LTM4609 datasheet(20/44 Pages) LINER

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LTC7812

7812fc

For more information www.linear.com/LTC7812

APPLICATIONS INFORMATION

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 2b), resistor R1 should be placed close to

the switching node, to prevent noise from coupling into

sensitive small-signal nodes.

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. 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, DIL. To calculate the

sense resistor value, use the equation:

RSENSE =

VSENSE(MAX)

IMAX +

DIL

When using the controller in very high duty cycle condi-

tions, the maximum output current level will be reduced

due to the internal compensation required to meet stabil-

ity criterion for switching regulators operating at greater

than 50% duty factor. A curve is provided in the Typical

Performance Characteristics section to estimate this re-

duction in peak output current level depending upon the

operating duty factor.

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC7812 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, conduction loss through a sense resistor would

costseveralpointsofefficiencycomparedtoDCRsensing.

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.

2b. Using the Inductor DCR to Sense Current

2a. Using a Resistor to Sense Current

Figure 2. Current Sensing Methods

7812 F02a

LTC7812

INTVCC

BOOST

SENSE1+

(SENSE2–)

SENSE1–

(SENSE2+)

SGND

VIN1

(VOUT2)

VOUT1

(VIN2)

RSENSE

CAP

PLACED NEAR SENSE PINS

7812 F02b

LTC7812

INTVCC

BOOST

SENSE1+

(SENSE2–)

SENSE1–

(SENSE2+)

SGND

VIN1

(VOUT2)

VOUT1

(VIN2)

C1*

*PLACE C1 NEAR SENSE PINS

RSENSE(EQ) = DCR(R2/(R1+R2))

DCR

INDUCTOR

(R1||R2) • C1 = L/DCR

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