Changing The Output Voltage - Linear Technology DC1891A Demo Manual

DEMO MANUAL DC1891A
QUICK START PROCEDURE
Demonstration circuit 1891A is easy to set up to evaluate
the performance of the LTC3862-2 circuit. Refer to Figure 1
for proper measurement equipment setup and follow the
procedure below:
NOTE: When measuring the input or output voltage
ripple, care must be taken to avoid a long ground lead
on the oscilloscope probe. Measure the input or output
voltage ripple by touching the probe tip directly across
the V or V and GND terminals. See Figure 2 for
IN OUT
the proper scope probe technique.
1. With power off, connect the input power supply to V
and GND. Make sure that the input power supply has
sufficient current rating at minimum input voltage for
the required output load.
2. Turn on the power at the input.
NOTE: Make sure that the input voltage does not exceed
60V.
3. Check for the proper output voltage.
V = 12V, ±3%.
OUT
If there is no output, temporarily disconnect the load
to make sure that the load is not set too high.
2
4. Once the proper output voltage is established, adjust the
load within the operating range and observe the output
voltage regulation, ripple voltage, efficiency and other
parameters.
5. The DC1891A is equipped with an input capacitor C
that is optional and is used to help with filtering when
the board is connected to the lab supply with long leads.
The capacitor C
IN4
source is close and has low source impedance.

CHANGING THE OUTPUT VOLTAGE

IN
To set the output voltage lower or higher than 12V, change
the bottom voltage divider resistor connected to the FB
pin of U1 (see the Schematic Diagram). Also, check the
MOSFET, output diode and capacitor voltage ratings if the
output voltage is set higher than 12V.
The optional Q5 circuit is used to get the circuit running.
Once the circuit is running, the 12V output is used to
bias U1 via D2. The start-up circuit Q5 is turned off by Q6
when V gets to the level set by D3, R14 and R16 that
OUT
activates Q6. Please contact the LTC factory for details.
can be removed if the input power
IN4
dc1891af