LTC3829 datasheet(18/40 Pages) LINER

18 / 40 page

LTC3829

3829f

applicaTions inForMaTion

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:

DCR

at T

SENSE EQUIV

MAX

L MAX

(

)

(

)

(

)

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

0.47µF. This forces R1|| R2 to around 2k, 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:

R R

DCR at

C C

1 2

(

) •

The sense resistor values are:

R R

1 2

−

;

•

The LTC3829 also features a DCR temperature compen-

sation circuit by using a NTC temperature sensor. See

the Inductor DCR Sensing Temperature Compensation

section for details.

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

and will occur in continuous mode at the maximum input

voltage:

LOSS

IN MAX

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

•

Inductor DCR Sensing Temperature Compensation

and the ITEMP Pin

Inductor DCR current sensing provides a lossless method

of sensing the instantaneous current. Therefore, it can

provide higher efficiency for applications of high output

currents. However, the DCR of the inductor, which is the

small amount of DC winding resistance of the copper,

typically has a positive temperature coefficient. As the

temperature of the inductor rises, its DCR value increases.

The current limit of the controller is therefore reduced.

The LTC3829 offers a method to counter this inaccuracy

by allowing the user to place an NTC temperature sensing

resistor near the inductor to actively correct this error. The

ITEMP pin, when left floating, is at a voltage around 5V and

DCR temperature compensation is disabled. The ITEMP

pin has a constant 10µA precision current flowing out the

pin. By connecting an NTC resistor from the ITEMP pin to

SGND,themaximumcurrentsensethresholdcanbevaried

over temperature according the following equation:

SENSEMAX ADJ

SENSE MAX

ITEMP

(

)

(

) •

. –

1 8

1 3

where:

VSENSEMAX(ADJ)isthemaximumadjustedcurrentsense

threshold.

VSENSE(MAX) is the maximum current sense threshold

specified in the Electrical Characteristics table. It is

typically 75mV, 50mV or 30mV depending on the set-

ting ILIM pins.

VITEMP is the voltage of the ITEMP pin.

The valid voltage range for DCR temperature compensa-

tion on the ITEMP pin is between 0.5V to 0.2V, with 0.5V

or above being no DCR temperature correction and 0.2V

the maximum correction. However, if the duty cycle of the

controller is less than 25%, the ITEMP range is extended

from 0.5V to 0V.

LTC3829 Datasheet(PDF) 18 Page - Linear Technology

LTC3829 Datasheet(HTML) 18 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

Datasheet Download

Link URL