LTM4677 datasheet(41/116 Pages) AD

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LTC3883/LTC3883-1

Rev. F

For more information www.analog.com

APPLICATIONS INFORMATION

purely inductive component. It was measured using two

scope probes and waveform math to obtain a differential

measurement. Based on additional measurements of the

inductor ripple current and the on-time, tON, and off-time,

tOFF,ofthetopswitch,thevalueoftheparasiticinductance

was determined to be 0.5nH using the equation:

ESL =

VESL(STEP)

∆IL

•

tON • tOFF

tON + tOFF

(1)

If the RC time constant is chosen to be close to the para-

sitic inductance divided by the sense resistor (L/R), the

resultant waveform looks resistive, as shown in Figure 20.

For applications using low maximum sense voltages,

check the sense resistor manufacturer’s data sheet for

information about parasitic inductance. In the absence of

data, measure the voltage drop directly across the sense

resistor to extract the magnitude of the ESL step and use

Equation1todeterminetheESL.However,donotoverfilter

the signal. Keep the RC time constant less than or equal to

the inductor time constant to maintain a sufficient ripple

voltage on VRSENSE for optimal operation of the current

loop controller.

INDUCTOR DCR CURRENT SENSING

For applications requiring the highest possible efficiency

at high load currents, the LTC3883 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 18a. The DCR of the inductor represents the small

amount of DC winding resistance of the copper, which

can be less than 1mΩ for today’s low value, high current

inductors. In a high current application requiring such an

inductor, conduction loss through a sense resistor would

cost a few points of efficiency compared to DCR sensing.

If the external (R1 + R3)||R2 • C1 time constant is chosen

to be exactly equal to the L/DCR time constant, assuming

R1 = R3, the voltage drop across the external capacitor,C1,

is equal to the drop across the inductor DCR multiplied by

R2/(R1+R2+R3). R2 scales the voltage across the sense

terminals for applications where the DCR is greater than

the target sense resistor value. The DCR value is entered

as the IOUT_CAL_GAIN in mΩ unless R2 is required. If

R2 is used:

IOUT _ CAL _ GAIN = DCR •

R1+ R2 + R3

If there is no need to attenuate the signal, R2 can be

removed. To properly dimension the external filter com-

ponents, the DCR of the inductor must be known. It

can be measured using an accurate RLC meter, but the

DCR tolerance is not always the same and varies with

temperature. Consult the manufacturers’ data sheets

for detailed information. The LTC3883 will account for

temperature variation if the correct parameter is entered

into the MFR_IOUT_CAL_GAIN_TC command. Typically

the resistance has a 3900ppm/°C coefficient.

C2 can be optimized for a flat frequency response, assum-

ing R1 = R3 by the following equation:

C2 = [2R1 • R2 • C1-L/DCR • (2R1+R2)]/R12

Using the inductor ripple current value from the Inductor

ValueCalculationsection,thetargetsenseresistorvalueis:

RSENSE(EQUIV) =

VSENSE(MAX)

IMAX +

∆IL

Figure 19. Voltage Measured Directly Across RSENSE

Figure 20. Voltage Measured After the RSENSE Filter

500ns/DIV

VSENSE

20mV/DIV

3883 F19

VESL(STEP)

500ns/DIV

VSENSE

20mV/DIV

3883 F20

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