LTC3788
25
3788fc
applicaTions inForMaTion
PC Board Layout Debugging
Start with one controller on at a time. It is helpful to use
a DC-50MHz current probe to monitor the current in the
inductor while testing the circuit. Monitor the output
switching node (SW pin) to synchronize the oscilloscope
to the internal oscillator and probe the actual output volt-
age. Check for proper performance over the operating
voltage and current range expected in the application.
The frequency of operation should be maintained over the
input voltage range down to dropout and until the output
load drops below the low current operation threshold—
typically 10% of the maximum designed current level in
Burst Mode operation.
The duty cycle percentage should be maintained from cycle
to cycle in a well designed, low noise PCB implementation.
Variation in the duty cycle at a subharmonic rate can sug-
gest noise pickup at the current or voltage sensing inputs
or inadequate loop compensation. Overcompensation of
the loop can be used to tame a poor PC layout if regulator
bandwidth optimization is not required. Only after each
controller is checked for its individual performance should
both controllers be turned on at the same time. A particu-
larly difficult region of operation is when one controller
channel
is nearing its current comparator trip point while
the other channel is turning on its bottom MOSFET. This
occurs around the 50% duty cycle on either channel due
to the phasing of the internal clocks and may cause minor
duty cycle jitter.
Reduce V
IN
from its nominal level to verify operation with
high duty cycle. Check the operation of the undervoltage
lockout circuit by further lowering V
IN
while monitoring
the outputs to verify operation.
Investigate whether any problems exist only at higher out-
put currents or only at higher input voltages. If problems
coincide with high input voltages and low output currents,
look for capacitive coupling between the BOOST, SW, TG,
and possibly BG connections and the sensitive voltage
and current pins. The capacitor placed across the current
sensing pins needs to be placed immediately adjacent to
the pins of the IC. This capacitor helps to minimize the
effects of differential noise injection due to high frequency
capacitive coupling.
An embarrassing problem, which can be missed in an
otherwise properly working switching regulator results
when the current sensing leads are hooked up backwards.
The output voltage under this improper hook-up will still
be maintained
, but the advantages of current mode control
will
not be realized. Compensation of the voltage loop will
be much more sensitive to component selection. This
behavior can be investigated by temporarily shorting out
the current sensing resistor—don’t worry, the regulator
will still maintain control of the output voltage.
