LTC3370 datasheet(24/44 Pages) LINER

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LTC3372

Rev. A

For more information www.analog.com

APPLICATIONS INFORMATION

Resistor Current Sensing

A typical sensing circuit using a discrete resistor is shown

in Figure 3a. RSENSE is chosen based on the required

output current.

This LTC3372’s current comparator has a fixed maximum

current limit threshold of 75mV (typical). 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 +

ΔIL

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimum value (68mV) for the Maximum Current Sense

Threshold (VSENSE(MAX)) in the Electrical Characteristics

table.

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. The maximum current sense threshold

vs duty cycle curve is provided in the Typical Performance

Characteristics section to estimate this reduction in peak

inductorcurrentdependingupontheoperatingdutyfactor.

Inductor DCR Current Sensing

Forapplicationsrequiringthehighestpossibleefficiencyat

high load currents, the LTC3372’s HV controller is capable

of sensing the voltage drop across the inductor DCR, as

showninFigure 3b.TheDCRoftheinductorrepresentsthe

small amount of DC resistance of the copper wire, 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.

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

ValueCalculationsection,thetargetsenseresistorvalueis:

RSENSE(EQUIV) =

VSENSE(MAX)

IMAX +

ΔIL

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimum value (68mV) for the Maximum Current Sense

Threshold (VSENSE(MAX)) in the Electrical Characteristics

table.

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, use the divider ratio (RD):

RD =

RSENSE(EQUIV)

DCRMAX at TL(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.

The equivalent resistance R1 || R2 is scaled to the room

temperature inductance and maximum DCR:

R1|| R2 =

(DCR at 20°C) • C1

High Voltage Buck Controller

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