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LTC7813

7813f

For more information www.linear.com/LTC7813

applicaTions inForMaTion

Low Value Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 2a. RSENSE is chosen based on the required

output current.

The current comparators have a maximum threshold

VSENSE(MAX) of 50mV, 75mV or 100mV. The current

comparator threshold voltage sets the peak of the induc-

tor current, yielding a maximum average output current,

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

ripple current, ∆IL. To calculate the sense resistor value,

use the equation:

RSENSE =

VSENSE(MAX)

IMAX +

∆IL

When using the buck controller in very low dropout condi-

tions, the maximum output current level will be reduced

due to the internal compensation required to meet stability

criteria for buck regulators operating at greater than 50%

duty factor. A curve is provided in the Typical Performance

Characteristics section to estimate this reduction in peak

inductorcurrentdependingupontheoperatingdutyfactor.

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC7813 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 2b. The DCR of the inductor represents the small

amount of DC winding resistance of the copper, which

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

inductors. In a high current application requiring such

an inductor, power loss through a sense resistor would

cost several points of efficiency compared to inductor

DCR sensing.

7813 F01

TO SENSE FILTER

NEXT TO THE CONTROLLER

INDUCTOR OR RSENSE

CURRENT FLOW

Figure 1. Sense Lines Placement with Inductor or Sense Resistor

Figure 2. Current Sensing Methods

7813 F02a

LTC7813

BOOST

SENSE1+

(SENSE2–)

SENSE1–

(SENSE2+)

GND

VIN1

(VOUT2)

VOUT1

(VIN2)

RSENSE

CAP

PLACED NEAR SENSE PINS

7813 F02b

LTC7813

BOOST

SENSE1+

(SENSE2–)

SENSE1–

(SENSE2+)

GND

VIN1

(VOUT2)

VOUT1

(VIN2)

C1*

*PLACE C1 NEAR SENSE PINS RSENSE(EQ) = DCR(R2/(R1+R2))

DCR

INDUCTOR

(R1||R2) • C1 = L/DCR

(2a) Using a Resistor to Sense Current

(2b) Using the Inductor DCR to Sense Current

If the external (R1||R2) • C1 time constant is chosen to be

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

across the external capacitor is equal to the drop across

theinductorDCRmultipliedbyR2/(R1+R2).R2scalesthe

voltage across the sense terminals for applications where

the DCR is greater than the target sense resistor value.

To properly dimension the external filter components, the

DCR of the inductor must be known. It can be measured

using a good RLC meter, but the DCR tolerance is not

always the same and varies with temperature; consult

the manufacturers’ data sheets for detailed information.

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