MAX16832A_12 Datasheet, PDF (8/10 Pages) Maxim Integrated Products – 2MHz, High-Brightness LED Drivers with Integrated MOSFET and High-Side Current Sense

2MHz, High-Brightness LED Drivers with
Integrated MOSFET and High-Side Current Sense
HYSTERETIC
MODE
fSW
I_LED
AVG. LED
CURRENT
∆I
t
V_DIM
t1
Figure 2. Current-Regulator Operation
t2
t
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
through the input cap, R_SENSE, the LED load, the
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-
tive terminal through R_SENSE, the LED load, the induc-
tor, and the freewheeling diode and back to the input
cap positive terminal. Note that the current through
R_SENSE, the LED load, and the inductor is basically DC
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
across R_SENSE. Any noise pickup on this node induces
erratic switching of the internal MOSFET (the IC will
operate at a much higher frequency). To help prevent
this, place R_SENSE as close as possible to CS and IN
and keep the sense traces short. It is especially impor-
tant to keep the square-wave switching currents in the
freewheeling diode away from R_SENSE. To minimize
interference, place the freewheeling diode on the oppo-
site side of the IC as R_SENSE and position the input
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.
8 _______________________________________________________________________________________