BD9206EFV Datasheet, PDF (5/13 Pages) Rohm – Simple Structure Constant Current Backlight Driver for LCD panels (Non-step type)

BD9206EFV
Technical Note
● Block functional descriptions
■PREREG, REF, 5VREG
PREREG is an circuit of constant voltage supplied to REF and 5VREG in which the voltage applied to VCC terminal is made
to be constant.
REF is a temperature-compensated reference voltage resource and used as reference voltage of TSD (Thermal Shutdown
Circuit).
5VREG is a 5V constant-voltage source and used as a power supply of constant-current driver.
The 5V constant voltage is output to VREG terminal. Moreover, it is recommended to attach a 1μF ceramic capacitor using
for phase correction, to VREG terminal.
■UVLO（Under Voltage Lock Out）
The LED driver is turned OFF when the VREG voltage is less than 2.9V(typ). The operation of lighting up is reset when
VREG becomes more than 3.0V(typ).
■ TSD（Thermal Shutdown Circuit）
TSD circuit protects the IC from thermo runaway or thermal damage.
TSD circuit detects the chip temperature and turns the circuit off if the chip temperature reaches 175℃. The hysteresis of
20℃ is set for TSD detection and release so as to prevent malfunction caused by temperature fluctuations.
■Current Driver（Constant-current driver）, Driver Current Setting
Current Driver(Constant-current driver) is an circuit that generates a constant current for lighting of LED.
Constant-current circuit of BD9206EFV consists of the constant current setting part and the constant current driver part.
The constant-current driver part operates in such a manner that the voltage of Point a is equal to the voltage of point b
because the part serves as a buffer, the input of which is the voltage VX that is set by the constant current setting part.
Therefore, the current ILED that flows into the VLED terminal is as follows:
ILED = Vb/RSET = Va/RSET = VX/RSET = VSET*A/RSET = VSET*B
（A and B are numerical constants）
For BD9206EFV, the numerical constants inside the IC are set in such a way that the following formula is brought into
existence:
ILED(mA) = VSET ＊10 (VSET=0.6～3.0V)
If VSET is fixed, then the Vb is fixed, therefore the current ILED always flows independent of the fixed voltage of VLED.
However, the constant current operation is stopped if the voltage of VLED terminal is less than 0.6V, so please ensure
VLED>0.6V.
Vo
VSET
[Constant【c定urre電nt s流etti設ng 定part部] 】
VX=VSET*A
Vo電ltag圧e
sh_変ift c_換ircuit
VX
回路
[Cons【ta定nt c電urre流nt dドrivラerイpaバrt] 部 】
Condenser for phase
compe^位ns^相atio^補n ^{償 用 コ ン デ ン サ}
a
+
IL E D
VLED
-
b
RSET
EN
Fig..5
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5/12
2009.07 - Rev.A