DC732A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
LTC2923
DESCRIPTION
Demonstration circuit 732 allows evaluating performance 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 supplies 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 (TA = 25°C)
PARAMETER
CONDITION
Supply Voltage
Rated operating limits
2.9 • VCC • 5.5V
Under voltage lockout
VCC rising
2.2V • VCC • 2.7V
ON Pin Threshold
Precision threshold accurately holds off power supplies until
both supplies are fully up
ON Pin Hysteresis
Gate Pin Current
VALUE
1.212 • VVON(TH) • 1.248V
30mV• ˜V ON(HYS) •150mV30A
Gate On : VGATE=0V, No Faults
-7µA •GATE • -13 µA
Gate Off:
7 µA • IGATE • 13 µA
VGATE =5V, No Faults
5mA • IGATE • 50mA
Gate Fault: VGATE =5V, Short-Circuit Fault
FET Drain-Source Overcurrent Voltage Threshold (VCCVRAMP)
FET Over Current Sensed by Drain-Source Voltage
Fixed Rail Current Capability
Limited by Si4412 MOSFET (To Control Higher Current Replace MOSFET with a Larger Device)
Track Pin Voltage
160mV • VDSTH • 240mV
4A Max
0.776V • VTRACK • 0.824V
Feedback Pin Compliance
Regulated Power Supplies Feedback Voltage Must be Less
Than 1.5V
0V • VFB • 1.5V
Feedback Node Control Current
VFB • 1.5V
1V MIN
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
OPERATING PRINCIPLES
The LTC2923 controls two regulated power supplies by
introducing a precision controlled current into the feedback 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 voltage is programmed below•0.6V or VREF of the supply is
not suitable for tracking by feedback node current injection. Sequencing and tracking via series pass FET using
the LTC2921 is a preferred approach.
NOTE: The LTC2923 VCC must be present and above
VCC(UVL) BEFORE the tracked supplies are enable or powered up. If VCC 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 accessible or does not provide the required high impedance
such as a linear LDO. An electronic circuit breaker function implemented in the LTC2923 protects the pass FET.
This function is activated after the pass FET is fully enhanced. The LTD2923 monitors the drain to source voltage and rapidly pulls the gate voltage to ground if FET
drops out of saturation.
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•VFB/REQ•1mA. Where REQ is the parallel (Thevenin) impedance of the feedback resistors and VFB is
the reference voltage of the power supply.
Condition 1 requires the slave power supply feedback 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.
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 controlled supply or using other voltage source is conditioned not to exceed VCC.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
QUICK START PROCEDURE
Demonstration Circuit DC732 provides two separate circuits, 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 schematics are showh in Fig.2 and board assembly=ies in
Fig.2.
NOTE: Because of the many options and circuit dependencies on the slave supplies involved, DC732A is provided 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 RTAX< RTBX as well as options for
RONA, RONB, and CGATE.
Initial information for the LTC2923 adjustment is:
- master power supply (unregulated )voltage, steady
state load current and capacitive load,
- power supply feedback node resistors RFA1, RFB1, RFA2,
RFB2,.
- defined power supply output voltage profile in the
power-up and power-down,
- all supply output voltage slew rate a=dV/dt (aMASTER,
aSLAVE_1, aSLAVE_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 RFA1, RFB1, RFA2, RFB2, 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 transient.
Note that that ON circuit resistors RONA and RONB are
configured for a 2.9V turn-on signal level.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 732
POWER SUPPLY TRACKING AND SEQUENCING CONTROLLER
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