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| 型號(hào)： | AN44 |
| 廠商： | Zetex Semiconductor |
| 元件分類： | LED驅(qū)動(dòng)器 |
| 英文描述： | A high power LED driver for low voltage halogen replacement |
| 中文描述： | 高功率LED低壓鹵素?zé)舾鼡Q驅(qū)動(dòng)程序 |
| 文件頁(yè)數(shù)： | 2/6頁(yè) |
| 文件大小： | 123K |
| 代理商： | AN44 |
AN44
www.zetex.com
2
Issue 2 - July 2006
Zetex Semiconductors plc 2006
So T
ON
can be calculated, as the voltage across L1 is obtained by subtracting the forward LED
voltage drop from V
IN
. Therefore, if L1 is smaller, T
ON
will be smaller for the same peak current
I
PEAK
and the same battery voltage V
IN
. Note that, while the inductor current is ramping up to
I
PEAK
, the current is flowing through the LED and so the average current in the LED is the sum of
the ramps during the T
ON
ramping up period and the T
OFF
ramping down period.
Off period, T
OFF
The T
OFF
of ZXSC300 and ZXSC310 is fixed internally at nominally 1.7μs. Note that, if relying on
this for current ramp calculations, the limits are 1.2μs min., 3.2μs max.
In order to minimize the conductive loss and switching loss, T
ON
should not be much smaller than
T
OFF
. Very high switching frequencies cause high dv/dt and it is recommended that the ZXSC300
and 310 are operated only up to 200 kHz. Given the fixed T
OFF
of 1.7μs, this gives a T
ON
of (5μs -
1.7μs) = 3.3μs minimum. However, this is not an absolute limitation and these devices have been
operated at 2 or 3 times this frequency, but conversion efficiency can suffer under these
conditions.
During T
OFF
, the energy stored in the inductor will be transferred to the LED, with some loss in the
Schottky diode. The energy stored in the inductor is:
x L x I
PEAK
2 [Joules]
Continuous and discontinuous modes (and average LED current)
If T
OFF
is exactly the time required for the current to reach zero, the average current in the LED
will be I
PEAK
/2. In practice, the current might reach zero before T
OFF
is complete and the average
current will be less because part of the cycle is spent with zero LED current. This is called the
‘discontinuous’ operation mode and is shown in Figure 2.
Figure 2
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