LTC3859AL datasheet(33/38 Pages) LINER

33 / 38 page

LTC3899

3899fa

For more information www.linear.com/LTC3899

APPLICATIONS INFORMATION

4. Are the SENSE– and SENSE+ leads routed together with

minimumPCtracespacing?Thefiltercapacitorbetween

SENSE+ and SENSE– should be as close as possible

to the IC. Ensure accurate current sensing with Kelvin

connections at the SENSE resistor.

5. Is the DRVCCanddecouplingcapacitorconnectedclose

to the IC, between the DRVCC and the ground pin? This

capacitor carries the MOSFET drivers’ current peaks.

6. Keep the switching nodes (SW1, SW2, SW3), top gate

(TG1, TG2, TG3), and boost nodes (BOOST1, BOOST2,

BOOST3) 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 LTC3899 and occupy

minimum PC trace area.

7. Useamodifiedstargroundtechnique:alowimpedance,

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 DRVCC decoupling

capacitor, the bottom of the voltage feedback resistive

divider and the GND pin of the IC.

PC Board Layout Debugging

Start with one controller 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

totheinternaloscillatorandprobetheactualoutputvoltage

as well. Check for proper performance over the operating

voltage and current range expected in the application. The

frequencyofoperationshouldbemaintainedovertheinput

voltage range down to dropout and until the output load

drops below the low current operation threshold—typi-

cally 25% of the maximum designed current level in Burst

Mode operation.

Thedutycyclepercentageshouldbemaintainedfromcycle

tocycleinawell-designed,lownoisePCBimplementation.

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 should multiple controllers be turned on at the same

time. A particularly difficult region of operation is when

one buck channel is nearing its current comparator trip

point when the other buck 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

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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