Quick Start Procedure - Linear Technology DC1878A Demo Manual

DEMO MANUAL DC1878A

Quick start proceDure

Demonstration circuit 1878A is easy to set up to evalu-
ate the performance of the LTC3789EGN. Please refer to
Figure 1 for proper measurement equipment setup and
follow the procedure below.
1. With power off, connect the input supply, load and
meters as shown in Figure 1. Preset the load to 0A and
V supply to 0V.
IN
2. Set the current limit of the input supply high enough
so that the bench supply does not go into current limit
mode. Figure 2 shows input current as a function of
input voltage, at full load. Input current can reach 8A
at 8V input voltage.
3. Place jumper in the following position:
4. Turn on the power at the input. Increase V
8V and 75V. The circuit will start and the output voltage
will regulate at 12V ±2%. The current draw at no load is
typically 18mA to 37mA depending on the input voltage.
5. Apply load and observe output voltage regulation, ripple
voltage, efficiency and other parameters. Efficiency and
power loss at full load is shown in Figure 3. Continuous
operation with full power and high input voltage may
require some airflow.
2
JP1
RUN
ON
to between
IN
Auxiliary Supply
The demo board DC1878A has a LTC3803 based flyback
converter to provide 8V/50mA bias voltage for LTC3789
and LTC4444-5. The snubber for the flyback primary wind-
ing provides a voltage that is above the input voltage. A
transistor Q9 is used to provide a small current source
to the top gate driver of LTC4444-5 so that 100% duty
cycle is possible.
If this current source is not used, for example if the flyback
auxiliary supply is replaced by an external 6V to 12V supply,
it is necessary to connect BOOST1 to 3.0V (add 3V Zener
diode D11, R11 = 1k, remove R16) and use a level shift
circuit (add D3 = BAS16, R12 = 1k, remove R6) between
TG1 and TINP (due to the 3.25V maximum threshold for
TINP). By setting BOOST1 to 3.0V, the LTC3789 will issue
narrow refresh pulses on BG1 which keeps the top gate
driver powered also in BOOST mode.
In addition, DC1878A demo board has layout for an optional
bias supply intended for a future monolithic high voltage
buck regulator. Additional information will be provided
when this part is released.
dc1878af