6
Rev. 0
EVALUATE DC2688A POWER STAGE
Demonstration circuit 2688A is easy to set up to evalu-
ate the performance of the LT8698S/LT8698S-1. Refer to
Figure 7 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 long ground lead
on the oscilloscope probe. Measure the output voltage
ripple by touching the probe tip directly across the
VB
US
and GND terminals. See Figure 7 and Figure 8
for proper measurement technique.
1. Check that JP6 and JP7 are placed on the OFF position
to ensure that the on-board LDOs are disabled.
2. Place JP2, JP3, and JP4 all on the “Z” position, which
puts the LT8698S/LT8698S-1 in V
BUS
only mode of
operation with V
BUS
in regulation.
3. With power off, connect the input power supply to
V
EMI
and GND turrets. If the EMI/EMC performance
is not important, connect the input power supply to
V
IN
and GND turrets.
4. With power off, connect the loads from V
BUS
to GND.
5. Check that JP1 EN jumper is placed on the ON position.
6. Turn on the power at the input.
NOTE:
Make sure that the input voltage never
exceeds 42V.
7. Check for proper output voltages: V
BUS
voltage should
be between 5V to 6.1V and depends on the cable drop
QUICK START PROCEDURE
compensation settings, and D11 LED indicator lights
up in green. For more information on cable drop
compensation, please refer to LT8698S/LT8698S-1
data sheet.
NOTE:
If there is no output, temporarily disconnect
the load to make sure that the load is not set too high.
8. Once the proper output voltages are established,
adjust the load within the operating ranges and
observe the output voltage regulation, ripple voltage,
efficiency and other parameters.
9. An external clock can be added to the EXT_SYNC ter-
minal when SYNC function is used (JP5 on the FCM
W/O SSM OR SYNC position). Please make sure that
R37 resistor should be chosen to set the LT8698S/
LT8698S-1 switching frequency equal to or below the
lowest synchronization frequency. JP5 can also set
the LT8698S/LT8698S-1 in spread-spectrum mode
(JP5 on the FCM W/SSM position) or pulse skipping
mode (JP5 on the PSK position).
10. Check STATUS pin voltage on the STATUS turret in
relation to the I
BUS
load current. If I
BUS
load is lower
than 100mA typically, STATUS pin should be low, and
the STATUS LED lights up in yellow; if I
BUS
load is
higher than 130mA typically, STATUS pin should be
high, and the STATUS LED does not light up.
11. Check I
MON
point voltage, in relation to I
BUS
load cur-
rent. Measure the voltage on the IMON turret: the
IMON voltage should be equal to 0.46V/A in propor-
tion to the I
BUS
load current.
