Changing The Output Voltage; Reset Circuit - Linear Technology 968A-D Quick Start Manual

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 968A-D

CHANGING THE OUTPUT VOLTAGE

To set the output voltage lower than 28V, change
the bottom voltage divider resistor connected to
LT4430 FB pin (see the simplified schematic on
page 5). For example, to get 24V output, change
R38 resistor value to 7.15k. However, keep in mind
that changing the transformer as well may increase
the efficiency. The schematic on page 6 shows all of
the components on the PCB. The schematic on
page 6 can be used as a reference if optional cir-
cuits need to be implemented.
To get higher than 28V output voltages transformer
with higher turns ratio may be required. Also, out-
put MOSFETs with higher voltage ratings may be
required. Please contact LTC factory for details.
PRIMARY MOSFET DRIVER LTC4440
The DC968 has an LTC4440 MOSFET driver U2. The
LTC4440 can be disabled if the efficiency is not im-
portant. If U2 is disabled, the DELAY pin of LT1952
should be readjusted to optimize the efficiency.
Please contact LT factory for assistance.

RESET CIRCUIT

The DC968 is equipped with an active reset circuit.
The active reset circuit is used in order to improve
the efficiency by 1% to 2%.
If the efficiency is not important, active reset circuit
can be disabled. In that case, MOSFET Q2 will need
to be replaced with a MOSFET with higher voltage
rating. Please contact LTC factory for help in select-
ing appropriate MOSFET. The active reset circuit
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Figure 2. Scope Probe Placement for Measuring Input or Output Ripple
ISOLATED DC/DC POWER CONVERTER
does not always help with the efficiency and is dis-
abled on DC968-A, -B and -C demo boards.
LT1952 DC968-D 28V @4A Output Efficiency
96%
95%
94%
93%
92%
91%
90%
1 2 Iout [A]
Figure 3. High efficiency of DC968A-D allows the
board to be used in thermally critical applications
with outputs up to 5A.
OUTPUT LOAD STEP RESPONSE
The load step response of DC968A-D is very fast
even though relatively small amount of output ca-
pacitance is present (6.8uF ceramic and 330uF elec-
trolytic). This is thanks to fast error amplifier of
LT4430, optimal amount of current slope compen-
sation of LT1952, fast opto coupler and fast error
amplifier of LT1952. If higher load steps need to be
handled more output capacitance can be added in
order to keep the voltage transients at the desired
level. The load step transients are shown in Figure
4.
3 4 5
3