2MHz, High-Brightness LED Drivers with

Integrated MOSFET and High-Side Current Sense

8

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Freewheeling-Diode Selection

For stability and best efficiency, a low forward-voltage

drop diode with fast reverse-recovery time and low

capacitance is recommended. A Schottky diode is a

good choice as long as its breakdown voltage is high

enough to withstand the maximum operating voltage.

PCB Layout Guidelines

Careful PCB layout is critical to achieve low switching

losses and stable operation. In normal operation, there

are two power loops. One is formed when the internal

MOSFET is on and the high current flows from ground

inductor, and the internal MOSFET back to ground. The

second loop is formed when the internal MOSFET is off

and the high current circulates from the input cap posi-

tor, and the freewheeling diode and back to the input

cap positive terminal. Note that the current through

with some triangular ripple (low noise). The high-noise,

large signal, fast transition switching currents only flow

through the freewheeling diode to the input cap positive

terminal, or through the MOSFET to ground and then to

the input cap positive terminal. Without a proper PCB

layout, these square-wave switching currents can cre-

ate problems in a hysteretic LED driver.

The current control depends solely on the voltage

erratic switching of the internal MOSFET (the IC will

operate at a much higher frequency). To help prevent

and keep the sense traces short. It is especially impor-

tant to keep the square-wave switching currents in the

interference, place the freewheeling diode on the oppo-

capacitor near the diode so it can return the high fre-

quency currents to ground. The layout in Figure 3

should be used as a guideline. The dashed line shows

the path of the high frequency components that cause

disruption in operation. For a good thermal design, the

exposed pad on the IC should solder to a large pad

with many vias to the backside ground plane.

t

t

t2

∆I

t1

AVG. LED

CURRENT

HYSTERETIC

MODE

Figure 2. Current-Regulator Operation