HV9919 Supertex, Inc., HV9919 Datasheet - Page 5

• Manufacturer Part Number: HV9919
• Manufacturer: Supertex, Inc.

Selecting Buck Inductor L
The HV9919 regulates the LED output current using an input
comparator with hysteresis (Figure 1). 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
Figure 1
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
MOSFET Selection
MOSFET selection is based on the maximum input operat-
R
R
V
V
SENSE
SENSE
RS,LO
RS,HI
L =
S
(V
:
IN
IN
, output current I
f
I
V
- V
S
LED
V
DIM
IN
OUT
∆I
) • V
O
t
DPDL
OUT
-
(V
LED
IN
, and operating switching
- V
t
∆I
DPDH
OUT
O
) • t
DPDL
-
V
IN
t
∆I
DPDH
DS(ON)
O
T
S
=
,
5
1
f
S
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
enough to withstand the maximum operating voltage. The
forward current rating of the diode must be at least equal to
the maximum LED current.
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
DPDL
OUT
∆I
∆I = ∆I
, t
= V
O
DPDH
=
IN
ΔI
V
/2.
are the propagation delays. Note, that the cur-
O
RS,HI
+
R
(V
SENSE
- V
IN
RS,LO
- V
L
OUT
ΔI
)t
DPDL
O
+
V
IN
t
L
DPDH
HV9919
O
. This ripple
t
t
IN