BD9470AEFV Datasheet, PDF (25/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
●OVP/SCP setting method
The OVP pin is an input pin for overvoltage protection and short circuit protection of DC/DC output voltage.
The OVP pin is a high impedance type and no pull-down resistor inside, resulting in unstable potential in the open circuit
state. To avoid this problem, be sure to make input voltage setting with the use of a resistive divider.
Conditions for each OVP protections are as listed in the table below.
Protection name
OVP Timer SET /
OVP Cancel
OVP Detect /
DCDC STOP
SCP
Protection
pin
OVP
OVP
OVP
Detection
Condition
OVP>2.9V
OVP>3.0V
OVP<0.1V
Release
Condition
OVP<2.9V
OVP<3.0V
OVP>0.1V
Timer
Operation
Yes
No
Yes
Protection type
All latch
Only DCDC converter
stops during detection
All latch
FAIL pin
GND
OPEN
GND
The following describes the setting procedures of that VOUT pin voltage to be detected is set by the use of resistive
dividers R1 and R2 as shown in the circuit diagram below.
○OVP detection setting method
Assuming that a voltage causing VOUT to abnormally rise
and detecting OVP is “VOVPDET”,
R1 and R2 setting will be made by the following equation.
R1 = R2[kΩ]× ^{(VOVPDET [V ] − 3.0[V ])} [kΩ]　
3.0[V ]
○Timer set・OVP release setting equation
When R1 and R2 setting is determined by the equation
shown above, OVP release voltage VOVPCAN will be
given by the following equation:
VOVP_CAN
= 2.9V
× ^{(R1[kΩ] + R2[kΩ])}
R2[kΩ]
[V ]　
○SCP detection equation
When R1 and R2 setting is determined by the equation
shown above, SCP setting voltage VSCPDET will be given
by the following equation.
VSCP_DET
= 0.1V
× ^{(R1[kΩ] + R2[kΩ])}
R2[kΩ]
[V ]　
VOUT
OVP
R1
DCDC_STOP_COMP
+
3.0V
-
DQ
OVP_TIMER_COMP
+
2.9V
-
R2
H:STOP
L:ACT
N
DRIVER
SCP_TIMER_COMP
+
0.1V
-
CP
Timer
65536
CP
Timer
65536×8
Figure 29. OVP block diagram
○Setting example
Assuming that normal VOUT voltage is 40V, OVP detection voltage VOVPDET is 48V, and R2 resistance is 10kΩ, R1
resistance is calculated by the following equation
R1 = R2[kΩ]× ^{(VOVPDET [V ] − 3.0[V ])} = 10[kΩ]× ^{(48[V ] − 3[V ])} = 150 [kΩ]
3.0[V ]
3[V ]
When OVP release voltage VOVPCAN setting is made with the said R1 and R2, it will be given by the following equation
VOVP_CAN
=
2.9[V ]× ^{(R1[kΩ] + R2[kΩ])}
R2[kΩ]
=
2.9[V ]× ^{10[kΩ] +150[kΩ][}V ]　= 46.4
10[kΩ]
[V ]
SCP detection voltage is given by the following equation
VSCP_DET
=
0.1[V ]× ^{(R1[kΩ] + R2[kΩ])}
R2[kΩ]
=
0.1[V ]× ^{10[kΩ] +150[kΩ][}V ]　= 1.6
10[kΩ]
[V ]
Give consideration to IC variations as well as individual component variations, and then evaluate on actual systems.
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TSZ02201-0F10C1002000-1-2
19.Oct.2013 Rev.003