BD9394FP Datasheet, PDF (15/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9394FP, BD9394EFV
Datasheet
●•LSP setting procedure
Making a change to the LSP pin input voltage will allow the threshold for LED short circuit protection to be changed.
The LED short circuit detection voltage is set to 6V (Typ.) with the LSP pin being in the open-circuited state.LSP pin
input voltage setting should be made in the range of 0.8V to 3V.
The relation between the LSP pin voltage and the LED short circuit protection detection voltage is given by the following
equation.
LED
= 5 × VLSP [V ]
SHORT
Since the LSP pin divides 3V within the IC using resistive dividers (see the circuit diagram shown below), connecting an
external resistor to the LSP pin will produce resistance combined with the internal IC resistance.
Consequently, to make LSP pin voltage setting using external resistive dividers, it is recommended to connect them
having resistance little affected by the internal resistance. (Smaller resistance makes the LSP pin increasingly less likely
to be affected by the internal resistance, but this results in more power consumption. Careful attention should be paid to
this matter.)
cLSP detection voltage setting equation
If the setting of LSP detection voltage VLSP is made by dividing the REG58 voltage by the use of resistive dividers R1
and R2, VLSP will be given by the following equation:
LED_SHORT
=
⎜⎜⎝⎛ ^{REG58[V ]×}
R2[kΩ]
(R1[kΩ] + R2[kΩ]
⎟⎟⎠⎞
×5
[V ]　L(1)
However, this equation includes no internal IC resistance. If internal resistance is taken into account, the detection
voltage VLSP will be given by the following equation:
LED_SHORT
=
⎜⎜⎝⎛
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])
⎟⎟⎠⎞
×5
[V ]　K(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 6V, R1 comes to 38.3kΩ
and R2 comes to 10kΩ.
When calculating LSP detection voltage taking into account internal IC resistance by Equation (2), it will be given as:
VLSP
=
⎜⎜⎝⎛
(38.3[kΩ]
×
10[kΩ]×1000[kΩ]× (5.8[V ] × 2000[kΩ] + 3[V ]× 38.3[kΩ])
2000[kΩ]× (10[kΩ] + 1000[kΩ]) + 10[kΩ]×1000[kΩ]× (38.3[kΩ]
+
2000[kΩ]) ^{⎟⎟⎠⎞}
×
5
=
5.992[V
]
The difference is given as:
(5.992[V ] − 6[V ])/ 6[V ]×100 = −0.13%
As a result, this setting will be little affected by internal impedance.
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TSZ22111・15・001
15/28
TSZ02201-0F1F0C100180-1-2
09.Nov.2012 Rev.002