BD9420F Datasheet, PDF (14/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●LED short voltage setting(BSx pin, LSP pin)
The detection voltage of LED short when BSx pin higher than 9V while
LSP pin is in OPEN state.
It is possible to change the LED short detection voltage, please input
(0.3V～2.5V) to LSP pin.
The relationship between LED short detection voltage 「VLED_short」, and
LSP pin voltage 「V_LSP」 as below equation.
V_LSP
[V
]

VLED_short
10
[V
]
In addition, because LSP pin has split 4V of terminal resistance inside IC.
（Refer to upper right schematic）, it will be combined resistance value of IC’s internal resistance and connecting to
external resistor. For this reason, when configure the voltage of LSP in resistor divider, it is recommended to connect a
resistor which has a little impact from internal resistance value.（Resistance is less susceptible to internal resistor, but
care must be taken because power consumption will become large.）
○LSP setting procedure
Below equation shows how to calculate the detection voltage of VLSP by using R1,R2 resistor divide which connect to
REG50 voltage.
However, this equation is without considering IC resistance. If internal resistance is taken into account, the detection
voltage VLSP will be given by the following equation:
VLSP


R2[k] R4[k]
(R1[k] R3[k] R2 
REG50[V ]
R4  R2[k]
R3  REF[V ] R1[k]
 R4[k] R1[k]  R3[k]

10
[V ]　(2)
Please set 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 reference.
【Setting example】
Assuming that LSP is approximated by Equation (1) in order to set LSP detection voltage to 5V, R1 comes to 51k and R2
comes to 3.6k.
When calculating LSP detection voltage taking into account internal IC resistance by Equation (2), it will be given as:
The difference is given as:
As a result, this setting will be little affected by internal impedance.
※For the selection of DC/DC components, please also consider the inaccuracy of each componentts.
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TSZ02201-0F1F0C100300-1-2
31.Jan.2014 Rev.002