ZXLD1374 Ver la hoja de datos (PDF) - Diodes Incorporated.
Número de pieza
componentes Descripción
Fabricante
ZXLD1374
Diodes Incorporated.
ZXLD1374 Datasheet PDF : 39 Pages
| |||
A Product Line of
Diodes Incorporated
ZXLD1374
Application Information (cont.)
This will give small enough ILED error for most practical purposes. Satisfactory operation will be obtained if VRS is more than about 80mV. This
means GI_ADJ should be greater than (1-DMIN) * 80/225 = (1- DMIN) * 0.355.
There is also a maximum limit on VRS which gives a maximum limit for GI_ADJ. If VRS exceeds approximately 300mV, or 133% of 225mV, the
STATUS output may indicate an over-current condition. This will happen for larger DMAX. Therefore, together with the requirement of Equation
13, the recommended range for GI_ADJ is
0.355 ( 1-DMIN) < GI_ADJ < 1.33 ( 1-DMAX )
An optimum compromise for GI_ADJ has been suggested, i.e.
Equation 15
GI_ADJAUTO = 1 - DMAX
Equation 16
This value has been used for the “Automatic” setting of the web Calculator. If 1-DMAX is less than 0.2, then GI_ADJ is set to 0.2. If 1- DMAX is
greater than 0.5 then GI_ADJ is set to 0.5.
Once GI_ADJ has been selected, a value of RGI1 can be selected from Equation 12.
Equation 2:
RGI1
=
RGI1⎜⎜⎝⎛
1
− GI
GI _
_ ADJ
ADJ
⎟⎟⎠⎞
Then RGI2 is calculated as follows, rearranging
Equation 17
For example to drive 12 LEDS at a current of 350mA from a 12V supply requires Boost configuration. Each LED has a forward voltage of 3.2V
at 350mA, so Vout = 3.2*12 = 38.4V. From Equation 6, the duty cycle is approximately
(VOUT − VIN) = (38.4 − 12) = 0.6875
VOUT
38.4
From Equation 16, we set GI_ADJ to 1 – D = 0.3125.
If RGI1 = 33kΩ, then from Equation 17, RGI2 = 33000 * ( 1 -0.3125 ) / 0.3125 = 72.6kΩ. Let us choose the preferred value RGI2 = 75kΩ. Now
GI_ADJ is adjusted to the new value, using Equation 2.
GI _ ADJ = ⎜⎛ RGI1 ⎟⎞ = 33k = 0.305
⎝ RGI1+ RGI2 ⎠ 33k + 75K
Now we calculate Rs from Equation 11. Assume ADJ is connected to REF.
RSBoostBB
=
⎜⎜⎝⎛
0.225
ILED
⎟⎟⎠⎞GI
_
ADJ⎜⎜⎝⎛
V ADJ
VREF
⎟⎟⎠⎞
=
0.225
0.35
* 0.305
=
0.196Ω
A preferredvalue of RSBoostBB = 0.2Ω will give the desired LED current with an error of 2% due to the preferred value selection.
Table 1 shows typical resistor values used to determine the GI_ADJ ratio with E24 series resistors.
Table 1
GI Ratio
0.2
0.25
0.3
0.35
0.4
0.45
0.5
RGI1
30kΩ
33kΩ
39kΩ
30kΩ
100kΩ
51kΩ
30kΩ
RG2
120kΩ
100kΩ
91kΩ
56kΩ
150kΩ
62kΩ
30kΩ
This completes the LED current setting.
ZXLD1374
Document number: DS35032 Rev. 3 - 2
21 of 39
www.diodes.com
September 2012
© Diodes Incorporated
Share Link: