LTC3869 datasheet(20/46 Pages) LINER

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LTC3875

3875fb

For more information www.linear.com/LTC3875

APPLICATIONS INFORMATION

R1 • C1 is the filter time constant of the SNSD+ pin.

R2 • C2 is the filter time constant of the SNSA+ pin.

For example, for a 12VIN, 1.2V/30A step-down buck con-

verter running at 400kHz frequency, a 0.15µH, 0.4mΩ

inductor is chosen. This inductor provides 15A peak-to-

peak ripple current, which is 50% of the 30A full load

current. At full load, the inductor peak current is 30A +

15A/2 = 37.5A.

IL(PK) • DCR = 37.5A • 0.4mΩ = 15mV.

In this case, choose the 20mV ILIM setting which is the

closest but higher than 15mV to provide margin for cur-

rent limit.

Select the two R/C sensing network:

Filter on SNSD+ pin: R1 • C1 = L/DCR,

Filter on SNSA+ pin: R2 • C2 = (L/DCR)/5.

In this case, the ripple sense signal across SNSA+ and

SNS– pins is ΔILP-P • DCR • 5 = 15A • 0.4mΩ • 5 = 30mV.

This signal should be more than 15mV for good signal-to-

noise ratio. In this case, it is certainly sufficient.

The peak inductor current at current limit is:

ILIM(PK) = 20mV/DCR = 20mV/0.4mΩ = 50A.

The average inductor current, which is also the output

current, at current limit is :

ILIM(AVG) = ILIM(PK) – ΔILP-P/2 = 50A – 15A/2 = 42.5A.

Toensurethattheloadcurrentwillbedeliveredoverthefull

operatingtemperaturerange,thetemperaturecoefficientof

DCR resistance, approximately 0.4%/°C, should be taken

into account. The LTC3875 features a DCR temperature

compensationcircuitthatusesanNTCtemperaturesensing

resistor for this purpose. See the Inductor DCR Sensing

Temperature Compensation section for details.

Typically, C1 and C2 are selected in the range of 0.047µF

to 0.47µF. If C1 and C2 are chosen to be 100nF, and an

inductor of 150nH with 0.4mΩ DCR is selected, R1 and R2

will be 4.64k and 931Ω respectively. The bias current at

SNSD+ and SNSA+ is about 30nA and 500nA respectively,

and it causes some small error to the sense signal.

There will be some power loss in R1 and R2 that relates to

the duty cycle, and will be the most in continuous mode

at the maximum input voltage:

PLOSS R

( )=

VIN(MAX) – VOUT

(

)•V

OUT

Ensure that R1 and R2 have a power rating higher than this

value. However, DCR sensing eliminates the conduction

loss of a sense resistor; it will provide a better efficiency

at heavy loads. To maintain a good signal-to-noise ratio

for the current sense signal, using ∆VSENSE of 15mV be-

tween SNSA+ and SNS– pins or an equivalent 3mV ripple

on the current sense signal. The actual ripple voltage

across SNSA+ and SNS– pins will be determined by the

following equation:

∆VSENSE =

VOUT

VIN

•

VIN – VOUT

R2•C2• fOSC

Inductor DCR Sensing Temperature Compensation

with NTC Thermistor

For DCR sensing applications, the temperature coefficient

of the inductor winding resistance should be taken into

account when the accuracy of the current limit is criti-

cal over a wide range of temperature. The main element

used in inductors is copper; that has a positive tempco

of approximately 4000ppm/°C. The LTC3875 provides

a feature to correct for this variation through the use of

the TCOMP/ITEMP pin. There is a 30µA precision current

source flowing out of the TCOMP/ITEMP pin. A thermistor

with a NTC (negative temperature coefficient) resistance

can be used in a network, RITEMP (Figure 4b) connected to

maintain the current limit threshold constant over a wide

operating temperature. The TCOMP/ITEMP voltage range

that activates the correction is from 0.7V or less. If this

pin is floating, its voltage will be at INTVCC potential, about

5.5V.WhentheTCOMP/ITEMPvoltageishigherthan0.7V,

the temperature compensation is inactive. Floating the

ENTMPB pin enables the temperature compensation

function.

The following guidelines will help to choose components

for temperature correction. The initial compensation is for

25°C ambient temperature:

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