LTC3891 datasheet(32/40 Pages) LINER

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LTC3890-2

38902f

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 switch-

ing 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 maintained over the

input voltage range down to dropout and until the output

load drops below the low current operation threshold—

typically 15% 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 implementa-

tion. Variation in the duty cycle at a subharmonic rate can

suggest noise pickup at the current or voltage sensing

inputs or inadequate loop compensation. Overcompen-

sation 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 perfor-

mance should both controllers be turned on at the same

time. A particularly difficult region of operation is when

one controller channel is nearing its current comparator

trip point when the other channel is turning on its top

MOSFET. This occurs around 50% duty cycle on either

channel due to the phasing of the internal clocks and may

cause minor duty cycle jitter.

Reduce VIN from its nominal level to verify operation of

the regulator in dropout. Check the operation of the un-

dervoltage lockout circuit by further lowering VIN 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. If problems are encountered with

high current output loading at lower input voltages, look

for inductive coupling between CIN, Schottky and the top

MOSFET components to the 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.

APPLICATIONS INFORMATION

LTC3891 Datasheet(PDF) 32 Page - Linear Technology

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