BD9470AEFV Datasheet, PDF (26/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●LSP setting procedure
LED SHORT threshold voltage can be adjusted by setting LSP pin voltage.
LED SHORT detection voltage is set to 9V when LSP pin=OPEN state.
Please set input voltage of LSP pin from 0.3V~2.5V range.
The relation between LSP pins and LED SHORT protection voltage as below.
VLSP_SHORT
=
VLED_SHORT
10
　[V ]
Also, LSP pin divides 4V within the IC using resistive dividers (see the circuit diagram shown below)
Therefore, connecting an external resistor to the LSP pin will produce resistance combined with the internal IC resistance.
Consequently, LSP pin voltage setting using external resistive dividers, it is recommended to connect them having
resistance little affected by the internal resistance.(Smaller resistance have less influence on internal resistance, but will
result in larger power consumption.)
REG58=5.8V
REF=4V
R3=
R1
3100kΩ LSP
+
R4
900kΩ
CLSP _R2
900kΩ
-
100kΩ
LEDx
Figure 30. LSP Block diagram
○LSP detection voltage setting
If the setting of LSP detection voltage VLSP is made by dividing the REG58V voltage by the use of resistive dividers
R1and R2, VLSP will be given by the following equation.
VLSP
=

REG58[V
]×
R2[kΩ]
(R1[kΩ] + R2[kΩ]

×10
[V ]　(1)
However, this equation includes no internal IC resistance. If internal resistance is taken into account, detection voltage
VLSP will be given by the following equation.
VLSP
=

R2[kΩ]× R4[kΩ]× (REG58[V ]× R3 + REF[V ]× R1[kΩ])
(R1[kΩ]× R3[kΩ]× (R2 + R4)+ R2[kΩ]× R4[kΩ]× (R1[kΩ] + R3[kΩ])

×10
[V ]　(2)
Make setting of R1 and R2 resistance so that a difference between resistance values found by Equations (1) and (2) will
come to approximately 2% or less as a guide.
○Setting example
Assuming that LSP is approximated by Equation (1) in order to set LSP detection voltage to 5V, R1 comes to 53kΩ andR2
comes to 5kΩ.LSP detection voltage taking into account internal IC resistance by Equation (2), it will be given as
VLSP
=

(53[kΩ]
×
5[kΩ]× 900[kΩ]× (5.8[V ]× 3100[kΩ] + 4[V ]× 53[kΩ])
3100[kΩ]× (5[kΩ] + 900[kΩ]) + 5[kΩ]× 900[kΩ]× (53[kΩ]
+
3100[kΩ])

×10
=
5.033V
[V
]
The difference is given as:
(5.033[V ] − 5[V ])/ 5[V ]×100 = 0.66%
As a result, this setting will be little affected by internal impedance.
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003