LT1511CSW datasheet(13/16 Pages) LINER

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LT1511

APPLICATIONS INFORMATION

Figure 7. Lower VBOOST

AVV

DRIVER =

( )()() +









()

38 43 3

55 15

011

The average IVX required is:

DRIVER

011

Fused-lead packages conduct most of their heat out the

leads. This makes it very important to provide as much PC

board copper around the leads as is practical. Total

thermal resistance of the package-board combination is

dominated by the characteristics of the board in the

immediate area of the package. This means both lateral

thermal resistance across the board and vertical thermal

resistance through the board to other copper layers. Each

layer acts as a thermal heat spreader that increases the

heat sinking effectiveness of extended areas of the board.

Total board area becomes an important factor when the

area of the board drops below about 20 square inches. The

graph in Figure 8 shows thermal resistance vs board area

for 2-layer and 4-layer boards with continuous copper

planes. Note that 4-layer boards have significantly lower

thermal resistance, but both types show a rapid increase

for reduced board areas. Figure 9 shows actual measured

lead temperatures for chargers operating at full current.

Battery voltage and input voltage will affect device power

dissipation, so the data sheet power calculations must be

used to extrapolate these readings to other situations.

Vias should be used to connect board layers together.

Planes under the charger area can be cut away from the

rest of the board and connected with vias to form both a

For example, VX = 3.3V then:

3.5mA V

1.5mA V

7.5mA

0.012 I

55 V

BIAS

BAT

DRIVER

BAT

()( )+

()

+ ()

()( )

[]

()(

) +









()

= () (

)(

)

()( )( )( )

VBAT

RSW = Switch ON resistance ≈ 0.16Ω

tOL = Effective switch overlap time ≈ 10ns

f = 200kHz

Example: VIN = 15V, VBAT = 8.4V, IBAT = 3A;

3.5mA 15

1.5mA 8.4

8.4

7.5mA

0.012 3

0.27W

3 8.4

55 15

0.33W

0.16 8.4

15 3 200kHz

0.81 0.09

0.9W

BIAS

DRIVER

()( )+

()

+ ()

()( )

[]=

()( ) +









()

= () (

)( )

()( )(

)

=+=

−

Total Power in the IC is: 0.27 + 0.33 + 0.9 = 1.5W

Temperature rise will be (1.5W)(30

°C/W) = 45°C. This

assumes that the LT1511 is properly heat sunk by con-

necting the seven fused ground pins to expanded traces

and that the PC board has a backside or internal plane for

heat spreading.

The PDRIVER term can be reduced by connecting the boost

diode D2 (see Figure 1) to a lower system voltage (lower

than VBAT) instead of VBAT.

Then PDRIVER =

()(

)( ) +









()

BAT

BOOST

SPIN

1511 • F07

LT1511

IVX

µF

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