LTC7899 datasheet(42/50 Pages) AD

42 / 50 page

Data Sheet

LTC7899

analog.com

Rev. 0

42 of 50

The duty cycle percentage is maintained from cycle to cycle in a well-designed, low noise PCB implementation.

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 an improper PCB layout if

regulator bandwidth optimization is not required.

Reduce |VIN–| from its nominal level to verify operation of the regulator at maximum 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 output currents or only at higher |VIN–| voltages. If problems

coincide with high |VIN–| and low output currents, look for capacitive coupling between the BOOST, SW, TGxx, and

possibly BGxx connections and the sensitive voltage and current pins. Place the capacitor across the current sensing

pins next 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 |VIN–| 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 the common GND path voltage pickup between these

components and the |VIN–| pin of the IC.

A problem that 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 is maintained, but the advantages

of current mode control are not realized. Compensation of the voltage loop is more sensitive to component selection.

This behavior can be investigated by temporarily shorting out the current sensing resistor. The regulator maintains

control of the output voltage.

Figure 43. Branch Current Waveforms

GND

VOUT

VIN–

RSENSE

CIN

COUT

RIN

LTC7899 Datasheet(PDF) 42 Page - Analog Devices