BD9285F Datasheet, PDF (19/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
Datasheet
●3.4.5. how to set UVLO
Under voltage lock out pin for the input voltage of the power stage. More than 3.0V(typ.), IC starts boost operation and
stops lower than 2.8V(typ.).
The UVLO pin is high impedance, because the internal resistance to a certain bias is not connected.
So, the bias by the external components is required, even if UVLO function is not used, because the open connection of
this pin is not fixed the potential.
The resistor value can be calculated by the below formula, if the VIN voltage is monitored, and that is divided by the
resistor R1, R2 like the below diagram.
○UVLO detection equality
If VIN decreases, R1, R2 value is expressed the following formula by
VIN
the VINdet, the detect voltage of UVLO.
R1 = R2[kΩ]× ^{(VINDET[V ] − 2.8[V ])} [kΩ]　
2.8[V ]
○UVLO release equality
By using the R1, R2 in the above equality, the release voltage of UVLO
can be expressed as following.
VIN_CAN
= 3.0V
×
(R1[kΩ] + R2[kΩ])
R2[kΩ]
[V ]　
ON/ OFF
+
2.8V/3.0V
-
R1
UVLO
R2
CUVLO
[setting example]
If the normal input voltage, VIN is 24V, the detect voltage of UVLO is 18V,
R2 is 30k ohm, R1 is calculated as following.
Figure 26. UVLO setting example
R1 = R2[kΩ]× ^{(VINDET[V ] − 2.8[V ])} = 30[kΩ]× ^{(18[V ] − 2.8[V ])} = 163 [kΩ]
2.8[V ]
2.8[V ]
By using these R1, R2, the release voltage of UVLO, VINcan can be calculated as following.
VIN_CAN
= 3.0[V ]×
(R1[kΩ] + R2[kΩ])
R2[kΩ]
= 3.0[V ]×
30[kΩ] +163[kΩ][_V
30[kΩ]
]　= 19.3
[V ]
www.rohm.com
© 2012 ROHM Co., Ltd. All rights reserved.
TSZ22111・15・001
19/36
TSZ02201-0F1F0C100030-1-2
11.Jul.2012 Rev.001