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LTC3807

3807fa

For more information www.linear.com/LTC3807

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 comparator has a maximum threshold

VSENSE(MAX) determined by the ILIM setting. 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 +

DIL

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimumvaluefortheMaximumCurrentSenseThreshold

(VSENSE(MAX))intheElectricalCharacteristicstable(30mV,

50mV or 75mV, depending on the state of the ILIM pin).

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due

to the internal compensation required to meet stability

criterion for buck regulators operating at greater than

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

mance Characteristics section to estimate this reduction

in peak inductor current depending upon the operating

duty factor.

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC3807 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 resistance of the copper wire, which can be

lessthan1mΩfortoday’slowvalue,highcurrentinductors.

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.

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.

Using the inductor ripple current value from the Inductor

Value Calculation section, the target sense resistor

value is:

RSENSE(EQUIV) =

VSENSE(MAX)

IMAX +

DIL

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimumvaluefortheMaximumCurrentSenseThreshold

(VSENSE(MAX))intheElectricalCharacteristicstable(30mV,

50mV or 75mV, depending on the state of the ILIM pin).

Next, determine the DCR of the inductor. When provided,

use the manufacturer’s maximum value, usually given at

20°C. Increase this value to account for the temperature

coefficient of copper resistance, which is approximately

0.4%/°C. A conservative value for TL(MAX) is 100°C.

To scale the maximum inductor DCR to the desired sense

resistor value (RD), use the divider ratio:

RD =

RSENSE(EQUIV)

DCRMAX atTL(MAX)

C1 is usually selected to be in the range of 0.1μF to 0.47μF.

ThisforcesR1||R2toaround2k,reducingerrorthatmight

have been caused by the SENSE+ pin’s ±1μA current.

APPLICATIONS INFORMATION

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