LM3554TMXSLASHNOPB datasheet(31/47 Pages) TI1

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R3 is then:

071

(

)

- 1

047

)

100

298

273

(

)

TRIP

- V

BIAS

)

TRIP

(

TRIP

( )

298

273

PEAK

LOAD

+ '

where

L =

IN x

(

)

OUT - V

OUT

LM3554

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SNVS549C – JUNE 2009 – REVISED FEBRUARY 2016

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8.2.2.3 Inductor Selection

The LM3554 is designed to use a 2.2-µH inductor. Table 13 lists various inductors and their manufacturers that

can work well with the LM3554. When the device is boosting (VOUT > VIN) the inductor is typically the biggest

area of efficiency loss in the circuit. Therefore, choosing an inductor with the lowest possible series resistance is

important. Additionally, the saturation rating of the inductor must be greater than the maximum operating peak

current of the LM3554. This prevents excess efficiency loss that can occur with inductors that operate in

saturation and prevents over heating of the inductor and possible damage. For proper inductor operation and

circuit performance ensure that the inductor saturation and the peak current limit setting of the LM3554 is greater

than IPEAK can be calculated by:

where

•

ƒSW = 2 MHz

•

η can be found in Typical Characteristics

(3)

Table 13. Recommended Inductors

MANUFACTURER

PART NUMBER

DIMENSIONS

(L×W×H)(mm)

ISAT

TOKO

2.2 µH

FDSE0312-2R2M

3 × 3 ×1.2

2 A

TDK

2.2 µH

VLS252012T-2R2M1R3

2 × 2.5 ×1.2 mm

1.5 A

Coilcraft

2. 2µH

LPS4018-222ML

3.9 × 3.9 × 1.7 mm

2.3 A

8.2.2.4 NTC Thermistor Selection

NTC thermistors have a temperature to resistance relationship of:

where

•

β is given in the thermistor datasheet

•

R25C is the thermistors value at 25°C

(4)

Figure 43 is chosen so that it is equal to:

where

•

R(T)TRIP is the thermistor value at the temperature trip point

•

VBIAS is shown in Figure 43

•

VTRIP = 1.05V (typical)

(5)

Choosing R3 here gives a more linear response around the temperature trip voltage. For example, with VBIAS =

2.5 V, a thermistor whose nominal value at 25°C is 100 k

Ω and a β = 4500 K, the trip point is chosen to be 93°C.

The value of R(T) at 93°C is:

(6)

Figure 42 shows the linearity of the thermistor resistive divider of the previous example.

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