LTC7103 datasheet(21/48 Pages) AD

21 / 48 page

LTC7871

Rev. 0

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provides better efficiency at heavy loads. To maintain a

good signal-to-noise ratio for the current sense signal,

use a minimum of 10mV between SNSA+ and SNS– pins

or the equivalent of 2mV ripple on the current sense signal

for duty cycles less than 40%. The actual ripple voltage

across SNSA+ and SNS– pins will be determined by the

following equation:

∆VSENSE =

VLOW

VHIGH

•

VHIGH – VLOW

R1• C1• fOSC

Figure 3.

LTC7871

7871 F03

SNS–

SNSD+

SNSA+

VLOW

Inductor DCR Sensing

Sensing Using an RSENSE Resistor

The LTC7871 can be used with an external RSENSE resis-

tor to sense current accurately. The external components

required to accomplish this are shown in Figure 4. The

SNSD+ pin senses directly across the RS resistor through

R3 and C3 network. The R1, R2, and C1 network provide

the current signal path to the SNSA+ pin. Internally the

signals from the AC and DC paths are combined for accu-

rate current sensing and low jitter performance. Resistor

R2 is used to divide down the DC component of the signal

seen by SNSA+ due to the DCR of the inductor. As a rule

of thumb, R2 needs to be 10 times smaller than R1 so

the DCR value can be safely ignored.

Figure 4.

LTC7871

7871 F04

SNSD+

SNS–

SNSA+

VLOW

RSENSE Resistor Sensing

constant matched to L/DCR of the inductor. For a specific

output requirement, choose the inductor with the DCR

that satisfies the maximum desired sense voltage, and

uses the relationship of the sense pin filters to output

inductor characteristics as depicted below.

DCR

VSENSE(MAX)

IMAX +

∆IL

DCR

= 5 • R1• C1= R2 • C2

where:

VSENSE(MAX) is the maximum sense voltage for a given

ILIM threshold

∆IL is the Inductor ripple current

L and DCR are the output inductor characteristics

R1 • C1 is the filter time constant of the SNSA+ pin

R2 • C2 is the filter time constant of the SNSD+ pin

To ensure that the load current will be delivered over the

full operating temperature range, the temperature coef-

ficient of DCR resistance, approximately 0.4%/°C, should

be taken into account.

Typically, C1 and C2 are selected in the range of 0.047μF

to 0.47μF. If C1 and C2 are chosen to be 0.1μF, and an

inductor of 10μH with 2mΩ DCR is selected, R1 and R2

will be 10k and 49.9k, respectively. The bias current at

SNSD+ and SNSA+ is less than 1μA, and it introduces a

small error to the sense signal.

There will be some power loss in R1 that relates to the

duty cycle, and will be the most in continuous mode at

the maximum VHIGH voltage:

PLOSS R

( ) =

VHIGH(MAX) – VLOW

(

)• VLOW

Ensure that R1 has a power rating higher than this value.

Care has to be taken for voltage coefficients of these resis-

tors at high VHIGH voltages. Multiple resistors can be used

in series to minimize this effect. However, DCR sensing

eliminates the conduction loss of a sense resistor; it also

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