LTC4449 datasheet(49/108 Pages) LINER

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LTC3882-1

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

RL1

SW1

RSENSE1

VOUT1

COUT1

VIN

CIN

RIN

RL0

BOLD LINES INDICATE

HIGH SWITCHING

CURRENT. KEEP LINES

TO A MINIMUM LENGTH.

SW0

38821 F24

RSENSE0

VOUT0

COUT0

Figure 24. High Frequency Paths and Branch Current Waveforms

for most inductors suitable to LTC3882-1 applications, is

between 0.3mΩ and 1mΩ. If the filter RC time constant is

chosen to be exactly equal to the L/DCR time constant of

the inductor, the voltage drop across the external capaci-

tor is equal to the voltage drop across the inductor DCR.

Check the manufacturer’s data sheet for specifications

regarding the inductor DCR in order to properly dimension

the external filter components. The DCR of the inductor

can also be measured using a good RLC meter.

Use the nominal or measured value of DCR to program

IOUT_CAL_GAIN (in mΩ). The temperature coefficient

of the inductor’s DCR is typically high, like copper. Again,

consult the manufacturer’s data sheet. The LTC3882-1 can

adjust for this non-ideality if the correct MFR_IOUT_CAL_

GAIN_TC value is programmed. Typically this coefficient

is around 3900ppm/°C.

Resistor R1 should be placed close to the switch node,

to prevent noise from coupling into sensitive small-signal

nodes. Capacitor C1 should be placed close to the IC pins.

Anexampleofdiscreteresistorsensingofoutputcurrentis

shown in Figure 26. Previously, the parasitic inductance of

the sense resistor could represent a relatively small error.

New high current density solutions may utilize low sense

resistor values producing sense voltages less than 20mV.

In addition, inductor ripple currents greater than 50%

with operation up to 1MHz are becoming more common.

Under these conditions, the voltage drop across the sense

resistor’s parasitic inductance is no longer negligible. An

RC filter can be used to extract the resistive component

of the current sense signal in the presence of parasitic

inductance. For example, Figure 27 illustrates the voltage

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