DC948A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
LTC2926
DESCRIPTION
The demonstration circuit board DC948A is intended to
evaluate the performance of the LTC2926 MOSFETControlled Power Supply Tracker. The board contains one
LTC2926 Tracker, three power MOSFETs for tracking and
sequencing three power supply rails, a few resistors for
configuring tracking profiles and one small signal
MOSFET for voltage drop compensation in the “master”
rail. As assembled, DC948A operates with +5.0V as the
“master” rail, and +3.3V and +2.5V as ”slave” rails. The
board features “master” supply slew rate of SM=100V/s,
and “slave” supply slew rates of SS=150V/s. The first
“slave” supply power up starts 12.5ms later than “master” supply and second “slave” starts 25ms later.
The LTC2926 is designed to provide compensation for
voltage drop across each power MOSFET and its traces,
when an individual power supply is used for each rail.
This compensation is achieved with automatic remote
sense switching. Two integrated switches for “slave”
supplies and a control signal source for the external
switch are implemented in the LTC2926.
The power supply tracking and sequencing profiles
known as coincident tracking, ratiometric tracking, offset
tracking, or supply sequencing are accomplished by selection of the four resistor values per channel (RTA, RTB,
RFA, and RFB).
Design files for this circuit board are available. Call
the LTC factory.
, LTC, and LT are registered trademarks of Linear Technology Corporation.
PERFORMANCE SUMMARY
SYMBOL
VCC
VCC(UVLO)
VON(TH)
VON(CLR)
VMASTER
VSLAVE1
VSLAVE2
SMGATE
SSLAVE1
SSLAVE2
IS_MIN
PARAMETER
Input Supply Voltage
Input Supply Undervoltage Lockout
ON Pin Threshold Voltage
ON Pin Fault Clear Threshold Voltage
Master Supply Voltage
Slave Supply 1 Voltage
Slave Supply 2 Voltage
Master Gate Voltage Slew Rate
Slave Supply 1 Slew Rate
Slave Supply 2 Slew Rate
Slave Supply Minimal Load Current
CONDITIONS
Operating Range
VCC Rising
VON Rising
VON FALLING
MIN
2.9
2.2
1.20
0.465
TYP
3.3
2.4
1.23
0.500
5.0
3.3
2.5
100
150
150
20
MAX
5.5
2.6
1.26
0.535
UNITS
V
V
V
V
V
V
V
V/s
V/s
V/s
mA
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
OPERATING PRINCIPLES
The “master” rail is controlled like a typical hot swap circuit, where the “master” gate voltage slew rate defines
the output slew rate. With the 10uA gate driver current
capability, 0.1uF gate capacitor CMGATE placed on the
board, and negligible MOSFET gate capacitance (1.5nF)
the “master” slew rate is 100V/s.
For managing each “slave” rail the LTC2926 Tracker contains individual tracking and gate controller cells.
The tracking cell servos the TRACK pin to 0.8V by providing the required current into the Track resistor to keep
the voltage on the TRACK pin equal to 0.8V. The current
supplied by the TRACK pin is mirrored to the feedback FB
pin. The TRACK pin node is connected to the RAMPBUF
pin and GND with external resistors RTB and RTA respectively.
The gate controller cell servos the FB pin to 0.8V by driving the gate of the external N-channel power MOSFET.
This establishes the “slave” output voltage based on the
TRACK pin current and the feedback divider resistors.
This cell is a bang-bang control system with the inner
command (reference) signal equal to 0.8V and feedback
provided from the feedback pin node, which is connected
to two resistors: RFA to GND and RFB to the “slave” rail
output. When the “slave” output reaches its nominal output, the feedback divider should provide the FB pin with a
voltage a little bit lower than the inner command. In this
case, the gate controller cell develops maximum gate
voltage to enhance the external MOSFET.
Figure 1, borrowed from the Data Sheet, demonstrates
the structure of the tracking and gate controller cells and
simplifies understanding their interaction.
SSLAVE=SMASTER• RFB/RTB.
The relation RFB/RTB for initially installed components
for both channels equals 1.5.
The master signal ramps up and the slave supplies track
the master signal, when the ON pin signal rises above
1.23V.
The board allows two options for VCC source selection.
Jumper JP1 ON position connects the master supply to
VCC node, and the OFF position allows the use of an external voltage source.
There are also two options for the RAMP pin signal
source. Jumper JP2 placed in the MASTER position connects master rail output voltage to the RAMP pin.
Jumper JP2 EXTERNAL position allows the use of an
external source.
After master and slave gate drivers reach their maximum
voltages, the LTC2926 closes integrated remote sense
switches and pulls up the external FET gate with a 10uA
current source as an external remote switch control signal.
For the proper LTC2926 operation, each “slave” rail output should be loaded to consume a current, which exceeds 50-100 times the steady state individual feedback
divider current. Initially populated DC948A requires a
100-150 Ohm resistive load or 150-200uF capacitive
load.
Refer to the LTC2926 data sheet for detailed equations
and design examples.
The “slave” output voltage slew rate is defined as:
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
QUICK START PROCEDURE
Demonstration circuit DC948A is easy to set up to
evaluate the performance of the LTC2926. Refer to Figure 2 for proper measurement equipment setup and
follow the procedure below:
1. Place jumpers in the following positions:
JP1 in position ON
JP2 in position MASTER
2. With power off, connect the +5.0V power supply to
the SUPPLY0 and GND turrets, the +3.3V supply to
the SUPPLY1 and GND turrets, the +2.5V supply to
the SUPPLY2 and GND turrets.
3. Connect the ON signal source with the ON/OFF turret.
4. Load each output rail (MASTER, SLAVE1, and
SLAVE2 turrets) with a 50-Ohm resistor or equivalent electronic load..
5. Place the scope probes on the output loads
(MASTER, SLAVE1, and SLAVE2) and ON/OFF turret. Be sure that the ON control signal is low.
6. Turn on the +5.0V, +3.3V, and +2.5V supplies.
7. Switch the ON signal from low to high.
8. The power-up output voltages should correlate with
the transient shown in Figure 3. Acceptable tolerance in the slew rate value and timing position is
±30%.
9. Switch the ON signal from high to low.
10. The power-down output voltages should correlate
with transient shown in Figure 4. Acceptable tolerance in the slew rate value and timing position is
±30%.
11. With any other “master” voltage and “slave” voltages set, calculate the following component parameters: RTA1, RTB1, RFA1, RFB1, RTA2, RTB2, RFA2,
RFB2, and CMGATE based on the required profiles
and timing.
12. Replace the appropriate components on the board
and repeat the described above actions.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
Figure 1. Simplified Tracking Cell and Gate Controller Cell Structure
Figure 2. The DC948A connection with Power Supplies, loads, and ON signal source
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
Figure 3. Power-up mode
Figure 4. Power-down mode
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 948
MOSFET-CONTROLLED POWER SUPPLY TRACKER
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