LT3782AIUFD datasheet(13/20 Pages) LINER

13 / 20 page

LT3782A

3782af

From a known power dissipated in the power MOSFET, its

junction temperature can be obtained using the following

formula:

TJ = TA + PFET • RTH(JA)

The RTH(JA) to be used in this equation normally includes

the RTH(JC) for the device plus the thermal resistance from

the case to the ambient temperature (RTH(CA)). This value

of TJ can then be compared to the original, assumed value

used in the iterative calculation process.

Input Capacitor Choice

The input capacitor must have high enough voltage and

ripple current ratings to handle the maximum input voltage

and RMS ripple current rating. The input ripple current in

a boost circuit is very small because the input current is

continuous. With 2-phase operation, the ripple cancellation

will further reduce the input capacitor ripple current rating.

The ripple current is plotted in Figure 5. Please note that

the ripple current is normalized against:

norm =

L•f

Output Capacitor Selection

The voltage rating of the output capacitor must be greater

than the maximum output voltage with sufﬁcient derat-

ing. Because the ripple current in output capacitor is a

pulsating square wave in a boost circuit, it is important

that the ripple current rating of the output capacitor

be high enough to deal with this large ripple current.

Figure 6 shows the output ripple current in the 1- and 2-

phase designs. As shown, the output ripple current of a

2-phase boost circuit reaches almost zero when the duty

cycle equals 50% or the output voltage is twice as much as

the input voltage. Thus the 2-phase technique signiﬁcantly

reduces the output capacitor size.

Figure 6. Normalized Output RMS Ripple Currents in Boost

Converter: 1-Phase and 2-Phase. IOUT Is the DC Output Current.

0.1

0.9

3782A F06

0.3

0.5

0.7

0.8

0.2

0.4

0.6

3.25

3.00

2.75

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

DUTY CYCLE OR (1-VIN/VOUT)

1-PHASE

2-PHASE

norm =

L•f

The RMS Ripple Current is About 29% of

the Peak-to-Peak Ripple Current.

Figure 5. Normalized Input Peak-to-Peak Ripple Current

DUTY CYCLE

1.00

0.90

0.80

0.60

0.70

0.50

0.40

0.30

0.20

0.10

0.8

3782A F05

0.2

0.4

0.6

1.0

1-PHASE

2-PHASE

APPLICATIONS INFORMATION

LT3782AIUFD Datasheet(PDF) 13 Page - Linear Technology

LT3782AIUFD Datasheet(HTML) 13 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

Datasheet Download

Link URL