Zxld1370, Application information – Diodes ZXLD1370 User Manual
Page 19
ZXLD1370
Document number: DS32165 Rev. 5 - 2
19 of 39
September 2012
© Diodes Incorporated
ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
The additional degree of freedom allows us to select GI_ADJ within limits but this may affect overall performance a little. As mentioned above,
the working voltage range at the GI pin is restricted. The permitted range of GI_ADJ in Boost or Buck-boost configuration is
0.2 < GI_ADJ < 0.5
Equation 13
The mean voltage across the sense resistor is
V
RS
= I
COIL
R
S
Equation 14
Note that if GI_ADJ is made larger, these equations show that R
S
is increased and V
RS
is increased. Therefore, for the same coil current, the
dissipation in R
S
is increased. So, in some cases, it is better to minimize GI_ADJ. However, consider Equation 5. If ADJ is connected to REF,
this becomes
⎟
⎠
⎞
⎜
⎝
⎛
−
=
D
1
ADJ
_
GI
225
.
0
V
RS
This shows that V
RS
becomes smaller than 225mV if GI_ADJ < 1 - D. If also D is small, V
RS
can become too small. For example if D = 0.2, and
GI_ADJ is the minimum value of 0.2, then V
RS
becomes 0.225* 0.2 / 0.8 = 56.25 mV. This will increase the LED current error due to small
offsets in the system, such as mV drop in the copper printed wiring circuit, or offset uncertainty in the ZXLD1370. If now, GI_ADJ is increased to
0.4 or 0.5, V
RS
is increased to a value greater than 100mV. This will give small enough I
LED
error for most practical purposes. Satisfactory
operation will be obtained if V
RS
is more than about 80mV. This means GI_ADJ should be greater than (1-D
MIN
) * 80/225 = (1- D
MIN
) * 0.355.
There is also a maximum limit on V
RS
which gives a maximum limit for GI_ADJ. If V
RS
exceeds approximately 300mV, or 133% of 225mV, the
STATUS output may indicate an over-current condition. This will happen for larger D
MAX
.
Therefore, together with the requirement of Equation 13, the recommended range for GI_ADJ is
0.355 ( 1-D
MIN
) < GI_ADJ < 1.33 ( 1-D
MAX
) Equation 15
An optimum compromise for GI_ADJ has been suggested, i.e.
GI_ADJ
AUTO
= 1 - D
MAX
Equation 16
This value has been used for the “Automatic” setting of the web Calculator. If 1-D
MAX
is less than 0.2, then GI_ADJ is set to 0.2. If 1- D
MAX
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.
Then RGI2 is calculated as follows, rearranging Equation 2:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −
=
ADJ
_
GI
ADJ
_
GI
1
R
R
1
GI
2
GI
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
(
)
6875
.
0
4
.
38
12
4
.
38
V
V
V
OUT
IN
OUT
=
⎟
⎠
⎞
⎜
⎝
⎛
−
=
−
From Equation 16, we set GI_ADJ to 1 – D = 0.3125.
IF R
GI1
= 33kΩ, then from Equation 17,
Ω
=
⎟
⎠
⎞
⎜
⎝
⎛ −
=
k
6
.
72
3125
.
0
3125
.
0
1
x
33
R
2
GI
Let us choose the preferred value R
GI2
= 75kΩ. Now GI_ADJ is adjusted to the new value, using Equation 2.
305
.
0
k
75
k
33
k
33
2
RGI
1
RGI
1
RGI
ADJ
_
GI
=
+
=
⎟
⎠
⎞
⎜
⎝
⎛
+
=