AT9919 Supertex, Inc., AT9919 Datasheet - Page 5

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

One possible application of the ADIM feature of the AT9919
may include protection of the LED load from over-tempera-
ture by connecting an NTC thermistor at ADIM, as shown in
Figure 1.
Setting LED Current with External Resistor R
The output current in the LED is determined by the external
current sense resistor (R
CS. Disregarding the effect of the propagation delays, the
sense resistor can be calculated as:
Selecting Buck Inductor L
The AT9919 regulates the LED output current using an input
R
R
V
V
SENSE
SENSE
RS(LO)
RS(HI)
R
SENSE
Supertex inc.
NTC
≈
I
V
2
LED
1
DIM
•
(V
RS(HI)
Figure 1
SENSE
I
LED
+ V
GND
VDD
ADIM
) connected between VIN and
AT9919
RS(LO)
t
DPDL
)
1235 Bordeaux Drive, Sunnyvale, CA 94089
=
200mV
I
LED
t
DPDH
SENSE
Figure 2
T
S
=
5
1
f
comparator with hysteresis (Figure 2). As the current through
the inductor ramps up and the voltage across the sense re-
sistor reaches the upper threshold, the voltage at GATE goes
low, turning off the external MOSFET. The MOSFET turns on
again when the inductor current ramps down through the
freewheeling diode until the voltage across the sense resis-
tor equals the lower threshold. Use the following equation to
determine the inductor value for a desired value of operating
frequency f
where:
and t
rent ripple ∆I in the inductor L is greater than ∆I
can be calculated from the following equation:
For the purpose of the proper inductor selection, note that
the maximum switching frequency occurs at the highest V
and V
S
DPDL
OUT
L =
∆I
∆I = ∆I
, t
= V
O
DPDH
S
=
(V
:
IN
Tel: [phone]
V
/2.
IN
are the propagation delays. Note, that the cur-
O
I
f
RS(HI)
S
- V
+
V
R
IN
(V
SENSE
OUT
∆I
- V
IN
O
)V
- V
RS(LO)
OUT
L
OUT
-
)t
(V
DPDL
I
O
www.supertex.com
IN
- V
+
V
∆I
OUT
OUT
O
L
)t
t
DPDH
DPDL
-
V
O
AT9919
. This ripple
OUT
∆I
O
t
DPDH
t
t
IN