LT3757 datasheet(28/36 Pages) LINER

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LTC3787

3787fc

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 VIN from its nominal level to verify operation with

high duty cycle. Check the operation of the undervoltage

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.

An embarrassing problem which can be missed in an oth-

erwise 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.

APPLICATIONS INFORMATION

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