HV9918DB1 Supertex, HV9918DB1 Datasheet - Page 5

• Manufacturer Part Number: HV9918DB1
• Description: LED Lighting Development Kits HI CURR LED DRIVER DEMOBOARD
• Manufacturer: Supertex

One possible application of the ADIM feature of the HV9918
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 HV9918 regulates the LED output current using an input
comparator with hysteresis (Figure 2). As the current through
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
t
DPDL
I
LED
+ V
GND
VDD
ADIM
) connected between VIN and
HV9918
RS(LO)
●
)
1235 Bordeaux Drive, Sunnyvale, CA 94089
=
200mV
I
LED
t
DPDH
SENSE
Figure 2
T
S
=
5
1
f
the inductor ramps up and the voltage across the sense re-
sistor reaches the upper threshold, the internal MOSFET at
SW turns off. The MOSFET turns on again when the inductor
current ramps down through the freewheeling diode until the
voltage across the sense resistor 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
●
=
(V
IN
Tel: [phone]
V
ΔI
/2.
IN
are the propagation delays. Note, that the cur-
O
f
RS(HI)
S
- V
+
V
R
IN
(V
SENSE
OUT
∆I
- V
IN
O
)V
- V
RS(LO)
OUT
L
OUT
ΔI
)t
-
●
DPDL
O
(V
www.supertex.com
IN
+
- V
V
∆I
OUT
OUT
O
L
t
)t
DPDH
DPDL
S
:
-
HV9918
O
V
. This ripple
OUT
∆I
O
t
DPDH
t
t
IN