LTC3875 datasheet(22/46 Pages) LINER

22 / 46 page

LTC3875

3875fb

For more information www.linear.com/LTC3875

APPLICATIONS INFORMATION

VSENSEMAX(ADJ) = VSENSE(MAX) •

2.2– VITEMP

1.5

VITEMP = 30µA • (RS + RP||RNTC)

IDC(MAX) = Maximum average inductor current.

TC is the inductor temperature.

The resulting current limit should be greater than or equal

toIMAXforinductortemperaturesbetween25°Cand100°C.

Typical values for the NTC compensation network are:

• NTC RO = 100k, B-constant = 3000 to 4000

• RS ≈ 3.92k

• RP ≈ 24.3k

GeneratingtheIMAXversusinductortemperaturecurveplot

first using the above values as a starting point, and then

adjusting the RS and RP values as necessary, is another

approach. Figure 6 shows a curve of IMAX versus inductor

temperature. For PolyPhase® applications, tie the TCOMP/

ITEMP pins together and calculate for an TCOMP/ITEMP

pin current of 30µA • #phases.

For the most accurate temperature detection, place the

thermistors next to the inductors as shown in Figure 7.

Take care to keep the TCOMP/ITEMP pins away from the

switch nodes.

Slope Compensation and Inductor Peak Current

Slope compensation provides stability in constant fre-

quency architectures by preventing sub-harmonic oscil-

lations 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 re-

sults in a reduction of maximum inductor peak current for

duty cycles > 40%. However, the LTC3875 uses a 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.

Figure 6. Worst-Case IMAX vs Inductor Temperature Curve with

and without NTC Temperature Compensation

CONNECT TO

ITEMP1

NETWORK

CONNECT TO

ITEMP2

NETWORK

RNTC2

GND

RNTC1

GND

3875 F07a

VOUT1

SW1

VOUT2

SW2

(7a) Dual Output Dual Phase DCR Sensing Application

(7b) Single Output Dual Phase DCR Sensing Application

Figure 7. Thermistor Locations. Place Thermistor Next to

Inductor(s) for Accurate Sensing of the Inductor Temperature,

but Keep the ITEMP Pins away from the Switch Nodes and

Gate Traces

RNTC

3875 F07b

VOUT

SW1

SW2

INDUCTOR TEMPERATURE (°C)

–40

–20

3875 F06

120

100

RS = 3.92k

RP = 24.3k

NTC THERMISTOR:

RO = 100k

TO = 25°C

B = 4334

CORRECTED IMAX

UNCORRECTED

IMAX

NOMINAL

IMAX

LTC3875 Datasheet(PDF) 22 Page - Linear Technology

LTC3875 Datasheet(HTML) 22 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 41 42 43 44 45 46

Datasheet Download

Link URL