DC1294A - Demo Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A
ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL
LT3080EDD
DESCRIPTION
Demonstration circuit 1294A shows adjustable linear
regulators in parallel using LT3080. Architected as a
precision current source and voltage follower, it allows this new regulator to be used in many applications requiring high current, adjustability to zero output, and no heat sink. Also the device brings out the
collector of the pass transistor to allow low dropout
operation when used with multiple supplies.
A key feature of the LT3080 is the capability to supply a wide output voltage range. By using a reference
current through a single resistor, the output voltage
can be programmed to any level between zero and
36V. The DC1294A has a reduced input voltage 25V
due to input capacitor voltage rating, and is capable
of delivering up to 4.4A output current by paralleling
4 LT3080s with only 20m PCB ballast resistors.
The DC1294A can be used as a high current linear
regulator, post regulator for switching supply, vari-
able voltage supply or low output voltage power supply.
Internal protection circuitry includes current limiting
and thermal limiting.
LT3080 regulator is offered in 8-lead MSOP (with an
Exposed Pad for better thermal characteristics), a
3mm × 3mm DFN, 5-lead TO-220 and a simple-touse 3-lead SOT-223 version.
The LT3080 datasheet gives a complete description
of the part, operation and application information.
The datasheet should be read in conjunction with this
quick start guide for working on or modifying the
demo circuit 1294A.
Design files for this circuit board are available.
Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Table 1. Performance Summary ( TA = 25oC )
PARAMETER
CONDITION
VALUE
Minimum Vin Voltage
Vout=3.3V
3.8V
Maximum Vin Voltage
Minimum Vcontrol Voltage
25V
Vout=3.3V
4.9V
Maximum Vcontrol Voltage
25V
Output Voltage
3.3V ±3%
Minimum Output Current
4mA
Maximum Output Current
Vin-Vout<2.5V
4.4A
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A
ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL
QUICK START PROCEDURE
The DC1294A is easy to set up to evaluate the performance of the LT3080. Refer to Figure 1. for proper
measurement equipment setup and following the
procedures below:
1.
Apply 3.8V across Vin (to Gnd), and 4.9V
across Vcontrol. Draw 4.4A of load current.
The measured Vout should be 3.3V ± 3%
(3.2V to 3.4V).
2.
Vary Vin from 3.8V to 25V, Vcontrol from
4.9V to 25V and the load current from 4mA to
4.4A. Vout should measure 3.3V ± 3% (3.2V
to 3.4V).
Note: Make sure the power dissipation is limited below the thermal limit.
Figure 1. Proper Measurement Equipment Setup
Figure 2. Measuring Input or Output Ripple
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A
ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL
INPUT VOLTAGE RANGE
The guaranteed Vin dropout voltage is 0.5V at 4.4A,
the guaranteed Vcontrol dropout voltage is 1.6V at
4.4A. The maximum Vin and maximum Vcontrol is
reduced to 25V due to the input capacitor voltage rating.
DUAL SUPPLY OR SINGLE SUPPLY
Use two separate supplies for Vin and Vcontrol, a low
dropout voltage can be achieved on the Vin pin and the
power dissipation is minimized. Alternatively, Tying the
Vcontrol to Vin through a zero ohm jump resistor R1
on board, a single supply is sufficient to drive the
demo circuit 1294A. AVOID USING A LONG WIRE TO
TIE VCONTROL AND VIN.
OUTPUT CURRENT
The output current will decrease at high input-tooutput differential. The actual current output is further
limited by the thermal shutdown function, which is
related to the board thermal dissipation.
THERMAL IMAGE
An example thermal image shows the temperature distribution on board. The test is done in still air at room temperature with average 1.7W power dissipation in each LT3080.
Thermoteknix
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A
ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL
BYPASS CAPACITOR
Since the SET pin is a high impedance node, unwanted
signals may couple into the SET pin and cause erratic
behavior. This will be most noticeable when operating
with minimum output capacitors at full load current.
The easiest way to remedy this is to bypass the SET
pin with a small amount of capacitance from SET to
ground, 10pF to 20pF is sufficient. A 0.01uF bypass
capacitor is used on the demo board to provide a lownoise output. Please refer to datasheet for details.
PARALLEL OPERATION
A small ballast resistor is required for parallel operation, such as 20m . A short PCB trace can easily
serve that purpose. Refer to the datasheet for trace
length calculation. THE BOTTOM PAD OF IC IS THE
OUTPUT. The bottom pad can be tied to the output pin
respectively, however, each bottom pad should isolated from the others, thus force the output current go
through the ballast resistors.
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QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1294A
ADJUSTABLE LDO LINEAR REGULATORS IN PARALLEL
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