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LTC3853

3853fc

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For example, Figure 3 illustrates the voltage waveform

across a 2mΩ sense resistor with a 2010 footprint for the

1.2V/15Aconverteroperatingat100%load.Thewaveform

is the superposition of a purely resistive component and a

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 and off-time of

the top switch, the value of the parasitic inductance was

determined to be 0.5nH using the equation:

ESL

VESL(STEP)

∆IL

tON • tOFF

tON + tOFF

IftheRCtimeconstantischosentobeclosetotheparasitic

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

ing waveform looks resistive again, as shown in Figure 4.

For applications using low maximum sense voltages,

check the sense resistor manufacturer’s data sheet for

information about parasitic inductance. In the absence of

APPLICATIONS INFORMATION

data, measure the voltage drop directly across the sense

resistor to extract the magnitude of the ESL step and use

the equation above to determine the ESL. However, do not

over-filter. Keep the RC time constant less than or equal

to the inductor time constant to maintain a high enough

ripple voltage on VRSENSE.

The above generally applies to high density/high current

applications where I(MAX) > 10A and low values of induc-

tors are used. For applications where I(MAX) < 10A, set

RF to 10Ω and CF to 1000pF. This will provide a good

starting point.

The filter components need to be placed close to the IC.

The positive and negative sense traces need to be routed

as a differential pair and Kelvin connected to the sense

resistor.

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC3853 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 2b. 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

costseveralpointsofefficiencycomparedtoDCRsensing.

If the external R1||R2 • C1 time constant is chosen to be

exactly equal to the L/DCR time constant, the voltage drop

across the external capacitor is equal to the drop across

theinductorDCRmultipliedbyR2/(R1+R2).R2scalesthe

voltage across the sense terminals for applications where

the DCR is greater than the target sense resistor value.

To properly dimension the external filter components, the

DCR of the inductor must be known. It can be measured

using a good RLC meter, but the DCR tolerance is not

always the same and varies with temperature; consult

the manufacturers’ data sheets for detailed information.

Using the inductor ripple current value from the Inductor

Value Calculation section, the target sense resistor value is:

RSENSE(EQUIV) =

VSENSE(MAX)

I(MAX) +

∆IL

500ns/DIV

VSENSE

20mV/DIV

3853 F03

VESL(STEP)

500ns/DIV

VSENSE

20mV/DIV

3853 F04

Figure 3. Voltage Waveform Measured

Directly Across The Sense Resistor

Figure 4. Voltage Waveform Measured After the

Sense Resistor Filter. CF = 1000pF, RF = 100Ω

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