LTC3835-1 Linear Technology, LTC3835-1 Datasheet - Page 23

• Manufacturer Part Number: LTC3835-1
• Description: Low IQ Synchronous Step-Down Controller
• Manufacturer: Linear Technology

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PC Board Layout Debugging
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 voltage as well. Check for proper performance over
the operating voltage and current range expected in the
application. The frequency of operation should be main-
tained over the input voltage range down to dropout and
until the output load drops below the low current opera-
tion 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.
Reduce V
of the regulator in dropout. Check the operation of the
undervoltage lockout circuit by further lowering V
monitoring the outputs to verify operation.
APPLICATIONS INFORMATION
IN
from its nominal level to verify operation
V
IN
R
IN
BOLD LINES INDICATE HIGH SWITCHING
CURRENT. KEEP LINES TO A MINIMUM LENGTH.
C
IN
Figure 9. Branch Current Waveforms
D1
IN
while
SW
Investigate whether any problems exist only at higher output
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 pos-
sibly 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. If
problems are encountered with high current output loading
at lower input voltages, look for inductive coupling between
C
sensitive current and voltage sensing traces. In addition,
investigate common ground path voltage pickup between
these components and the SGND pin of the IC.
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 hookup 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.
IN
L1
, Schottky and the top MOSFET components to the
R
SENSE
C
OUT
V
OUT
3835-1 F09
LTC3835-1
R
L1
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