AT9919 Supertex, Inc., AT9919 Datasheet - Page 6

• Manufacturer Part Number: AT9919
• Manufacturer: Supertex, Inc.

MOSFET Selection
MOSFET selection is based on the maximum input operat-
ing voltage V
frequency. Choose a MOSFET that has a higher breakdown
voltage than the maximum operation voltage, low R
and low total charge for better efficiency. MOSFET threshold
voltage must be adequate if operated at the low end of the
input-voltage operating range.
Freewheeling Diode Selection
The forward voltage of the freewheeling diode should be
as low as possible for better efficiency. A Schottky diode
is a good choice as long as the breakdown voltage is high
enough to withstand the maximum operating voltage. The
forward current rating of the diode must be at least equal to
the maximum LED current.
Pin Description
Pin #
TAB
1
2
3
4
5
6
7
8
Supertex inc.
IN
, output current I
RAMP
GATE
ADIM
GND
GND
VDD
DIM
VIN
Pin
CS
Description
Current sense input. Senses LED string current.
Input voltage 4.5 to 40V DC.
Analog PWM dimming ramp output.
Analog 0~2.0V signal input for analog control of PWM dimming.
PWM signal input.
Internally regulated supply voltage. Connect a capacitor from VDD to ground.
Device ground.
Drives gate of external MOSFET.
Must be wired to pin 7 on PCB.
LED
, and operating switching
1235 Bordeaux Drive, Sunnyvale, CA 94089
DS(ON)
,
6
LED Current Ripple
The LED current ripple is equal to the inductor current ripple.
In cases when a lower LED current ripple is needed, a ca-
pacitor can be placed across the LED terminals.
PCB Layout Guidelines
Careful PCB layout is critical to achieve low switching losses
and stable operation. Use a multilayer board whenever pos-
sible for better noise immunity. Minimize ground noise by
connecting high-current ground returns, the input bypass
capacitor ground lead, and the output filter ground lead to
a single point (star ground configuration). The fast di/dt loop
is formed by the input capacitor C
and the MOSFET. To minimize noise interaction, this loop
area should be as small as possible. Place R
as possible to the input filter and VIN. For better noise immu-
nity, a Kelvin connection is strongly recommended between
CS and R
area ground plane for improved power dissipation.
SENSE
Tel: [phone]
. Connect the exposed tab of the IC to a large-
www.supertex.com
IN
, the free-wheeling diode
SENSE
AT9919
as close