LTC3834-1 Datasheet, PDF(23/28 Page) Linear Technology – 30μA IQ Synchronous Step-Down Controller

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LTC3834-1 Datasheet, PDF (23/28 Pages) Linear Technology – 30μA IQ Synchronous Step-Down Controller

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APPLICATIO S I FOR ATIO

LTC3834-1

VIN

RIN

CIN

RSENSE

VOUT

COUT

RL1

3834-1 F09

BOLD LINES INDICATE HIGH SWITCHING

CURRENT. KEEP LINES TO A MINIMUM LENGTH.

Figure 9. Branch Current Waveforms

PC Board Layout Debugging

It is helpful to use a DC-50MHz current probe to monitor

the current in the inductor while testing the circuit. Moni-

tor 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

application. The frequency of operation should be main-

tained over the input voltage range down to dropout and

until the output load drops below the low current opera-

tion thresholdâtypically 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 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. Over-

compensation of the loop can be used to tame a poor PC

layout if regulator bandwidth optimization is not required.

Reduce VIN from its nominal level to verify operation of the

regulator in dropout. Check the operation of the under-

voltage lockout circuit by further lowering VIN while moni-

toring the outputs to verify operation.

Investigate whether any problems exist only at higher

output currents or only at higher input voltages. If prob-

lems 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 en-

countered 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 com-

mon ground path voltage pickup between these compo-

nents 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 back-

wards. 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 tempo-

rarily shorting out the current sensing resistorâdonât

worry, the regulator will still maintain control of the

output voltage.

38341f

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