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LT3645

36V 500mA Step-Down Regulator and 200mA LDO

Linear Technology 

LT3645

APPLICATIONS INFORMATION

P1 has a finite on-resistance which will result in power

dissipation and some loss in efficiency. For higher buck

output voltage applications, a smaller PFET may be used

since the gate drive will be higher.

PCB Layout

For proper operation and minimum EMI, care must be taken

during printed circuit board layout. Figure 13 shows the

recommended component placement with trace, ground

plane, and via locations.

Note that large, switched currents flow in the LT3645’s VIN

and SW pins, the catch diode (D1), and the input capacitor

(C1). The loop formed by these components should be as

small as possible and tied to system ground in only one

place. These components, along with the inductor and

output capacitor, should be placed on the same side of the

circuit board, and their connections should be made on

that layer. Place a local, unbroken ground system ground in

only one place. These components, along with the inductor

and output capacitor, should be placed on the same side

of the circuit board, and their connections should be made

OUT1

EN/UVLO

NPG EN2

OUT2

C3

R2

FB1

R1

R4

FB2

C4

R3

C2

BOOST

SW

D1

DA

L1

C1 VIN

C5

VCC2

VIN

MAIN PCB

BOARD

POWER

+

VIA TO LOCAL GROUND PLANE

OUTLINE OF LOCAL GROUND PLANE

Figure 13.

3645 F13

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 C1). The SW and BOOST nodes should

be kept as small as possible. Finally, keep the FB nodes

small so that the ground pin and ground traces will shield

them from the SW and BOOST nodes. Include vias near

the exposed GND pad of the LT3645 to help remove heat

from the LT3645 to the ground plane.

High Temperature Considerations

The die temperature of the LT3645 must be lower than

the maximum rating of 125°C (150°C for H-grade). This

is generally not a concern unless the ambient tempera-

ture is above 85°C. For higher temperatures, extra care

should be taken in the layout of the circuit to ensure good

heat sinking at the LT3645. The maximum load current

should be derated as the ambient temperature approaches

125°C. The die temperature is calculated by multiplying the

LT3645 power dissipation by the thermal resistance from

junction to ambient. Power dissipation within the LT3645

can be estimated by calculating the total power loss from

an efficiency measurement and subtracting the catch diode

loss. The resulting temperature rise at full load is nearly

independent of input voltage. Thermal resistance depends

upon the layout of the circuit board, but 68°C/W is typical

for the QFN (UD) package, and 40°C/W is typical for the

MSE package. Thermal shutdown will turn off the Buck

and LDO when the die temperature exceeds 160°C, but

it is not a warrant to allow operation at die temperatures

exceeding 125°C (150°C for H-grade).

Other Linear Technology Publications

Application Notes 19, 35, and 44 contain more detailed

descriptions and design information for step-down regu-

lators and other switching regulators. The LT1376 data

sheet has an extensive discussion of output ripple, loop

compensation, and stability testing. Design Note 318

shows how to generate a bipolar output supply using a

step-down regulator.

3645f

18

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