Auto-Track™
Sequencing
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PTB78520W
20-A, 18–60-V Input Auto-Track
Compatible Isolated DC/DC Converter
Description
The PTB78520W is a 20-A rated,
wide-input (18-60 V) isolated DC/DC
converter that incorporates Auto-Track™
power-up sequencing. This allows these
modules to simultaneously power up with
any other downstream non-isolated, AutoTrack compliant module.
The PTB78520W module provides
two outputs, each regulated to the same
voltage. During power up, the voltage at
Bus’ rises first, allowing this output
‘V
O
to provide input power to any downstream
non-isolated module. The voltage from
Seq’ is then allowed to rise simulta-
‘V
O
neously, under the control of Auto-Track,
along with the outputs from the downstream modules.
Whether used to facilitate power-up
sequencing, or operated as a stand-alone
module, the PTB78520W includes many
features expected of high-performance
DC/DC converter modules. The combi-
SLTS226A – JULY 2004 – REVISED OCTOBER 2005
Features
• Wide-Input Voltage Range:
18 V to 60 V
• 20 A Total Output Current
• 90% Efficiency
• Wide-Adjust Output Voltage:
1.8 V to 3.6 V
• Over-Current Protection
• Output Over-Voltage Protection
• Over-Temperature Shutdown
• Output Enable Control
nation of input-output isolation and a
wide-input voltage range, allows operation
from either +24 V or –48 V. The wideoutput adjust enables the output voltage
to be set to to any voltage over the range,
1.8 V to 3.6 V, using a single external
resistor. Precise output voltage regulation is assured using Smart-Sense. This
is a differential remote sense that will
intelligently regulate the sequenced output, depending on its sequence status.
Other operational features include an
input under-voltage lockout (UVLO) and
an output enable control. Over-current,
over-voltage, and over-temperature protection assures the module’s ability to
survive any load fault.
Typical applications include distributed
power architectures in both telecom and
computing environments, particularly
complex digital systems requiring powersequencing of multiple power supply rails.
• Auto-Track Compatible
Sequenced Output
• Smart-Sense Remote Sensing
• Under-voltage Lockout
• Industry Standard Footprint
• Surface Mountable
• 1500 VDC Isolation
• Agency Approvals (Pending):
UL/cUL 60950, EN 60950
Pin-Out Information
Pin Function
1+V
IN
2VO Enable *
3–V
IN
4 Track
5VO Com
6 (–) Sense
7 Vo Adjust
8 (+) Sense
9VO Bus
10 VO Seq
Shaded functions indicate signals
electrically common with the input.
* Denotes negative logic:
Low (–V
) = Normal operation
IN
Open = Output off
Typical Application
+V
I
–V
I
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1
+V
I
2
V
Enable
O
3
–V
I
PTB78520W
+Sense
VO Adjust
V
–Sense
Track
V
Seq
O
VO Bus
COM
O
Simultaneo us Powerup
V
C2
100 µF
1
V
2
V1 =3.3 V
V2 =1.8 V
4
8
10
9
V
PTH03050W
I
4
2
Track
GNDInhibit Adjust
15
V
O
R1
5.49 k
63
7
5
R
SET
887
6
C1
100 µF
PTB78520W
20-A, 18–60-V Input Auto-Track
Compatible Isolated DC/DC Converter
Ordering Information
Output Voltage
Code Voltage
W 1.8 V to 3.6 V
Notes: (1) Reference the applicable package reference drawing for the dimensions and PC board layout
(2) “Standard” option specifies 63/37, Sn/Pb pin solder material.
(PTB78520❒xx)
Package Options
Code Description Pkg Ref.
AH Horiz. T/H (ERP)
AS SMD, Standard
(PTB78520x❒❒)
(2)
Pin Descriptions
+VIN: The positive input for the module with respect to
–VIN. When powering the module from a negative input
voltage, this input is connected to the input source ground.
–V
: The negative input supply for the module, and the
IN
0-V reference for the ‘VO Enable’ input. When powering
the module from a positive source, this input is connected
to the input source return.
Vo Enable*: An open-collector (open-drain) negative logic
input that is referenced to –V
to –Vin potential to enable the output voltage. A high-
. This input must be pulled
IN
impedance connection will disable the module output. If
the output enable feature is not used, pin 2 should be
permanently connected to –V
produce an output whenever a valid input source is applied.
. The module will then
IN
Vo Bus: Produces a positive power output with respect
COM’. This is the main output from the converter
to ‘V
O
when operated in a stand-alone configuration. It is dcisolated from the input power pins and is the first output
to rise when the converter is either powered or enabled.
In power-up sequencing applications, this output can
provide a 3.3-V standby source to power the downstream
non-isolated modules.
Vo Seq: This is the sequenced output voltage from the
converter. This voltage can be directly controlled from
the Track pin. During power up, V
the Track pin voltage, typically 20 ms after the VO Bus
Seq will rise with
O
output has reached regulation.
Vo COM: This is the output power return for both the
‘V
Bus’ and ‘VO Seq’ output voltages. This node should
O
be connected to the load circuit common.
Track: The voltage at this pin directly controls the voltage
at the ‘V
to sequence the voltage at ‘V
outputs from any downstream non-isolated modules that
are powered from the converter’s ‘+VO Bus’ output. In
these applications, the ‘Track’ pin is simply connected
to the track control of each of the non-isolated modules. The ‘Track’ pin of the PTB78520W has an internal
transistor, which holds it at ‘V
approximately 20 ms after the ‘VO Bus’ output is in
regulation. Following this delay, the ‘Track’ voltage and
‘V
Seq’ will rise simultaneously with the output voltage
O
from all the non-isolated modules, that are under the control of Auto-Track.
Vo Adjust: A resistor must be connected between this pin
and ‘–Sense’ to set the converter’s output voltage. A
0.05-W rated resistor may be used, with tolerance and
temperature stability of 1% and 100 ppm/°C, respectively.
If this pin is left open, the converter output voltage will
default to its lowest value. The specification table gives
the preferred resistor values for the popular bus voltages.
+Sense: The ‘+Sense’ pin can be connected to either the
‘V
Bus’ or ‘VO Seq’ outputs. When connected to ‘VO Seq’,
O
remote sense compensation will be delayed until the
power-up sequence is complete. The voltage at ‘V
will also be raised slightly. The pin may be left open
circuit, but connecting it to one of the output terminals
improves load regulation of that output.
–Sense: Provides the converter with a remote sense capability when used in conjunction with +Sense. For optimum
output voltage accuracy this pin should
to ‘VO COM’. This pin is also the reference connection for the output voltage set-point resistor.
SLTS226A – JULY 2004 – REVISED OCTOBER 2005
(1)
(ERQ)
Seq’ regulated output. It is primarily used
O
Seq’ with the regulated
O
COM’ potential for
O
always be connected
Bus’
o
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PTB78520W
20-A, 18–60-V Input Auto-Track
Compatible Isolated DC/DC Converter
SLTS226A – JULY 2004 – REVISED OCTOBER 2005
Environmental & Absolute Maximum Ratings
Characteristics Symbols Conditions Min Typ Max Units
Input Voltage V
Track Input Voltage V
Track Input Current I
Operating Temperature Range T
IN
TRACK
(max) From external source — — 10
TRACK
A
Over-Temperature Protection OTP PCB temperature (near pin 1) — 115 — °C
Solder Reflow Temperature T
Storage Temperatur e T
REFLOW
S
Mechanical Shock Per Mil-STD-883D, Method 2002.3 T/H — 250 — Gs
Mechanical Vibration Mil-STD-883D, Method 2007.2 T/H — 15 — Gs
Weight — — 28.5 — grams
Flammability — Meets UL 94V-O
Notes: (i) When the Track input is fed from an external voltage source, the input current
(ii) During solder reflow of SMD package version, do not elevate the module PCB, pins, or internal component temperatures above a peak of 235 °C.
Specifications (Unless otherwise stated, T
Characteristic Symbol Conditions Min Typ Max Units
Output Current I
Input Voltage Range V
Set Point Voltage Tolerance VO tol — ±0.6
Temperature Variation Reg
Line Regulation Reg
Load Regulation Reg
Total Output Voltage Variation ∆V
Output Voltage Adjust Range ∆V
Efficiency η I
Vo Ripple (pk-pk) V
Transient Response t
Track Input (pin 4)
Input Current I
Open Circuit Voltage V
Track Slew Rate Capability dV
Output Enable Input (pin 2) Referenced to –VIN (pin 3)
Input High Voltage V
Input Low Voltage V
Input Low Current I
Standby Input Current IIN standby pin 2 open — 2 — mA
No-Load Input Current IIN no-load pins 2 & 3 connected, Io
Over-Current Threshold I
Output Over-Voltage Protection OVP Output shutdown and latch off — 125 — %V
Under-Voltage Lockout UVLO 15.5 17 18 V
Switching Frequency ƒ
Internal Input Capacitance C
External Output Capacitance C
Isolation Voltage Input-output & input-case 1,500 — — Vdc
Capacitance Input-output — 1,000 — pF
Resistance Input-output 10 — — MΩ
Reliability MTBF Telcordia TR-332 1.2 — — 106 Hrs
Notes: (1) See SOA curves or consult factory for appropriate derating.
(2) When load current is supplied from the V
(3) The set-point voltage tolerance is affected by the tolerance and stability of R
with 100 ppm/°C temperature stability.
(4) When controlling the Track input from an external source the slew rate of the applied signal
the voltage to completely rise to the voltage at the Vo Bus output, at no less than the minimum specified rate, may thermally overstress the converter.
(5) The ‘VO Enable’ input has an internal pull-up, and if left open the converter output will be turned off. A discrete MOSFET or bipolar transistor is
recommended to control this input. The open-circuit voltage is approximately 20% of the input voltage. If the output enable feature is not used, this pin
should be permanently connected to –V
bus Over VIN range 0 — 20
O
IO seq 0 — 10
IO tot Sum total IO bus + IO seq 0 — 20 A
IN
temp
line
load
tot Includes set-point, line, load, — ±1.5 ±3
O
ADJ
R
TR
∆V
TR
TRACK
TRACK
TRACK
IH
IL
IL
TRIP
S
IN
OUT
Surge (100 ms maximum) — — 75 V
0 — VO Bus + 0.3 V
Over VIN Range –40 — +85 °C
Surface temperature of module or pins — — 235
— –40 — +125 °C
1 msec, ½ Sine, mounted SMD — 150 —
20-2000 Hz, PCB mounted SMD — 5 —
must be limited. A 2.74-kΩ value series resistor is recommended.
=25 °C, VIN =24 V, VO =3.3 V, CO =0 µF, and IO =IOmax)
A
PTB78520W
Over IO Range 18 48 60 V
(3)
–40° ≤TA ≤ +85°C — ±0.8 — %V
Over VIN range — ±1 — mV
Over IO range — ±1 — mV
–40° ≤TA ≤ +85°C
Over Vin range 1.8 — 3.6 V
=10 A R
O
=887 Ω,VO =3.3 V — 90 —
SET
=6.98 kΩ,VO =2.5 V — 88.5 — %
R
SET
=35.7 kΩ,VO =2.0 V — 87 —
R
SET
R
=open cct. VO =1.8 V — 86.5 —
SET
20 MHz bandwidth — 20 — mV
1 A/µs load step, 50% to 100% IOmax — 75 — µs
VO over/undershoot — ±3 — %V
pin connected to VO COM — — –0.13 mA
0—V
/dt 0.1
(4)
— 1 V/ms
2 — open
–0.2 — +0.8
— –240 — µA
=0 — 85 — mA
TOT
Shutdown, followed by auto-recovery — 30 — A
Over VIN range 225 275 325 kHz
—3 —µF
Between +Vo and –V
o
0 5,000 µF
50% stress, TA =40°C, ground benign
output, the module will exhibit higher power dissipation and slightly lower operating efficiency.
O SEQ
. The stated limit is unconditionally met if R
SET
must be greater than the minimum limit. Failure to allow
. See application notes for other interface considerations.
IN
(i)
(ii)
(1)
(1) (2)
—%V
(3)
Bus V
O
(5)
has a tolerance of 1%,
SET
mA
°C
A
%V
V
O
O
O
pp
O
O
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PTB78520W
Typical Characteristics
20-A, 18–60-V Input Auto-Track
Compatible Isolated DC/DC Converter
Characteristic Data; VIN =24 V (See Note A)
Efficiency vs. Load Current (IO Bus)
100
90
80
70
Efficiency - (%)
60
50
0 4 8 12 16 20
Output Ripple vs. Load Current (IO Bus)
50
40
30
20
Ripple - (mV)
10
Io Bus - (A)
SLTS226A – JULY 2004 – REVISED OCTOBER 2005
Characteristic Data; VIN =48 V (See Note A)
Efficiency vs. Load Current (IO Bus)
100
V
OUT
3.3V
2.5V
2.0V
1.8V
V
OUT
3.3V
2.5V
2.0V
1.8V
90
80
70
Efficiency - (%)
60
50
0 4 8 121620
Output Ripple vs. Load Current (I
50
40
30
20
Ripple - (mV)
10
Io Bus - (A)
Bus)
O
V
OUT
3.3V
2.5V
2.0V
1.8V
V
OUT
3.3V
2.5V
2.0V
1.8V
0
048121620
Power Dissipation vs. Load Current (IO Bus)
12
10
8
6
4
Power Dissipation - (W)
2
0
048121620
Io Bus - (A)
Io Bus - (A)
0
048121620
Power Dissipation vs. Load Current (IO Bus)
12
10
V
OUT
3.3V
2.5V
2.0V
1.8V
8
6
4
Power Dissipation - (W)
2
0
0 5 1 0 15 20
Io Bus - (A)
Io Bus - (A)
V
OUT
3.3V
2.5V
2.0V
1.8V
Note A: All data listed in the above graphs has been developed from actual products tested at 25 °C. This data is considered typical data for the DC-DC Converter.
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PTB78520W
Typical Characteristics
20-A, 18–60-V Input Auto-Track
Compatible Isolated DC/DC Converter
SLTS226A – JULY 2004 – REVISED OCTOBER 2005
Safe Operating Areas; VIN =24 V (See Note B) Safe Operating Areas; VIN =48 V (See Note B)
Load Current from ‘+VO Bus’ Output
90
80
70
60
50
40
Ambient Temperature (°C)
30
20
0 4 8 121620
Load Current from ‘+VO Seq’ Output
90
80
70
60
50
40
Ambient Temperature (°C)
30
20
0246810
Output Current (A)
Output Current (A)
Airflow
400LFM
200LFM
100LFM
Nat conv
Airflow
100LFM
Nat conv
Load Current from ‘VO Bus’ Output
90
80
70
60
50
40
Ambient Temperature (°C)
30
20
0 4 8 121620
Load Current from ‘VO Seq’ Output
90
80
70
60
50
40
Ambient Temperature (°C)
30
20
0246810
Output Current (A)
Output Current (A)
Airflow
400LFM
200LFM
100LFM
Nat conv
Airflow
200LFM
100LFM
Nat conv
Note B: SOA curves represent operating conditions at which internal components are at or below manufacturer’s maximum rated operating temperature.
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Application Notes
PTB78520W
Operating Features and System Considerations
for the PTB78520W DC/DC Converter
Over-Current Protection
To protect against load faults these converters incorporate
output over-current protection. Applying a load to the
output that exceeds the converter’s over-current threshold
(see applicable specification) will cause the output voltage
to momentarily fold back, and then shut down. Following
shutdown the module will periodically attempt to automatically recover by initiating a soft-start power-up.
This is often described as a “hiccup” mode of operation,
whereby the module continues in the cycle of successive
shutdown and power up until the load fault is removed.
Once the fault is removed, the converter automatically
recovers and returns to normal operation.
Output Over-Voltage Protection
The converter continually monitors for an output overvoltage (OV) condition, directly across the ‘+VO Bus’
output. The OV threshold automatically tracks the output voltage setpoint to a level that is 25% higher than
that set by the external R
voltage adjust resistor. If
SET
the output voltage exceeds this threshold, the converter
is immediately shut down and remains in a latched-off
state. To resume normal operation the converter must be
actively reset. This can only be done by momentarily
removing the input power to the converter. For failsafe
operation and redundancy, the OV protection uses circuitry that is independent of the converter’s internal
feedback loop.
Differential Output Voltage Sense
A differential remote sense allows a converter’s regulation circuitry to compensate for limited amounts of IR
drop, that may be incurred between the converter and
load, in either the positive or return PCB traces. Connecting the (+)Sense and (–)Sense pins to the respective
positive and ground reference of the load terminals will
improve the load regulation of the converter’s output
voltage at that connection point. The (–)Sense pin should
always be connected to the ‘VO COM’. The (+)Sense pin
may be connected to either the ‘+VO Bus’ or ‘+VO Seq’
outputs.
When the (+)Sense pin is connected to the ‘VO Seq’
output, the voltage at ‘Vo Bus’ voltage will regulate
slightly higher. Depending on the load conditions on the
‘VO Seq’ output, the voltage at ‘VO Bus’ may be up to
100 mV higher than the converter’s set-point voltage.
In addition, the Smart-Sense feature (incorporated into
the PTB78520 converter) will only engage sense compensation to the ‘VO Seq’ output when that output voltage is
close to the set-point. During other conditions, such as
power-up and power-down sequencing events, the sense
circuit automatically defaults to sensing the ‘VO Bus’
voltage, internal to the converter.
Leaving the (+)Sense and (–)Sense pins open will not
damage the converter or load circuitry. The converter
includes default circuitry that keeps the output voltage in
regulation. If the remote sense feature is not used, the
(–)Sense pin should
always be connected to ‘Vo COM’.
Note: The remote sense feature is not designed to compensate
for the forward drop of non-linear or frequency dependent
components that may be placed in series with the converter
output. Examples include OR-ing diodes, filter inductors,
ferrite beads, and fuses. When these components are enclosed
by the sense pin connections they are effectively placed inside
the regulation control loop, which can adversely affect the
stability of the converter.
Over-Temperature Protection
Over-temperature protection is provided by an internal
temperature sensor, which monitors the temperature of
the converter’s PCB (close to pin 1). If the PCB temperature exceeds a nominal 115 °C, the converter will
shut down. The converter will then automatically restart
when the sensed temperature drops back to approximately
105 °C. When operated outside its recommended thermal derating envelope (see data sheet SOA curves), the
converter will typcially cycle on and off at intervals from
a few seconds to one or two minutes. This is to ensure
that the internal components are not permanently damaged from excessive thermal stress.
Under-Voltage Lockout
The Under-Voltage Lock-Out (UVLO) is designed to
prevent the operation of the converter until the input
voltage is close to the minimum operating voltage. The
converter is held off when the input voltage is below the
UVLO threshold, and turns on when the input voltage
rises above the threshold. This prevents high start-up
current during normal power-up of the converter, and
minimizes the current drain from the input source during low input voltage conditions. The converter will
meet full specifications when the minimum specified
input voltage is reached. The UVLO circuitry also overrides the operation of the Vo Enable control. Only when
the input voltage is above the UVLO threshold will the
Vo Enable control be functional.
Primary-Secondary Isolation
These converters incorporate electrical isolation between
the input terminals (primary) and the output terminals
(secondary). All converters are tested to a withstand voltage of 1500 VDC. This complies with UL/cUL 60950
and EN 60950 and the requirements for operational isolation. It allows the converter to be configured for either a
positive or negative input voltage source. The data sheet
‘Pin Descriptions’ section provides guidance as to the
correct reference that must be used for the external control signals.
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Application Notes
PTB78520W
Output Voltage Adjustment
The ‘V
higher than 1.8 V. For output voltages other than 1.8 V a
single external resistor, R
between the ‘V
A 0.05-W rated resistor can be used. The tolerance should
be 1%, with a temperature stability of 100 ppm/°C (or
better). Place the resistor close to the converter and
connect it directly between pins 7 & 6 using dedicated
PCB traces (see typical application). Table 1-1 gives the
preferred value of the external resistor for a number of
standard voltages, along with the actual output voltage
that this resistance value provides.
For other output voltages the value of the required adjust
resistor may be calculated using the following formula.
Adjust’ control sets the output voltages to a value
O
, must be connected directly
Adjust’ (pin 7) and ‘(–)Sense’ (pin 6) pins.
O
SET
R
SET
Table 1-1; Preferred Values of R
V
(Standard) R
SET
3.6 V 0 Ω 3.604V
3.3 V 887 Ω 3.303 V
2.5 V 6.98 kΩ 2.503 V
2.0 V 35.7 kΩ 2.003 V
1.8 V Open 1.805 V
= 6.49 kΩ · 1.225 V – 4.42 kΩ
V
– 1.805 V
SET
for Standard Output Voltages
SET
(Pref’d Value) V
SET
(Actual)
SET
Input Current Limiting
The converter is not internally fused. For safety and
overall system protection, the maximum input current to
the converter must be limited. Active or passive current
limiting can be used. Passive current limiting can be a
fast acting fuse. A 125-V fuse, rated no more than 10 A,
is recommended. Active current limiting can be implemented with a current limited “Hot-Swap” controller.
Thermal Considerations
Airflow may be necessary to ensure that the module can
supply the desired load current in environments with
elevated ambient temperatures. The required airflow
rate may be determined from the Safe Operating Area
(SOA) thermal derating chart (see converter specifications).
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Application Notes
PTB78520W
Using the Output Enable Control on the PTB78520
Auto-Track Compatible DC/DC Converter
The ‘VO Enable’ (pin 2) control is an active low input that
allows the output voltage from the converter to be turned
on and off while it is connected to the input source. The
‘VO Enable’ input is referenced to the –VIN (pin 3) 1, on
the primary side of the converter’s isolation, and has its
own internal pull up. The open-circuit voltage is approximately 20% of the applied input source voltage.
For the converter to function normally pin 2 must be
pulled low to –V
then produce a regulated voltage whenever a valid source
voltage is applied between +V
If the voltage at pin 2 is allowed to rise above VIH(min),
(see specification table), the output from the converter
will be turned off.
Figure 1-1 is an application schematic that shows the
typical use of the Output Enable function. Note the discrete
transistor (Q
transistor is recommended to control this input. Table 1-1
gives the threshold requirements.
When placed in the “Off” state the output will neither
source or sink output current. The load voltage will then
decay as the output capacitance is discharged by the load
circuit. With the output turned off, the current drawn
from the input source is typically reduced to 2 mA.
Table 1-1; Output Enable Control Requirements
Parameter Min Typ Max
Enable (VIH)2 V——
Disable (VIL) — — 0.8 V
V
[Open-Circuit] — — 13.5 V
O/C
IIN [pin 1 at –VIN] — — –0.6 mA
Notes:
1. The Output Enable control uses –VIN (pin 3) as its ground
reference. All voltages are with respect to –V
2. Use an open-collector (or open-drain) discrete transistor
to control the Output Enable input. A pull-up resistor is
not necessary. To disable the converter the control pin
should be pulled low to less than +0.8 V. If the Output
Enable feature is not used, pin 2 should be permanently
connected to –V
3. The converter incorporates an “Under-Voltage Lockout”
(UVLO). The UVLO does not allow the converter to
power up until the input voltage is close to its minimum
specified operating voltage. This is regardless of the state
of the Output Enable control. Consult the specifications
for the UVLO thresholds.
potential 2. The converter output will
IN
(pin 1) and –VIN (pin 3) 3.
IN
). Either a discrete MOSFET or bipolar
1
1
.
IN
(pin 3).
IN
Figure 1-1; Output Enable Operation
4
Track
+V
IN
1 = Enable
–V
IN
R
10 k
PTB78520W
1
+V
IN
2
V
Enable
O
Q
1
1
BSS138
3
–V
IN
+Sense
V
Seq
O
VO Bus
VO Adjust
COM
V
O
–Sense
8
10
9
7
R
2
887
5
6
Turn-On Time: In the circuit of Figure 1-1, turning Q
off allows the voltage at pin 2 to rise to its internal pullup voltage. This disables the converter output. When Q
is then turned on, it applies a low-level voltage to pin 2,
and enables the output of the converter. The converter
produces a regulated output voltage within 50 ms. Figure 1-2 shows the output response of a PTB78520W
is turned on. The turn on of Q1 corresponds to
after Q
1
the drop in the Q
Vds waveform. Although the output
1
voltage rise-time is short (<10 ms), the indicated delay
) will vary depending upon the input voltage and
time (t
d
the module’s internal timing. The output voltage of
the PTB78520W was set to 3.3 V. The waveforms were
measured with 24-Vdc input voltage, and a 10 A resistive
load.
Figure 1-2; Output Enable Power-Up Characteristic
V Bus (1 V/Div)
o
Iin (1 A/Div)
Q Vds (5 V/Div)
1
HORIZ SCALE: 5 ms/Div
t
d
L
O
A
D
1
1
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Application Notes
PTB78520W
Configuring the PTB78520 DC/DC Converter to
Power-Up Sequence with POL Modules
Overview
The PTB78520 DC/DC converter has two outputs,
Seq’ and ‘VO Bus’. ‘VO Bus’ is the main output
‘V
O
from the converter. ‘VO Seq’ is an output that is derived
from ‘V
voltages during power-up. Both outputs are regulated
to the same set-point voltage, except that the rise in the
‘V
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
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 presented 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 voltage 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.
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.
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 waveform.
The Track input of the PTB78520W has a pull-up resistor 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 modules time to initialize so that their outputs can rise with
the ‘VO Seq’ output.
Bus’ and can be sequenced with other supply
O
Seq’ output is controlled by a pin called ‘Track’, and
O
1.
In these applications, the PTB78520W incor-
2
4, 5
3
Figure 2-1; Block Diagram of PTB78520 Auto-Track Features
Vo Bus
To
feedback
error amp’
Supply
Supervisor
20 ms
Delay
Smart Sense
R
TRK
24.9 k
C
TRK
1 µF
(+)Sense
V
Seq
o
Track
V
COM
o
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
Seq’ output
O
(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
Bus’ output (pin 9) will be up to
O
100 mV higher.
3 The ‘V
Seq’ output cannot sink load current. This
O
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
Seq’
O
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
Bus’ output is in
O
regulation.
For technical support and further information visit http://power.ti.com
Application Notes
PTB78520W
Typical Power-Up Sequencing Configuration
Figure 2-2 shows how the PTB78520W (U
) can be
1
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 (U
& U3), each rated for up 6 A of output current.
2
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
Bus’ output. This is 20 A, less the current allocated
‘V
O
to the load circuit via the ‘VO 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
=887Ω). Note that this sets the output voltage of
(R
1
both the ‘VO Bus’ and ‘VO Seq’ outputs. The 3.3-V input
non-isolated modules, U2 and U3, can be set to any voltage 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 downstream non-isolated modules via the ‘Vo Bus’ output.
This is the output that rises first to allow the down-
Figure 2-3; PTB78520 Power-Up Waveforms with POL Modules
V Bus (1 V/Div)
o
V Seq (1 V/Div)
o
V 1 (1 V/Div)
POL
V 2 (1 V/Div)
POL
t =20 ms
d
HORIZ SCALE: 20 ms/Div
stream modules to complete their power-up initialization.
‘VO Seq’ (3.3 V), and the outputs V
1 (2.5 V) and V
POL
POL
(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 ‘VO 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.
2
Figure 2-2; Power-Up Sequencing Circuit With PTB78520W & Non-Isolated POL Modules
U1
4
Track
PTB78520W
8
+Sense
+V
I
–V
I
1
+V
I
2
V
Enable
O
3
–V
I
Seq
V
O
VO Bus
VO Adjust
COM
V
O
–Sense
10
9
7
R1
5
887
6
U2
C1
100 µF
U3
C2
100 µF
V
PTH03050W
IN
4
V
PTH03050W
IN
4
2
Track
GNDInhibit Adju st
15
2
Track
GNDInhibit Adju st
15
V
O
R2
2.21 k
V
O
R3
5.49 k
Vo Seq (3.3 V)
1 (2.5 V)
V
63
63
POL
C3
100 µF
V
POL
C4
100 µF
2 (1.8 V)
For technical support and further information visit http://power.ti.com
Application Notes
PTB78520W
Stand-Alone Operation
The combination of a wide-input and wide-output voltage range makes the PTB78520W an attractive product
as a stand-alone DC/DC converter. In these applications
the PTB78520W is not required to power up, or sequence
with, any non-isolated POL modules. The output voltage
can be adjusted to any value over the range, 1.8 V to 3.6 V,
and the Auto-Track features simply disregarded.
Figure 2-4 shows the the recommended configuration of
the PTB78520W when it is used as a stand-alone converter. As a sequenced output voltage is not required, the
main output, ‘V
rent. The ‘Track’ pin, and ‘VO Seq’ output are simply left
open circuit. The ‘(+)Sense’ pin can also be connected to
the ‘VO Bus’ output for improved load regulation.
When the PTB78520W is operated in this mode, the
output from ‘V
converter also exhibits slightly less power dissipation and
a corresponding improvment in operating efficiency.
Bus’, is used to supply all the load cur-
O
Bus’ rises promptly upon power up. The
O
Figure 2-4; PTB78520W Stand-Alone Configuration
4
Track
PTB78520W
1
+V
I
–V
I
+V
I
2
Enable
V
O
3
–V
I
+Sense
V
Seq
O
VO Bus
VO Adjust
V
COM
O
–Sense
8
10
9
7
R
SET
5
6
C1
100 µF
L
O
A
D
For technical support and further information visit http://power.ti.com
PACKAGE OPTION ADDENDUM
www.ti.com
23-Nov-2005
PACKAGING INFORMATION
Orderable Device Status
PTB78520WAD ACTIVE DIP MOD
(1)
Package
Type
Package
Drawing
Pins Package
Qty
Eco Plan
ERP 10 9 TBD Call TI Call TI
ULE
PTB78520WAH ACTIVE DIP MOD
ERP 10 9 TBD Call TI Level-1-235C-UNLIM
ULE
PTB78520WAS ACTIVE DIP MOD
ERQ 10 9 TBD Call TI Level-1-235C-UNLIM
ULE
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - The planned eco-friendly classification: Pb-Free (RoHS) or Green (RoHS & no Sb/Br) - please check
http://www.ti.com/productcontent for the latest availability information and additional product content details.
TBD: The Pb-Free/Green conversion plan has not been defined.
Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements
for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered
at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes.
Green (RoHS & no Sb/Br): TI defines "Green" to mean Pb-Free (RoHS compatible), and free of Bromine (Br) and Antimony (Sb) based flame
retardants (Br or Sb do not exceed 0.1% by weight in homogeneous material)
(2)
Lead/Ball Finish MSL Peak Temp
(3)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder
temperature.
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Addendum-Page 1
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