LTC7802EUFDM datasheet(29/34 Pages) AD

29 / 34 page

LTC7802

Rev. 0

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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

other 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 LTC7802 and occupy minimum PC trace area.

Minimize the inductance of the TG and BG gate drive

traces and their respective return paths to the controller

IC (SW and GND) by using wide traces and multiple

parallel vias.

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

decoupling capacitor, the bottom of the voltage feed-

back resistive divider and the GND pin of the IC.

For more detailed layout guidance, see Analog Devices

Application Notes AN136 “PCB Layout Considerations

for Non-Isolated Switching Power Supplies” and AN139

“Power Supply Layout and EMI”.

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

voltage as well. Check for proper performance over the

operating voltage and current range expected in the appli-

cation. 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 25% of the maximum designed cur-

rent 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.

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 indi-

vidual performance should both controllers be turned on

at the same time. A particularly difficult region of opera-

tion 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

undervoltage lockout circuit by further lowering VIN while

monitoring the outputs to verify operation. Investigate

whether any problems exist only at higher output cur-

rents 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 dif-

ferential 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, the top MOSFET, and the bottom

MOSFET components to the sensitive current and voltage

sensing traces. In addition, investigate common ground

path voltage pickup between these components and the

GND 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.

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