LTC7813 datasheet(16/36 Pages) LINER

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LTC7821

Rev A

For more information www.analog.com

APPLICATIONS INFORMATION

The “Typical Application” on the first page is a basic

LTC7821 application circuit. The LTC7821 can be con-

figured to use either DCR (inductor resistance) sensing

or resistor sensing. The choice between the two current

sensing schemes is largely a design trade-off between

cost, power consumption, and accuracy. DCR sensing is

popular because it saves an expensive current sensing

resistor and is more power efficient, especially in high

current applications. However, a current sensing resistor

providesthemostaccuratecurrentlimitfortheapplication.

Other external component selection is driven by the load

requirement, and begins with the selection of RSENSE (if

RSENSE is used). Next CFLY, CMID, and the power MOSFETs

are selected, followed by the input and output capacitors.

In addition to the power level, switching frequency plays

a role in selecting the balancing capacitance (CFLY and

CMID) and the inductance of the inductor.

ISNS+ and ISNS– Pins

The ISNS+ and ISNS– pins are the inputs to the current

comparators. The common mode input voltage range of

the current comparators is 0V to 36V. Both ISNS pins are

high impedance inputs with small leakage currents of less

than 1.2µA. When the ISNS pins ramp up from 0V to 2.4V,

small base currents flow out of the ISNS pins. When the

ISNS pins ramp down from 36V to 2V, the small base cur-

rents flow into the ISNS pins. The high impedance inputs

to the current comparators allow accurate DCR sensing.

However, care must be taken not to float these pins during

normal operation.

Filter components mutual to the sense lines should be

placed close to the LTC7821, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 6). Sensing cur-

rent elsewhere can effectively add parasitic inductance

and capacitance to the current sense element, degrading

the information at the sense terminals and making the

programmed current limit unpredictable. If DCR sensing

is used (Figure 7b), sense resistor R1 should be placed

closetotheswitchingnode,topreventnoisefromcoupling

into sensitive small-signal nodes. The capacitor C1 should

be placed close to the IC pins.

Resistor Current Sensing

ThehybridarchitectureoftheLTC7821generatesavoltage

rail of half the VIN supply to the step-down control loop.

Therefore the current ripple calculation and its operating

duty cycle is referred to the voltage at the MID pin which

is approximately at VIN/2.

A typical sensing circuit using a discrete resistor is shown

in Figure 7a. RSENSE is chosen based on the required

output current.

The current comparator has a maximum threshold of

50mV and its inputs have a common mode range of 0V

to 36V. The current comparator threshold sets the peak of

the inductor current, yielding a maximum average output

current IMAX equal to the peak value less half the peak-to-

peak ripple current, ΔIL. To calculate the sense resistor

value, use the equation:

RSENSE =

50mV

I(MAX)+

ΔIL

Because of possible PCB noise in the current sensing

loop, the AC current sensing ripple of ΔVSENSE = ΔIL •

RSENSE also needs to be verified in the design to get a

good signal-to-noise ratio.

In general, for a reasonably good PCB layout, a 10mV

ΔVSENSEvoltageisrecommendedasaconservativenumber

tostartwith,eitherforRSENSEorDCRsensingapplications,

for duty cycles less than 40%. For applications where

the inductor’s ripple current could be greater than 50%

and operating at 750kHz and above, the sense resistor’s

parasitic inductance has to be taken into consideration

since its contribution is no longer negligible.

Figure 6. Sense Lines Placement with Sense Resistor

RSENSE

COUT

TO SENSE FILTER,

NEXT TO THE CONTROLLER

7821 F06

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