ZXLD1371QESTTC Diodes Inc. / Zetex, ZXLD1371QESTTC Datasheet - Page 25

• Manufacturer Part Number: ZXLD1371QESTTC
• Description: MOSFET LED Controller TSSOP TSSOP-16EP T&R 2.5K
• Manufacturer: Diodes Inc. / Zetex

Specifications of ZXLD1371QESTTC

• Rohs: yes

ZXLD1371
Document number: DS35436 Rev. 1 - 2
Applications Information
Note that from considerations of ZXLD1371 input bias current, the recommended limits for R
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
The mean voltage across the sense resistor is
Note that if GI_ADJ is made larger, these equations show that R
same coil current, the dissipation in R
consider Equation 5 . If ADJ is connected to REF, this becomes
This shows that V
example if D = 0.2, and GI_ADJ is the minimum value of 0.2, then V
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
100mV.
Boost and Buck-boost topology
For Boost and Buck-boost topologies, the LED current depends
upon the resistors, R
above. There is more than one degree of freedom. That is to say,
there is not a unique solution. From Equation 4,
If ADJ is connected to REF, this becomes
GI_ADJ is given by Equation 2, repeated here for convenience:
Buck topology
In Buck mode, GI is connected to ADJ as in Figure 5. The LED
current depends only upon R
above,
If ADJ is directly connected to VREF, this becomes:
R
SBoostBB
R
SBUCK
=
RS
=
R
⎛
⎜ ⎜
⎝
⎛
⎜ ⎜
⎝
. 0
SBoostBB
becomes smaller than 225mV if GI_ADJ < 1 - D. If also D is small, V
I
. 0
LED
GI
I
225
S
LED
218
, R
_
R
⎞
⎟ ⎟
⎠
ADJ
GI1
SBUCK
⎞
⎟ ⎟
⎠
⎛
⎜ ⎜
⎝
GI
=
, and R
V
V
S
⎛
⎜ ⎜
⎝
=
REF
_
, V
ADJ
. 0
I
⎛
⎜
⎝
ADJ
LED
=
225
RGI
ADJ
⎞
⎟ ⎟
⎠
(cont.)
⎛
⎜ ⎜
⎝
. 0
⎛
⎜ ⎜
⎝
GI2
I
RGI
1
LED
⎞
⎟ ⎟
⎠
and V
218
V
V
S
+
REF
ADJ
GI
RGI
as in Equations 4 and 2
is increased. So, in some cases, it is better to minimize GI_ADJ. However,
1
22kΩ < R
0.2 < GI_ADJ < 0.5 Equation 13
⎞
⎟ ⎟
⎠
_
⎞
⎟ ⎟
⎠
REF
2
ADJ
Equation 10
V
⎞
⎟
⎠
. From Equation 1
RS
Equation 11
V
RS
= I
www.diodes.com
GI1
=
COIL
. 0
< 100kΩ Equation 12
225
25 of 42
R
S
⎛
⎜
⎝
Equation 14
GI
1
_
S
− D
ADJ
is increased and V
RS
⎞
⎟
⎠
becomes 0.225* 0.2 / 0.8 = 56.25 mV. This will
Figure 6. Setting LED current in Boost and
Figure 5. Setting LED Current in Buck
Diodes Incorporated
A Product Line of
RS
Buck-boost configurations
is increased to a value greater than
RS
is increased. Therefore, for the
Configuration
GI1
RS
are:
can become too small. For
© Diodes Incorporated
February 2012
ZXLD1371