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dc1739bcf
DEMO MANUAL DC1739B-C
OPERATING PRINCIPLES
QUICK START PROCEDURE
The LTC3765 active clamp forward controller and gate
driver is used on the primary and provides start-up, gate
drive, and protection functions. Once start-up is accom-
plished, the LTC3766 high efficiency, secondary-side
synchronous forward controller takes over, and provides
the LTC3765 with timing information and bias power
through a small pulse transformer.
When input voltage is applied, the LTC3765 commences
soft-start of the output voltage. When the secondary bias
source reaches the undervoltage threshold, the LTC3766
comes alive and takes control by sending encoded PWM
gate pulses to the LTC3765 through T3. These pulses
also provide primary bias power efficiently over a wide
input voltage range.
The transition from primary to secondary control occurs
at some fraction of the nominal output voltage. From then
on, operation and design is simplified to that of a simple
buck converter. Secondary control eliminates delays, tames
large-signal overshoot, and reduces output capacitance
needed to meet transient response requirements.
An optional LC filter stage on the input lowers rms input
current. The filter must have output impedance that is
less than the converter input impedance to assure stabil-
ity. This may require a damping impedance. (See Linear
Technology Application Note 19 for a discussion of input
filter stability.) A source with a 170mΩ or higher ESR
at the filter resonant frequency (~35kHz) is one way of
providing damping for the filter elements provided on the
DC1739B-C. For bench testing, an electrolytic capacitor
has been added at the input terminals to provide suitable
damping and ripple current capability. The values selected
have a filter resonant frequency that is below the converter
switching frequency, thus avoiding high circulating cur-
rents in the filter.
Demonstration circuit 1739B-C is easy to set up to evalu-
ate the performance of the LTC3765/LTC3766. Refer to
Figure 1 for proper measurement equipment setup and
follow the procedure below:
Note: When measuring the output voltage ripple, care must
be taken to avoid a long ground lead on the oscilloscope
probe. Measure the output voltage ripple by touching the
probe tip and ground ring directly across the last output
capacitor as shown in Figure 1.
1. Set an input power supply that is capable of 9V to 36V
to 9V. Then turn off the supply.
2. Direct an airflow of 200lfm across the unit for sustained
operation at full load.
3. With power off, connect the supply to the input terminals
+VIN and –VIN.
a. Input voltages lower than 9V can keep the converter
from turning on due to the undervoltage lockout
feature of the LTC3765 / LTC3766.
b. If efficiency measurements are desired, an ammeter
capable of measuring 10ADC or a resistor shunt can
be put in series with the input supply in order to
measure the DC1739B-C’s input current.
c. A voltmeter with a capability of measuring at least
36V can be placed across the input terminals in order
to get an accurate input voltage measurement.
4. Turn on the power at the input.
Note: Make sure that the input voltage never exceeds 36V.
5. Check for the proper output voltage of 12V. Turn off
the power at the input.
6. Once the proper output voltages are established, con-
nect a variable load capable of sinking 10A at 12V to
the output terVOUT and –VOUT. Set the current
for 0A.
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