LTC7813 datasheet(17/36 Pages) LINER

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LTC7821

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

In an application where the sense resistor’s parasitic in-

ductancecontributionisnegligible,asmallRCfilterplaced

near the IC is enough to reduce the effects of capacitive

and inductive noise coupled in the sense traces on the

PCB. A typical filter consists of two series 10Ω resistors

connected to a parallel 1000pF capacitor, resulting in a

time constant of 20ns. However, the same RC filter with

minor modifications can be used to extract the resistive

component of the current sense signal in the presence

of significant parasitic inductance in the sense resistor.

For example, Figure 8 illustrates the voltage waveform

across a 1mΩ sense resistor with a 2010 footprint for the

12V/20A converter operating at 100% load. The waveform

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 0.3nH using the equation:

ESL =

VESL(STEP)

ΔIL

•

tON• tOFF

tON+ tOFF

⎛

⎝

⎜

⎞

⎠

⎟

IftheRCtimeconstantischosentobeclosetotheparasitic

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

ing waveform looks resistive again, as shown in Figure 9.

Figure 7. Two Different Methods of Sensing Current

Figure 8. Voltage Waveform Measured Directly

Across the Sense Resistor

Figure 9. Voltage Waveform Measured After the

Sense Resistor Filter, CF = 1nF, RF = 100Ω

COUT

7821 F07a

ESL

ISNS+

ISNS–

PGND

INTVCC

BG2

SW3

BOOST3

TG2

MID_SENSE

MID

VOUT

SENSE RESISTOR

PLUS PARASITIC

INDUCTANCE

CF • 2RF < ESL/RS

POLE-ZERO

CANCELLATION

FILTER COMPONENTS

PLACED NEAR SENSE PINS

(7a) USING A RESISTOR TO SENSE CURRENT

COUT

7821 F07b

DCR

ISNS+

ISNS–

PGND

INTVCC

BG2

SW3

BOOST3

TG2

MID_SENSE

MID

VOUT

INDUCTOR

PLACE R1 NEXT

TO INDUCTOR

PLACE C1 NEAR

SNS+, SNS– PINS

(7b) USING INDUCTOR DCR TO SENSE CURRENT

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

500ns/DIV

VSENSE

5mV/DIV

7821 F08

500ns/DIV

VSENSE

5mV/DIV

7821 F09

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