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LT3592

500mA Wide Input Voltage Range Step-Down LED Driver with 10:1 Dimming

Linear Technology 

LT3592

APPLICATIONS INFORMATION

two poles in the loop. Rc provides a zero. With the recom-

mended output capacitor, the loop crossover occurs above

the RCCC zero. This simple model works well as long as the

value of the inductor is not too high and the loop crossover

frequency is much lower than the switching frequency.

With a larger ceramic capacitor that will have lower ESR,

crossover may be lower and a phase lead capacitor (CPL)

across the feedback divider may improve the transient

response. Large electrolytic capacitors may have an ESR

large enough to create an additional zero, and the phase

lead might not be necessary. If the output capacitor is

different than the recommended capacitor, stability should

be checked across all operating conditions, including DIM

and BRIGHT current modes, voltage control via FB, input

voltage, and temperature.

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 LT3592’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 LT3592 to help remove

heat from the LT3592 to the ground plane.

High Temperature Considerations

The die temperature of the LT3592 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, extra care should be taken in the layout of

the circuit to ensure good heat sinking at the LT3592. The

maximum load current should be derated as the ambient

temperature approaches 125°C. The die temperature is

calculated by multiplying the LT3592 power dissipation

by the thermal resistance from junction to ambient.

Power dissipation within the LT3592 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 76°C/W is typical for the

3mm × 2mm DFN (DDB10) package, and 38°C/W is typical

for the MS10E package.

Higher Output Voltages

At higher output voltages, the choice of output capacitor

becomes especially critical. Many small case size ceramic

capacitors lose much of their rated capacitance well below

BRIGHT

SHDN

VIN

SYS GND

3592 F11

Figure 11. A Good PCB Layout Ensures Proper, Low EMI Operation

3592fc

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