Linear Technology LTC2923 User manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
1
LTC2923
DESCRIPTION
Demonstration circuit 732 allows evaluating perform-
ance of the LTC2923 Power Supply Tracking Controller
which is intended to provide hot swapping of one not
regulated voltage source (master supply) and tracking
or sequencing of two other regulated power supplies
via current injection into their respective feedback
nodes.
The DC732 provides two different configurations on
one snap apart circuit board.
Circuit 1 provides for tracking of two regulated sup-
plies via feedback node current injection.
Circuit 2 provides an additional pass FET to control a
third fixed rail on a larger board. Both boards provide
very small footprints for easy installation into existing
systems.
Design files for this circuit board are available. Call
the LTC factory.
LTC is a trademark of Linear Technology Corporation
Table 1. Performance Summary (T
A
= 25°C)
PARAMETER CONDITION VALUE
Supply Voltage Rated operating limits 2.9 • V
CC
• 5.5V
Under voltage lockout V
CC
rising 2.2V • V
CC
• 2.7V
ON Pin Threshold Precision threshold accurately holds off power supplies until
both supplies are fully up 1.212 • V
VON(TH)
• 1.248V
ON Pin Hysteresis 30mV• ˜V
ON(HYS)
•150mV30A
Gate Pin Current
Gate On : V
GATE
=0V, No Faults
Gate Off: V
GATE
=5V, No Faults
Gate Fault: V
GATE
=5V, Short-Circuit Fault
-7µA •
GATE
• -13 µA
7 µA • I
GATE
• 13 µA
5mA • I
GATE
• 50mA
FET Drain-Source Overcurrent Voltage Threshold (V
CC
-
V
RAMP
) FET Over Current Sensed by Drain-Source Voltage 160mV • V
DSTH
• 240mV
Fixed Rail Current Capability Limited by Si4412 MOSFET (To Control Higher Current Re-
place MOSFET with a Larger Device) 4A Max
Track Pin Voltage 0.776V • V
TRACK
• 0.824V
Feedback Pin Compliance Regulated Power Supplies Feedback Voltage Must be Less
Than 1.5V 0V • V
FB
• 1.5V
Feedback Node Control Current V
FB
• 1.5V 1V MIN

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
2
OPERATING PRINCIPLES
The LTC2923 controls two regulated power supplies by
introducing a precision controlled current into the feed-
back node of the supplies.
A third fixed rail can be controlled via a series pass FET.
The power supply’s feedback node must be accessible
and should be high impedance.
NOTE: The feedback nodes of many linear regulators
(and of some boost converters) do not meet the above
high input impedance requirement when the output volt-
age is programmed below•0.6V or V
REF
of the supply is
not suitable for tracking by feedback node current injec-
tion. Sequencing and tracking via series pass FET using
the LTC2921 is a preferred approach.
NOTE : The LTC2923 V
CC
must be present and above
V
CC(UVL)
BEFORE the tracked supplies are enable or pow-
ered up. If V
CC
is not present, the slave supplies will
power up normally and uncontrolled.
The series pass FET can control a third primary supply
or a supply where the feedback node either is not acces-
sible or does not provide the required high impedance
such as a linear LDO. An electronic circuit breaker func-
tion implemented in the LTC2923 protects the pass FET.
This function is activated after the pass FET is fully en-
hanced. The LTD2923 monitors the drain to source volt-
age and rapidly pulls the gate voltage to ground if FET
drops out of saturation.
The LTC2923 is designed to track two slave supplies to
an externally generated ramp voltage applied to the
RAMP pin. This ramp voltage can be generated by using
the LTC2923 GATE pin constant current source to charge
a capacitor or using the output of the pass FET con-
trolled supply or using other voltage source is condi-
tioned not to exceed V
CC
.
A pair of resistors configures the behavior of each slave
supply relative to the master ramp signal. The choice of
resistors can control a slave supply to track the master
ramp signal coincidently, ratiometrically, with offset and
or time delay (sequencing).
The high impedance feedback current outputs of the
LTC2923 do not load the feedback node of the slave
supplies and therefore do not affect the slave power
supply stability or transient response.
The LTC2923 is able to control power supplies satisfying
these two conditions:
1. The feedback node potential must be lower than
1.5V when the supply is in regulation.
2. The power supply feedback divider current
should satisfy the following expression:
10µA•V
FB
/R
EQ
•1mA. Where R
EQ
is the parallel (Theve-
nin) impedance of the feedback resistors and V
FB
is
the reference voltage of the power supply.
Condition 1 requires the slave power supply feed-
back voltage to be within the compliance range of
the LTC2923 current source output.
Condition 2 requires the power supply feedback
node impedance to be within a range where a0 the
available control current can drive the slave power
supply output voltage to zero and b) the LTc2923
leakage current does not introduce errors in the
power supply output voltage exceeding ½% worst
case.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
3
QUICK START PROCEDURE
Demonstration Circuit DC732 provides two separate cir-
cuits, one with pass FET capable of handling a 4A supply
and a smaller board without the pass FET. Two circuits
can be separated by snapping them apart along the
score line. The back sides of the boards contain no parts
to facilitate mounting the boards into an existing system
with double sides adhesive tape if needed.
The board with drawing number DC732A-1 does not
have a power FET and the master ramp signal is derived
from the LTC2923 gate pin voltage. Both boards sche-
matics are showh in Fig.2 and board assembly=ies in
Fig.2.
NOTE: Because of the many options and circuit depend-
encies on the slave supplies involved, DC732A is pro-
vided without RTA1, RTB1, RTA2, and RTB2 installed.
Refer to the LTC2923 Data Sheet available at
www.linear.com for extensive design instructions on
calculating the values of R
TAX
< R
TBX
as well as options for
R
ONA
, R
ONB
, and C
GATE.
Initial information for the LTC2923 adjustment is:
- master power supply (unregulated )voltage, steady
state load current and capacitive load,
- power supply feedback node resistors R
FA1
, R
FB1
, R
FA2
,
R
FB2,
.
- defined power supply output voltage profile in the
power-up and power-down,
- all supply output voltage slew rate a=dV/dt (a
MASTER
,
a
SLAVE_1
, a
SLAVE_2
).
- desired delay time (in the offset tracking or in supply
sequencing).
1) Chose power MOSFET for master supply and install
on the board (only for board DC732A).
2) Calculate the resistor R
FA1
, R
FB1
, R
FA2
, R
FB2 ,
values and
gate pin capacitor value (C1 for DC732A board and C4
for DC732A-1 board) according to the LTC2923 Data
Sheet and install on the board.
3) Connect power supply feedback nodes with
FEEDBACK1 (TP4) and FEEDBACK2 (TP7) pins.
4) Connect power supply with board GND with TP8.
5) Connect load to MASTER OUTPUT VOLTAGE (TP9).
6) Apply MASTER INPUT VOLTAGE and observe tran-
sient.
Note that that ON circuit resistors R
ONA
and R
ONB
are
configured for a 2.9V turn-on signal level.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
4

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
5

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