LTC7841 datasheet(24/48 Pages) AD

24 / 48 page

LTC7841

Rev. 0

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APPLICATIONS INFORMATION

The Typical Application on the first page is a basic LTC7841

application circuit. The LTC7841 can be configured to use

either inductor DCR (DC resistance) sensing or a discrete

sense resistor (RSENSE) for current sensing. The choice

between the two current sensing schemes is largely a

design trade-off between cost, power consumption and

accuracy. DCR sensing is becoming popular because it

does not require current sensing resistors and is more

power-efficient, especially in high current applications.

However, current sensing resistors provide the most accu-

rate current limits and current readback(ADC) for the con-

troller. Other external component selection is driven by the

load requirement, and begins with the selection of RSENSE

(if RSENSE is used) and inductor value. Next, the power

MOSFETs are selected. Finally, input and output capacitors

are selected. Note that the two controller channels of the

LTC7841 should be designed with the same components.

SENSE+ and SENSE– Pins

The SENSE+ and SENSE– pins are the inputs to the cur-

rent comparators. The current sense resistor is normally

placed at the input of the boost controller in series with

the inductor.

The SENSE+ pin also provides power to the current com-

parator. It draws ~200μA during normal operation. There

is a small base current of less than 1μA that flows into

the SENSE– pin. The high impedance SENSE– input to the

current comparators allows accurate DCR sensing.

Filter components mutual to the sense lines should be

placed close to the LTC7841, and the sense lines should

run close together to a Kelvin connection underneath the

current sense element (shown in Figure 8). 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 9b), sense resistor R1 should be placed

close to the switching node, to prevent noise from cou-

pling into sensitive small-signal nodes.

Figure 8.

VIN

TO SENSE FILTER,

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

7841 F08

Sense Lines Placement with Inductor or Sense Resistor

Figure 9. Two Different Methods of Sensing Current

(a) Using a Resistor to Sense Current

(b) Using the Inductor DCR to Sense Current

LTC7841

INTVCC

BOOST

SENSE+

SENSE–

(OPTIONAL)

VBIAS

VIN

VOUT

SGND

7841 F09a

INDUCTOR

DCR

LTC7841

INTVCC

BOOST

SENSE+

SENSE–

VBIAS

VIN

VOUT

PLACE C1 NEAR SENSE PINS

SGND

7841 F09b

(R1||R2) • C1 =

DCR

RSENSE(EQ) = DCR •

R1 + R2

LTC7841 Datasheet(PDF) 24 Page - Analog Devices

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