LTC3853_15 datasheet(16/36 Pages) LINER

16 / 36 page

LTC3853

3853fc

For more information www.linear.com/LTC3853

To ensure that the application will deliver full load cur-

rent over the full operating temperature range, choose

the minimum value for the Maximum Current Sense

Threshold (VSENSE(MAX)) in the Electrical Characteristics

table (22mV, 42mV, or 65mV, depending on the state of

the ILIM pin).

Next, determine the DCR of the inductor. Where pro-

vided, use the manufacturer’s maximum value, usually

given at 20°C. Increase this value to account for the tem-

perature coefficient of 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 =

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.

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 to the extra switching losses incurred through R1.

However, DCR sensing eliminates a sense resistor, re-

duces conduction 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

DVSENSE of 10mV

to 15mV. For a DCR sensing application, the actual ripple

voltage will be determined by:

∆VSENSE =

VIN – VOUT

R1• C1

•

VOUT

VIN • fOSC

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant-

frequency architectures by preventing subharmonic

oscillations at high duty cycles. It is accomplished

internally by adding a compensating ramp to the inductor

current signal at duty cycles in excess of 40%. Normally,

this results in a reduction of maximum inductor peak cur-

rent for duty cycles >40%. However, the LTC3853 uses

a patented scheme that counteracts this compensating

ramp, which allows the maximum inductor peak current

to remain unaffected throughout all duty cycles.

Inductor Value Calculation

Given the desired input and output voltages, the inductor

value and operating frequency, fOSC, directly determine

the inductor’s peak-to-peak ripple current:

IRIPPLE =

VOUT

VIN

VIN – VOUT

fOSC •L









Lower ripple current reduces core losses in the inductor,

ESR losses in the output capacitors, and output voltage

ripple. Thus, highest efficiency operation is obtained at

low frequency with a small ripple current. Achieving this,

however, requires a large inductor.

A reasonable starting point is to choose a ripple current

that is about 40% of IOUT(MAX). Note that the largest ripple

current occurs at the highest input voltage. To guarantee

that ripple current does not exceed a specified maximum,

the inductor should be chosen according to:

≥

VIN – VOUT

fOSC •IRIPPLE

•

VOUT

VIN

APPLICATIONS INFORMATION

LTC3853 数据表(PDF) 16 Page - Linear Technology

LTC3853 数据表(HTML) 16 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

数据表下载

链接网址

LTC3853 数据表(PDF) 16 Page - Linear Technology

LTC3853 数据表(HTML) 16 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

数据表 下载

链接网址

数据表下载