LT8551 datasheet(24/32 Pages) AD

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LT8551

Rev 0

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APPLICATIONS INFORMATION

resistor (RSENSE) or inductor DC resistance (DCR). The

choice between the two current sensing schemes is

largely a design trade-off between cost, accuracy and

power consumption. DCR is becoming popular since it

saves expensive current sensing resistors and is more

power efficient, especially in high current applications.

However, current sensing resistors provide the most accu-

rate current limits for the controller. The ISPx/ISNx (i.e.

ISP, ISP1/2/3/4, ISN, ISN1/2/3/4) pins are the inputs to

the current sense amplifiers. The common mode input

voltage range of the current sense amplifier is from 0V

to 80V. The current sense resistor is normally placed at

the input of the LT8551 in series with the inductor. Each

ISPx pin also provides power to the current sense ampli-

fier when ISPx is higher than 1.1V (typical). It draws 0µA

to 100μA during normal operation. The ISNx pin current

is less than 1µA. The high impedance ISNx input to the

current amplifier allows accurate DCR current sensing.

Low Value Resistor Current Sensing: A typical RSENSE

inductor current sensing is shown in Figure 16a. The filter

components (RF, CF) need to be placed near the LT8551.

RF values greater than 100Ω should be avoided as this

may increase offset voltage. The filter time constant

(RF • CF) should be no more than 30nS. The positive and

negative sense traces need to be routed as a differential

pair close together and Kelvin (4-wire) connected under-

neath the sense resistor as shown in Figure 17. RSENSE

is chosen based on the maximum output current. Given

the maximum input current, I(MAX), maximum sense volt-

age, VSENSE(MAX), and maximum inductor ripple current,

∆IL(MAX), the value of the RSENSE can be chosen from the

following formula:

RSENSE =

VSENSE MAX

(

)

IMAX +

∆IL

IMAX depends on the required output current IOUT(MAX)

and can be calculated using:

IMAX =IOUT MAX

(

) •

VOUT

VIN

⎛

⎝⎜

⎞

⎠⎟

FILTER COMPONENTS

CLOSE TO THE ISPx/ISNx PINS

RSENSE

VIN

KELVIN SENSE

8551 F16a

ISNx

ISPx

MASTER

LT8551

R2 AND C CLOSE TO THE ISPx/ISNx PINS

R1 CLOSE TO THE SW NODE

OPTIONAL

DCR

VIN

KELVIN SENSE

8551 F16b

ISNx

ISPx

MASTER

LT8551

Figure 16. Inductor Current Sense Filter

DCR Inductor Current Sensing: For applications requir-

ing the highest possible efficiency, the LT8551 is capable

of sensing the voltage drop across the inductor DCR, as

shown in Figure 16b. The DCR of the inductor represents

the small amount of DC winding resistance, which can

be less than 1mΩ for today’s low value, high current

inductors. In high current applications requiring such an

inductor, conduction loss through a sense resistor would

cost several points of efficiency compared to DCR sens-

ing. The inductor DCR is sensed by connecting an RC

filter across the inductor. This filter typically consists of

one or two resistors (R1 and R2) and one capacitor (C).

If the external (R1||R2)•C time constant is chosen to be

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

across C will be:

VSENSE =IL •DCR•

+R2

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