ltc3834-1 Linear Technology Corporation, ltc3834-1 Datasheet - Page 23

• Manufacturer Part Number: ltc3834-1
• Description: 30?a Iq Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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. Moni-
tor 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 imple-
mentation. Variation in the duty cycle at a subharmonic
rate can suggest noise pickup at the current or voltage
sensing inputs or inadequate loop compensation. Over-
compensation of the loop can be used to tame a poor PC
layout if regulator bandwidth optimization is not required.
Reduce V
regulator in dropout. Check the operation of the under-
voltage lockout circuit by further lowering V
toring the outputs to verify operation.
Investigate whether any problems exist only at higher
output currents or only at higher input voltages. If prob-
IN
from its nominal level to verify operation of the
V
IN
R
IN
U
BOLD LINES INDICATE HIGH SWITCHING
CURRENT. KEEP LINES TO A MINIMUM LENGTH.
C
U
IN
W
Figure 9. Branch Current Waveforms
IN
while moni-
U
D1
SW
lems 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. If problems are en-
countered with high current output loading at lower input
voltages, look for inductive coupling between C
and the top MOSFET components to the sensitive current
and voltage sensing traces. In addition, investigate com-
mon ground path voltage pickup between these compo-
nents 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 back-
wards. 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 tempo-
rarily shorting out the current sensing resistor—don’t
worry, the regulator will still maintain control of the
output voltage.
L1
R
SENSE
C
OUT
V
OUT
3834-1 F09
LTC3834-1
R
L1
IN
, Schottky
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38341f