LT3653

9

3653f

Battery Charger Operation

overdrive the pin. The LT3653 is internally compensated

amplifier, G1. Check stability over the full input voltage

range, output load range and temperature.

Connect the HVOK node of the LT3653 to the high voltage

present pin of the charger. This is the WALL pin on the

LTC4098. The HVOK pin is capable of supplying up to 1mA

of drive current. When the HVOK pin is low the LT3653 is

not switching and the system output cannot be supported

by the LT3653 regulator. See the Typical Applications sec-

tion for different configurations.

PCB Layout

Proper operation and minimum EMI requires a careful

printed circuit board layout. Figure 1 shows the recom-

mended component placement with trace, ground plane

and via locations. Note that large, switched currents flow

and the input capacitor (C1). Keep the loop formed by

these components as small as possible and tied to system

ground in only one place. Place these components, along

with the inductor and output capacitor, on the same side

of the circuit board, with their connections made on that

layer. Place a local, unbroken ground plane below these

components, and tie this ground plane to system ground

at one location, ideally at the ground terminal of the output

capacitor C2. Make the SW and BOOST nodes as short

as possible. Include vias near the exposed GND pad of

the LT3653 to help remove heat from the LT3653 to the

ground plane.

High Temperature Considerations

The die temperature of the LT3653 must not exceed

the maximum rating of 125°C. This is generally not a

concern unless the ambient temperature is above 85°C.

For higher temperatures, take care in the layout of the

circuit to ensure good heat sinking of the LT3653. Derate

the maximum load current as the ambient temperature

approaches 125°C. The die temperature is calculated by

multiplying the LT3653 power dissipation by the thermal

resistance from junction to ambient. Power dissipation

within the LT3653 is estimated by calculating the total

power loss from an efficiency measurement and subtracting

the catch diode loss. Thermal resistance depends on the

layout of the circuit board, but 64°C/W is typical for the

(2mm

× 3mm) DFN (DCB) package.

Other Linear Technology Publications

Application Notes 19, 35 and 44 contain more detailed

descriptions and design information for Buck regulators

and other switching regulators. The LT1376 data sheet

has a more extensive discussion of output ripple, loop

compensation and stability testing.

APPLICATIONS INFORMATION

Figure 1. LT3653 PCB Layout

GND

C3

C1

C2

3

4

2

1

6

5

7

8

TO CHARGER:

HVOK

3653 F01