User Guide

DC3109A

EVALUATION BOARD HARDWARE

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Rev. 0 | 9 of 12

INTRODUCTION TO THE DC3109A

The DC3109A demonstration circuit features the LTC3306, a 1.75

A, synchronous step-down Regulator in 1.6 mm x 1 mm WLCSP.

The DC3109A has two options. The DC3109A-A features the

LTC3306A, operating in Burst Mode. The DC3109A-C features the

LTC3306C, operating in forced continuous mode at 2 MHz.

The LTC3306 is a monolithic, constant frequency, current mode

step-down DC-DC converter. A fixed frequency oscillator turns on

the internal top power switch at the beginning of each clock cycle.

Current in the inductor then increases until the top switch compara-

tor trips and turns off the top power switch. If the EN pin is low, the

LTC3306 is in shutdown and in a low quiescent current state. When

the EN pin is above its threshold, the switching regulator is enabled.

The maximum allowable operating frequency is influenced by the

minimum on time of the top switch, the ratio of VOUT to VIN and

the available inductor values. Equation (3) shows the maximum

allowable operating frequency:

fSWMAX = VOUT

VINMAX ×TONMIN

(3)

ACCURATELY MEASURING OUTPUT RIPPLE

OF THE LTC3306

With the fast edge rates of the circuit, high frequency noise can be

observed when measuring the output voltage with 1 mΩ terminated

oscilloscope probes. To better view the high frequency output

ripple, TP1, an U.FL, RECEPT, ST SMD, 0 Hz to 6 GHz, 50 Ω

connector, should be populated. The output ripple should be tested

with oscilloscopes of 400 MHz bandwidth and above, with a 50 Ω

coax cable connected as close to the output capacitor as possible,

and with oscilloscope channel terminated to 50 Ω at the scope.

This helps to reduce the noise coupling onto and displaying on the

scope.

The high frequency spikes are partially attributed to the interwinding

capacitance of the inductor and the voltage step is partially attribut-

ed to the inductance in the output capacitors. This can be reduced

by choosing low ESL capacitors or adding small low ESL capacitors

in parallel to the output capacitors as close to the inductor as

possible. To reduce the high frequency noise, optional C11 should

be populated with a 10 nF, 0402, X2Y capacitor. Adding capacitors

close to the load creates a π filter between the output capacitors,

trace inductance, and load decoupling capacitors and also helps to

reduce the ripple.

Figure 13. Output Ripple Without C11

Figure 14. Output Ripple with C11