LT8705 datasheet(29/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

5. Is the INTVCC decoupling capacitor connected close

to the IC, between the INTVCC and the power ground

pins? This capacitor carries the MOSFET drivers’ cur-

rent peaks. An additional 1μF ceramic capacitor placed

immediately next to the INTVCC and PGND pins can help

improve noise performance substantially.

6. Keep the switching nodes (SW1, SW2), top gate nodes

(TG1, TG2) and boost nodes (BOOST1, BOOST2) away

from sensitive small-signal nodes, especially from

the opposites channel’s voltage and current sensing

feedback pins. All of these nodes have very large and

fast moving signals and, therefore, should be kept on

the output side of the LTC3784 and occupy a minimal

PC trace area.

7. Use a modified “star ground” technique: a low imped-

ance, large copper area central grounding point on

the same side of the PC board as the input and output

capacitors with tie-ins for the bottom of the INTVCC

decouplingcapacitor,thebottomofthevoltagefeedback

resistive divider and the SGND pin of the IC.

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.

Thedutycyclepercentageshouldbemaintainedfromcycle

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 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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