BD9285F Datasheet, PDF (20/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
Datasheet
●3.4.6. how to set OVP, SCP
The OVP terminal is the input for over-voltage protection and short circuit protection of output voltage.
The OVP 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 OVP 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 VOUT voltage is monitored, and that is divided by the
resistor R1, R2 like the below diagram.
○OVP detection equality
If the VOUT is boosted abnormally, VOVPdet is the
detect voltage of OVP, R1, R2 can be expressed by the
following formula.
R1 = R2[kΩ]× ^{(VOVPDET [V ] − 3.0[V ])} [kΩ]　
3.0[V ]
OVP
+
VOUT
R1
OVP
○OVP release equality
By using the R1, R2 in the above equality, the release
voltage of OVP, VOVPcan can be expressed as
following.
VOVP_CAN
=
2.9V
×
(R1[kΩ] + R2[kΩ])
R2[kΩ]
[V ]　
-
SCP
-
0.2V/0.25V
+
R2
COVP
○SCP detection equality
In the same way, the detect voltage of SCP, VSCPdet is
VSCP_DET
=
0.2V
×
(R1[kΩ] + R2[kΩ])
R2[kΩ]
[V ]　
Figure 27. OVP/SCP setting example
[setting example]
If the normal output voltage, VOUT is 40V, the detect voltage of OVP is 48V, R2 is 10k ohm, R1 is calculated as
following.
R1 = R2[kΩ]× ^{(VOVPDET [V ] − 3.0[V ])} = 10[kΩ]× ^{(48[V ] − 3[V ])} = 150 [kΩ]
3.0[V ]
3[V ]
By using these R1, R2, the release voltage of OVP, VOVPcan can be calculated as following.
VOVP_CAN
= 2.9[V ]× ^{(R1[kΩ] + R2[kΩ])}
R2[kΩ]
= 2.9[V ]× ^{10[kΩ] +150[kΩ][}V ]　= 46.4
10[kΩ]
[V ]
Moreover, by using these R1, R2 the detect voltage of SCP, VSCPdet is
VSCP_DET
=
0.2[V ]×
(R1[kΩ] + R2[kΩ])
R2[kΩ]
=
0.2[V
]×
10[kΩ] +150[kΩ][_V
10[kΩ]
]　= 3.2
[V ]
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