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LT3502/LT3502A

3502fc

APPLICATIONS INFORMATION

If the output capacitor is different than the recommended

capacitor, stability should be checked across all operat-

ing conditions, including load current, input voltage and

temperature. The LT1375 data sheet contains a more

thorough discussion of loop compensation and describes

how to test the stability using a transient load.

PCB Layout

For proper operation and minimum EMI, care must be

taken during printed circuit board layout. Figure 11 shows

the recommended component placement with trace,

ground plane and via locations. Note that large, switched

currents ﬂow in the LT3502/LT3502A’s VIN and SW pins,

the catch diode (D1) and the input capacitor (C2). 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 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 as

small as possible. Finally, keep the FB node small so that

the ground pin and ground traces will shield it from the

SW and BOOST nodes. Include vias near the exposed GND

pad of the LT3502/LT3502A to help remove heat from the

LT3502/LT3502A to the ground plane.

High Temperature Considerations

The die temperature of the LT3502/LT3502A must be

lower than the maximum rating of 125°C. This is generally

not a concern unless the ambient temperature is above

85°C. For higher temperatures, care should be taken in

the layout of the circuit to ensure good heat sinking of

the LT3502/LT3502A. The maximum load current should

be derated as the ambient temperature approaches

125°C. The die temperature is calculated by multiplying

the LT3502/LT3502A power dissipation by the thermal

resistance from junction to ambient. Power dissipation

within the LT3502/LT3502A can be estimated by calculat-

ing 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 102°C/W

and 110ºC/W are typical for the (2mm × 2mm) DFN and

MS10 packages respectively.

Outputs Greater Than 7V

Note that for outputs above 7V, the input voltage range will

be limited by the maximum rating of the BOOST pin. The

sum of input and output voltages cannot exceed the BOOST

pin’s 50V rating. The 15V circuit (Figure 12) shows how to

overcome this limitation using an additional zener diode.

Other Linear Technology Publications

Application notes AN19, AN35 and AN44 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. Design

Note 100 shows how to generate a bipolar output supply

using a buck regulator.

Figure 11

Figure 12. 15V Step-Down Converter

BST

GND

VIN

VOUT

= VIA

3502 F11

SHDN

VIN

0.1μF

22pF

3502 F12

33μH

10V

SHDN

BOOST

LT3502A

GND

OFF ON

10μF

10k

180k

1μF

VIN

20V TO 40V

VOUT

15V

500mA

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