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LTC3787

3787fc

APPLICATIONS INFORMATION

Sense 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). When the ILIM pin is grounded, floating or

tied to INTVCC, the maximum threshold is set to 50mV,

75mV or 100mV, respectively. The current comparator

threshold sets the peak of the inductor current, yielding

a maximum average inductor current, IMAX, equal to the

peak value less half the peak-to-peak ripple current, ΔIL.

To calculate the sense resistor value, use the equation:

SENSE =

SENSE(MAX)

MAX +

ΔI

The actual value of IMAX for each channel depends on the

required output current IOUT(MAX) and can be calculated

using:

MAX =

OUT(MAX)

⎛

⎝⎜

⎞

⎠⎟

•

OUT

⎛

⎝⎜

⎞

⎠⎟

When using the controller in low VIN and very high voltage

output applications, the maximum inductor current and

correspondingly the maximum output current level will

be reduced due to the internal compensation required to

meet stability criterion for boost regulators operating at

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

Typical Performance Characteristics section to estimate

this reduction in peak inductor current level depending

upon the operating duty factor.

Inductor DCR Sensing

For applications requiring the highest possible efficiency

at high load currents, the LTC3787 is capable of sensing

the voltage drop across the inductor DCR, as shown in

Figure 2b. The DCR of the inductor can be less than 1mΩ

for high current inductors. In a high current application

requiring such an inductor, conduction loss through a

sense resistor could reduce the efficiency by a few percent

compared to 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

the inductor DCR multiplied by R2/(R1 + R2). R2 scales the

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 induct-

or value calculation section, the target sense resistor

value is:

SENSE(EQUIV) =

SENSE(MAX)

MAX +

ΔI

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimum value for the maximum current sense threshold

(VSENSE(MAX)).

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

use the manufacturer’s maximum value, usually given at

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

coefficient of resistance, which is approximately 0.4%/°C.

Aconservativevalueforthemaximuminductortemperature

(TL(MAX)) is 100°C.

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