LT1769IGN datasheet(13/16 Pages) LINEAR

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LT1769

1769fa

APPLICATIONS INFORMATION

Figure 7. Lower VBOOST

BOOST

SPIN

1769 F07

LT1769

IVX

µF

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

low thermal resistance system and to act as a ground plane

for reduced EMI.

Glue-on, chip-mounted heat sinks are effective only in

moderate power applications where the PC board copper

cannot be used, or where the board size is small. They offer

very little improvement in a properly laid out multilayer

board of reasonable size.

Higher Duty Cycle for the LT1769 Battery Charger

Maximum duty cycle for the LT1769 is typically 90%, but

this may be too low for some applications. For example, if

Example: VIN = 19V, VBAT = 12.6V, IBAT = 2A:

3.5mA 19

1.5mA 12.6

12.6

7.5mA

0.012 2000mA

0.35W

2 12.6

55 19

0.43W

0.16 12.6

19 2 200kHz

0.42 0.08

0.5W

BIAS

DRIVER

()( )+

()

+ ()

()(

)

[]=

()( ) +









()

= () (

)( )

()( )(

)

−

12 6

Total Power in the IC is: 0.35 + 0.43 + 0.5 = 1.3W

Temperature rise will be (1.3W)(35

°C/W) = 46°C. This

assumes that the LT1769 is properly heat sunk by con-

necting the eleven 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 7) to a lower system voltage (lower

than VBAT) instead of VBAT.

Then PDRIVER =

()(

)( ) +









()

BAT

For example, VX = 3.3V then:

DRIVER =

()( )( ) +









()

126

55 19

009

The average IVX required is:

DRIVER

009

The previous example shows the dramatic drop in driver

power dissipation when the boost diode (D2) is connected

to an external 3.3V source instead of the 12.6V battery.

PDRIVER drops from 0.43W to 0.09W resulting in an

approximately 12

°C drop in junction temperature.

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

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