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LTC3774

Dual, Multiphase Current Mode Synchronous Controller

Linear Technology 

LTC3774

APPLICATIONS INFORMATION

10000

1000

THERMISTOR RESISTANCE

RO = 100k

TO = 25°C

B = 4334 FOR 25°C/100°C

100

RITMP

RS = 20k

10 RP = 43.2k

100k NTC

1

–40 –20 0 20 40 60 80 100 120

INDUCTOR TEMPERATURE (°C)

3774 F05

Figure 5. Resistance Versus Temperature for the ITEMP Pin

Network and the 100k NTC

After determining the components for the temperature

compensation network, check the results by plotting IMAX

versus inductor temperature using the following equations:

( ( ) ) IMAX =

VSENSEMAX(ADJ) – ∆VSENSE / 2

DCR(MAX) at 25°C • 1+ TL(MAX) – 25°C • 0.4 / 100

where:

2.0V – VITEMP

VSENSEMAX(ADJ) = VSENSE(MAX) •

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

to IMAX for inductor temperatures between 25°C and 100°C.

These are typical values for the NTC compensation network:

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

• RS ≈ 20k

• RP ≈ 50k

Generating the IMAX versus inductor temperature curve plot

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 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.

25

20

CORRECTED

15

IMAX

NOMINAL

IMAX

10

RS = 20k

UNCORRECTED

IMAX

RP = 43.2k

5

NTC THERMISTOR:

RO = 100k

TO = 25°C

B = 4334

0

–40 –20 0 20 40 60 80 100 120

INDUCTOR TEMPERATURE (°C)

3774 F06

Figure 6. Worst-Case IMAX Versus Inductor Temperature Curve

with and without NTC Temperature Compensation

VOUT

RNTC

L1

SW1

3774 F07

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

For more information www.linear.com/LTC3774

3774f

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