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LTC3855

3855f

applicaTions inForMaTion

always the same and varies with temperature; consult the

manufacturers’ datasheets 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)

I(MAX) +

∆IL

To ensure that the application will deliver full load current

over the full operating temperature range, choose the

minimumvaluefortheMaximumCurrentSenseThreshold

(VSENSE(MAX))intheElectricalCharacteristicstable(25mV,

45mV, or 68mV, depending on the state of the ILIM pin).

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

or use LTC3855 DCR temperature compensation function.

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 =

RSENSE(EQUIV)

DCR(MAX) at TL(MAX)

C1 is usually selected to be in the range of 0.047µF to

0.47µF. This forces R1|| R2 to around 2kΩ, reducing error

that might have been caused by the SENSE pins’ ±1µA

current. TL(MAX) is the maximum inductor temperature.

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

temperature inductance and maximum DCR:

R1||R2

(DCR at 20

°C) • C1

The sense resistor values are:

R1|| R2

; R2

R1• RD

−RD

The maximum power loss in R1 is related to duty cycle,

and will occur in continuous mode at the maximum input

voltage:

PLOSS R1=

VIN(MAX) − VOUT

(

)•V

OUT

Ensure that R1 has a power rating higher than this value.

If high efficiency is necessary at light loads, consider this

power loss when deciding whether to use DCR sensing or

sense resistors. Light load power loss can be modestly

higher with a DCR network than with a sense resistor, due

totheextraswitchinglossesincurredthroughR1.However,

DCR sensing eliminates a sense resistor, reduces conduc-

tion losses and provides higher efficiency at heavy loads.

Peak efficiency is about the same with either method.

To maintain a good signal to noise ratio for the current

sense signal, use a minimum ∆VSENSE of 10mV for duty

cycles less than 40%. For a DCR sensing application, the

actual ripple voltage will be determined by the equation:

∆

−

R C

SENSE

OUT

IN OSC

1 1

•

NTC Compensated DCR Sensing

For DCR sensing applications where a more accurate

current limit is required, a network consisting of an NTC

thermistor placed from the ITEMP pin to ground will

provide correction of the current limit over temperature.

Figure 2b shows this network. Resistors RS and RP will

linearize the impedance the ITEMP pin sees. To implement

NTC compensated DCR sensing, design the DCR sense

filter network per the same procedure mentioned in the

previousselection,exceptcalculatethedividercomponents

using the room temperature value of the DCR. For a single

output rail operating from one phase:

1. Set the ITEMP pin resistance to 50k at 25°C. With

10µA flowing out of the ITEMP pin, the voltage on the

ITEMP pin will be 0.5V at room temperature. Current

limit correction will occur for inductor temperatures

greater than 25°C.

2. Calculate the ITEMP pin resistance and the maximum

inductor temperature which is typically 100°C. Use the

following equations:

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