LTC3621 datasheet(14/20 Pages) AD

14 / 20 page

LTC3621/LTC3621-2

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Efficiency Considerations

The percent efficiency of a switching regulator is equal to

the output power divided by the input power times 100%.

It is often useful to analyze individual losses to determine

what is limiting the efficiency and which change would

produce the most improvement. Percent efficiency can

be expressed as:

% Efficiency = 100% – (Loss1 + Loss2 + …)

where Loss1, Loss2, etc. are the individual losses as a

percentageofinputpower.Althoughalldissipativeelements

in the circuit produce losses, three main sources usually

account for most of the losses in LTC3621 circuits: 1) I2R

losses, 2) switching and biasing losses, 3) other losses.

1. I2R losses are calculated from the DC resistances of

the internal switches, RSW, and external inductor, RL.

In continuous mode, the average output current flows

through inductor L but is “chopped” between the

internal top and bottom power MOSFETs. Thus, the

series resistance looking into the SW pin is a function

of both top and bottom MOSFET RDS(ON) and the duty

cycle (DC) as follows:

RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 – DC)

TheRDS(ON)forboththetopandbottomMOSFETscanbe

obtained from the Typical Performance Characteristics

curves. Thus to obtain I2R losses:

I2R losses = IOUT2(RSW + RL)

Table 1. Inductor Selection Table

INDUCTOR

INDUCTANCE

(µH)

DCR

(mΩ)

MAX CURRENT

(A)

DIMENSIONS

(mm)

HEIGHT

(mm)

MANUFACTURER

IHLP-1616BZ-11 Series

1.0

2.2

4.7

4.5

3.25

1.7

4.3 × 4.7

Vishay

www.vishay.com

IHLP-2020BZ-01 Series

2.2

3.3

4.7

5.6

6.8

18.9

45.6

79.2

108

113

139

4.2

3.3

2.8

2.5

2.4

5.4 × 5.7

FDV0620 Series

2.2

3.3

4.7

5.7

3.2

2.8

6.7 × 7.4

Toko

www.toko.com

MPLC0525L Series

1.5

2.2

6.4

5.2

4.1

6.2 × 5.4

2.5

NEC/Tokin

www.nec-tokin.com

XFL4020 Series

1.0

1.5

2.2

3.3

4.7

10.8

14.4

21.3

34.8

52.2

5.1

4.4

3.5

2.5

4 × 4

2.1

Coilcraft

www.coilcraft.com

RLF7030 Series

1.5

2.2

3.3

4.7

6.8

8.8

9.6

6.4

6.1

5.4

4.1

3.4

2.8

6.9 × 7.3

3.2

TDK

www.tdk.com

WE-TPC 4828 Series

1.2

1.8

2.2

2.7

3.3

3.9

4.7

3.1

2.7

2.5

2.35

2.15

1.72

1.55

4.8 × 4.8

2.8

Würth Elektronik

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