
24.9 k
CTRK
1 µF
Track
V_o COM
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 ‘V_O 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 ‘V_O Bus’ output (pin 9) will be up to
100 mV higher.
3 The ‘V_O 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 ‘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 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-kΩ 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 ‘V_O Bus’ output is in
regulation.
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