AN44 ZETEX [Zetex Semiconductors], AN44 Datasheet - Page 3

• Manufacturer Part Number: AN44
• Description: A high power LED driver for low voltage halogen replacement
• Manufacturer: ZETEX [Zetex Semiconductors]

For continuous mode
If the current does not reach zero after 1.7µs, but instead falls to a value of I
is said to be in ‘continuous’ mode. The LED current will ramp up and down between I
(probably at different di/dt rates) and the average LED current will therefore be the average of
I
Design example
(Refer to Figure 1 and Table 1)
These equations make the approximation that the LED forward drop is constant throughout the
current ramp. In fact it will increase with current, but they still enable design calculations to be
made within the tolerances of the components used in a practical circuit. Also, the difference
between V
dependent on these voltages.
Note that, for an LED drop of 9.6V and a Schottky drop of 300mV, the time to ramp down from
680mA to zero would be:
Issue 2 - July 2006
© Zetex Semiconductors plc 2006
PEAK
Input = V
LED forward drop = V
V
Therefore V
The peak current = V
(R1 is R
T
T
TDIS
ON
IN
ON
and I
= V
= I
680mAx22µH
---------------------------------------
680mAx22µH
---------------------------------------
LED
MIN
SENSE
IN
PEAK
(
IN
9.6
and V
+V
, as shown in Figure 3.
2.4
= 12V
L
+
L
) = 24mV/50mR = 480mA
x L1/V(L1)
= (12 - 9.6) = 2.4
0.3
LED
)
is small compared to either of them, so the 6.2µs ramp time will be fairly
=
SENSE
=
LED
6.2µs
1.5µs
= 9.6V
/ R1
Figure 3
3
MIN
, then the device
www.zetex.com
MIN
AN44
and I
PEAK