QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 509
DUAL PHASE, SINGLE OUTPUT SYNCHRONOUS BUCK CONVERTER
LTC3729EUH
DESCRIPTION
Demonstration circuit 509 is a single output step down
converter featuring the LTC3729EUH. The LTC3729EUH
is a dual phase controller with a frequency range of
250kHz to 550kHz in a 32 lead 5mm
×
5mm QFN pack-
age. The QFN package has an exposed bottom pad that
is soldered to the PCB and provides the LTC3729EUH
with low thermal impedance. The input voltage range of
the DC509 is 5V to 14V and the output voltage is 2.5V
with a load rating of 25A.
The DC509A also contains a footprint for a 5.5V boost
converter bias supply whose output is connected to
EXTVCC. See the “O ” section below
for more details.
ptional Bias Supply
Design files for this circuit board are available. Call
the LTC factory.
Table 1.
Performance Summary
PARAMETER CONDITION VALUE
Minimum Input Voltage 5V
Maximum Input Voltage 14V
V
OUT
V
IN
= 5V to 14V, I
OUT
= 0A to 25A 2.5V ± 3%
QUICK START PROCEDURE
Demonstration circuit 509 is easy to set up to evaluate
the performance of the LTC3729EUH. Refer to Figure 1
for proper measurement equipment setup and follow the
procedure below:
1.
With power off, connect the input voltage source and
the load. The input cables should be sized to carry
current of at least 15A. The load cables need to be
sized for at least 30A.
2.
The jumpers should be placed in their default posi-
tions as shown in Figure 1.
3.
If a constant current electronic load is used, preset
the load current to about 1A or less before applying
power to the input. Otherwise, the foldback current
limit function of the LTC3729 will be triggered during
startup and the converter will remain in current limit
during steady-state operation.
4.
Apply power to the input and set the input voltage to
12V. The output voltage should be 2.5V ± 3%. In-
crease the load to 25A. The output voltage should
still be within regulation.
5.
Keep the input voltage at 12.0V and the load at 25A.
Measure the DC input current. It should be less than
6.1A.
6.
Adjust the input voltage and load current to the de-
sired levels within their limits and observe the regula-
tion, output ripple, load step response, efficiency and
other parameters.
7.
If desired, the switching frequency of the converter
may be adjusted with jumper JP1. The default posi-
tion is 550kHz and the other position is 250kHz.
“550kHz” is a slight misnomer; the actual PLLFLTR
signal will be 1.8V and this will result in a typical
switching frequency of roughly 500kHz. A typical
switching frequency of 550kHz can be achieved by
decreasing the value of R3 while in the 550kHz posi-
tion. By placing the jumper in the 250kHz position,
the PLLFLTR signal will be tied to ground. The typical
switching frequency will then be 250kHz.
8.
The phasing between controller #1 and CLKOUT and
between controller #1 and #2 can be set with jumper
JP2. The default position is 90, which will result in a
1
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 509
DUAL PHASE, SINGLE OUTPUT SYNCHRONOUS BUCK CONVERTER
90° phase shift between the CLKOUT signal and con-
troller #1 and 180° phase shift between controllers #1
and #2. Refer to the LTC3729 data sheet for more de-
tails.
NOTE:
When measuring the output voltage ripple, care
must be taken to avoid a long ground lead on the os-
cilloscope probe. Make the connections as short as
possible and measure the output ripple directly across
output capacitor C20 or C21. See Figure 2.
NOTE:
For input voltages from 5V to 7V, connect the
input voltage to EXTVCC. Otherwise, the INTVCC volt-
age will fall out of regulation and the operation of the
circuit may be affected.
+
–
+
–
+
–
+
–
+
–
+
–
LOAD
Figure 1.
Proper Setup for Measurement Equipment
Figure 2.
Measuring Output Ripple
2
QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 509
DUAL PHASE, SINGLE OUTPUT SYNCHRONOUS BUCK CONVERTER
OPTIONAL BIAS SUPPLY
The DC509A also contains a footprint for a 5.5V boost
converter bias supply whose output is connected to
EXTVCC. The input of this bias supply can be con-
nected either to VIN, if V
IN
<5V, or to VOUT for
5V
≤
V
IN
≤
14V. The latter connection will provide higher
efficiency. If this bias supply is stuffed, please keep the
following points in mind:
1.
The minimum input voltage for the optional bias
supply is 3.0V using the LT1613. If the input voltage
is less than 3.0V, the bias supply output may fall out
of regulation. This means that the bias supply input
cannot be connected to the main output if it is at the
nominal 2.5V.
2.
If the input voltage of the optional bias supply is
connected to VIN, then the board will need to be
slightly modified to ensure that EXTV
CC
does not
exceed V
IN
. These are the modifications:
a.
Remove resistor R12.
b.
Tie the (+) end of C14 to the (+) end of C15.
3.
If the input voltage of the optional bias supply is
connected to VOUT, precautions need to be taken to
make sure that EXTVCC does not exceed VIN during
turn-on of the main converter. Follow these steps
when turning on the main converter:
a.
Preset the load current to 1A or less.
b.
Tie the RUN/SS pin to ground.
c.
Turn-on the main input voltage and set to desired
level.
d.
Disconnect the RUN/SS pin from ground.
e.
Apply the desired load to the output.
4.
If a different power MOSFET configuration is used
or the switching frequency is increased, then be
sure to measure the current flowing into the
EXTVCC pin and make sure the current will not ex-
ceed the maximum output current of the bias sup-
ply. At 3.3V input, the maximum output current of
the bias supply is 200mA.
For further assistance, contact the factory.
3
5
5
4
4
3
3
2
2
1
1
D D
C C
B B
A A
23
27
13
28
1
2
5
7
10
16
6
4
19
32
8
17
9
11
12
14
20
22
26
25
24
30
31
21
29 15
18
3
VIN
CLKOUT
PGOOD
RUN/SS
EAIN
PLLFLTR
ITH
VDIFFOUT
NC
NC
SGND
PHASMD
PGND
NC
VOS-
BOOST2
VOS+
SENSE2-
SENSE2+
TG2
INTVCC
BG1
TG1
SW1
BOOST1
SENSE1+
SENSE1-
EXTVCC
NC SW2
BG2
PLLIN
1
32
4
65
1
2
3
5
4
SW
GND
FB
VIN
SHDN
