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dc1470af
DEMO MANUAL DC1470A
total to minimize the wire inductance. 1-inch is even better.
The cathode of the LED should return to LED– with R25
populated with a resistor in a few mΩ. It is recommended
to measure the voltage on R25 with a short 50Ω coax
cable directly into a BNC connector on the oscilloscope.
The current can then be calculated from the measured
voltage. To get accurate current, R25 should be measured
before it is mounted on the board. A current probe adds
more delays to the rise time so using a current probe is
not recommended. If fast rise time measurement is not
necessary, the cathode of the LED can be connected to
GND as shown in Figure 1.
R27 can be used to slow down the gate drive. Slower gate
drive helps to reduce ringing on the SW node without
noticeable effect on the efficiency. A 10Ω is usually more
than enough to completely damp any ringing.
DESCRIPTION
Demonstration circuit 1470A is easy to set up to evalu-
ate the performance of the LT3743. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
1. With power off, connect the input power supply to PV
IN
and GND. The input power supply should have a current
limit of 10A or more.
2. With power off, connect a LED between V
OUT
and GND
or between V
OUT
and LED–, if fast rise time is to be
verified. When returning LED to LED–, R25 needs to
be populated. To check voltage across R25, the best
set up is to solder a short 50Ω coax cable across R25.
Connect the other end of the cable directly into a BNC
connector on an oscilloscope. A less perfect but easier
set up is to touch the probe tip directly across the IS+
and IS– terminals with out the probe ground wire. See
Figure 2 for setup. The latter set up tends to have more
noise but is ok for non-critical measurements. The LED
connecting wires should be as short as possible, 1 inch
total being ideal.
3. Turn on the power at the input.
R15, R16 and C15 help filtering out voltage spikes seen
on the SENSE
+
or SENSE
–
pin. It is critical to have those
components on a board.
The LT3743 has a three-state control. It can change output
current among zero, a low level and a high level, all in a
few μs. The low current level of the demo board is set to
around 5A. The high current level is 20A. Both levels can
be adjusted by changing the voltage dividers on CTRL_H
and CTRL_L. Applying a PWM signal to CTRL_SEL will
toggle the output between 5A to 20A. While applying a PWM
signal to the PWM pin will toggle the output between either
0A to 5A or 0A to 20A depending on the CTRL_SEL level.
The LT3743 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this demo manual
for DC1470A.
QUICK START PROCEDURE
4. At this time, the output current will be slightly below
5A. If no output current is observed, turn off PV
IN
and
check the connections.
5. To increase the load current to 20A, turn off PV
IN
. Pull
CTRL_SEL high either by populating R21 or by using an
external voltage source. Make sure the load can handle
20A continuously and the load voltage is not too high
for the board. Then turn on PV
IN
.
6. Both the low current level, 5A and the high current
level 20A can be adjusted by changing the voltage on
CTRL_L and CTRL_H terminals between 0V and 2V.
7. To evaluate the transient between 0A and a non-zero
current level, apply a PWM signal to the PWM terminal.
The nonzero current level is dictated by the CTRL_SEL
and CTRL_H or CTRL_L voltages.
8. To evaluate the transient between two non-zero cur rent
levels, apply a PWM signal to the CTRL_SEL terminal.
9. To modify the demo board for other applications, please
contact Linear Applications Group for help.
