LTC4449 datasheet(19/38 Pages) LINER

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LTC3774

3774fc

For more information www.linear.com/LTC3774

After determining the components for the temperature

compensation network, check the results by plotting IMAX

versusinductortemperatureusingthefollowingequations:

IMAX =

VSENSEMAX(ADJ) – ∆VSENSE /2

DCR(MAX) at 25°C• 1+ TL(MAX) –25°C

(

)•0.4/100

(

)

where:

VSENSEMAX(ADJ) = VSENSE(MAX) •

2.0V –

VITEMP

2.8

1.5

VITEMP = 30µA • RS +RP ||RNTC

(

)

Use typical values for VSENSE(MAX).

The resulting current limit should be greater than or equal

toIMAXforinductortemperaturesbetween25°Cand100°C.

These are typical values for the NTC compensation network:

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

• RS ≈ 20k

• RP ≈ 50k

GeneratingtheIMAXversusinductortemperaturecurveplot

first using the above values as a starting point and then

adjusting the RS and RP values as necessary is another

APPLICATIONS INFORMATION

approach. Figure 6 shows a typical curve of IMAX versus

inductor temperature.

The same thermistor network can be used to correct for

temperatures less than 25°C. But make sure VITEMP is

greater than 0.6V for duty cycles of 25% or more, oth-

erwise temperature correction may not occur at elevated

ambients. For the most accurate temperature detection,

place the thermistors next to the inductor as shown in

Figure 7. Take care to keep the ITEMP pin away from the

switch nodes.

INDUCTOR TEMPERATURE (°C)

100

1000

10000

–40

100

120

–20

3774 F05

THERMISTOR RESISTANCE

RO = 100k

TO = 25°C

B = 4334 FOR 25°C/100°C

RITEMP

RS = 20k

RP = 43.2k

100k NTC

Figure 5. Resistance Versus Temperature for the ITEMP Pin

Network and the 100k NTC

INDUCTOR TEMPERATURE (°C)

–40

120

3774 F06

–20

80 100

CORRECTED

IMAX

NOMINAL

IMAX

UNCORRECTED

IMAX

RS = 20k

RP = 43.2k

NTC THERMISTOR:

RO = 100k

TO = 25°C

B = 4334

VOUT

RNTC

SW1

3774 F07

Figure 6. Worst-Case IMAX Versus Inductor Temperature Curve

with and without NTC Temperature Compensation

Figure 7. Thermistor Location. Place Thermistor Next to

Inductor for Accurate Sensing of the Inductor Temperature,

But Keep the ITEMP Pin Away from the Switch Nodes and Gate

Drive Traces

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