Diodes ZXLD1374 - Manual (Page 20)

ZXLD1374
Document number: DS35032 Rev. 3 - 2
20 of 39
www.diodes.com
September 2012
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ZXLD1374
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Buck Topology
In Buck mode, GI is connected to ADJ as in Figure 30 (for simplicity TADJ is
not shown. However if not used should be connected to REF).
The LED current depends only upon R
S
, V
ADJ
and V
REF
. From Equation 1 above,
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
I
218 . 0
R
REF
ADJ
LED
SBuck
Equation 10
If ADJ is directly connected to VREF, this becomes:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
I
218 . 0
R
LED
SBuck
Boost and Buck-Boost Topology
In Boost and Buck-boost mode GI is connected to ADJ through a voltage divider as in
figure 31 (for simplicity TADJ is not shown. However if not used should be connected to
REF).
R
S
ISM VIN
REF
ADJ
GI
SGND
Figure 30 Buck Configuration
The LED current depends upon the resistors, R
S
, R
GI1
, and R
GI2
as in Equations 4 and
2 above. There is more than one degree of freedom. That is to say, there is not a unique
solution. From Equation 4,
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
V
V
ADJ _ GI
I
225 . 0
R
REF
ADJ
LED
SBoostBB
Equation 11
If ADJ is connected to REF, this becomes
ADJ _ GI
I
225 . 0
R
LED
SBoostBB ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
=
GI_ADJ is given by Equation 2, repeated here for convenience:
⎟
⎠
⎞
⎜
⎝
⎛
+
=
2 RGI 1 RGI
1 RGI
ADJ _ GI
R
S
ISM VIN
REF
ADJ
GI
SGND
R
GI2
R
GI1
Figure 31 Boost and Buck-Boost
Connection
Note that from considerations of ZXLD1374 input bias current, the recommended limits for R
GI1
are:
22kΩ < R
GI1
< 100kΩ Equation 12
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 = 56.25mV. 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 ZXLD1371. If now, GI_ADJ is increased to 0.4 or
0.5, V
RS
is increased to a value greater than 100mV.