BD9483F Datasheet, PDF (14/32 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9483F,FV
Datasheet
○Sequence explanation of shut down
1. When ENA=L, DCDC and REG90 is stopped.
2. While ENA=L and REG90UVLO=H, DIMOUT asserts the same logic of PWM. And VOUT is discharged until
REG90=9.0V is reached to 6.0V by 500kΩ.
3. VOUT is enough discharged by ILED, ILED don’t get to flow.
4. REG90 voltage is reached under 6.0V(typ.), whole system is shutdown.
○Setting method of REG90 capacitance
Shutdown time TOFF is decided by the following equation.
T_OFF[sec] 
C
REG
[F]

R
REG
[]

In
REG90 _t0[V]
REG90 _UVLO [V]

C_REG [F]  500[k]  in
9.0[V]
6.0[V]

20.2 10⁵
 C_REG [sec]
When discharge function is used, PWM signal must be continued to input after ENA=L.
VOUT discharge time is longest when PWM is set on mininum DUTY.
Please set CREG capacitance value with margin so that the system is shutdown after VOUT is enough
discharged.
●3.4.3 The LED current setting
LED current can be adjusted by setting the resistance R_ISENSE which connects to ISENSE
pin.
○the relationship between R_ISENSE and ILED current
With DC dimming (ADIM<3.0V)
Without DC dimming (ADIM>3.0V)
R _ISENSE

ADIM[V]/3 _[]
I _LED [A]
R _ISENSE

1.0[V] _[]
I _LED [A]
[setting example]
If ILED current is 400mA as ADIM is 3.0V, we can calculate R_ISENSE as below.
R _ISENSE

ISENSE[V]
I _LED [A]

ADIM / 3[V]
I _LED [A]

3.0 / 3[V]
0.4[A]

2.5[]　　
●3.4.4 how to set DCDC oscillation frequency
R_RT which connects to RT pin set the oscillation frequency of DCDC.
○ the relationship between OSC and R_RT (ideal)
R _RT

15000
f_SW [kHz]
[k]
Vout
DIMOUT
Error AMP
ISENSE
1.0V
R_ISENSE
FB
Figure 22.
where fsw is the oscillation frequency of DCDC [kHz]
Frequency (fsw)
Ideal
GATE
Figure 23.
RT
R_RT
CS
Rcs
GND
Figure 24.
This equation is an ideal equation in which correction factors are not applied.
The adequate verification with an actual set needs to be performed to set frequency precisely.
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28.Nov.2013 Rev.003