BD9483F Datasheet, PDF (16/32 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter type)

BD9483F,FV
Datasheet
[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[k] ^{(VOVPDET[V] 3.0[V])}  10[k] ^{(48[V] 3[V])}  150 [k]
3.0[V]
3[V]
By using these R1, R2, the release voltage of OVP, VOVPcan can be calculated as following.
VOVP  2.9[V] ^{(R1[k]  R2[k])}  2.9[V] ^{10[k]  150[k][V]}　 46.4 [V]
CAN
R2[k]
10[k]
●3.4.7 how to set the interval until latch off (CP pin)
BD9483F,FV starts the counting up (charging CP pin) by the detection of FBMAX abnormal state, and BD9483F,FV falls
to the latch off state when the following interval has passed.
Only PWM=L input does not reset the timer counter, as the abnormal state continues.
LATCH_TIME = 1.0 * 10⁶ * C_cp [sec]
Where LATCH_TIME is the interval until latch off state
C_CP is the external capacitor of CP pin.
[setting example]
If the capacitor of CP pin is 0.47uF, the timer latch interval is as following.
LATCH_TIME = 1.0 * 10⁶ * C_cp [sec] = 1.0 * 10⁶ * 0.47 * 10^-6 [sec] = 470 [msec]
●3.5 DCDC parts selection
●3.5.1 how to set OCP / the calculation method for the current rating of DCDC parts
BD9483F,FV stops the switching by the OCP detect, when the CS pin voltage is
more than 0.4V. The resistor value of CS pin, R_CS need to be considered by the
coil L current. And the current rating of DCDC external parts is required more
than the peak current of the coil.
VIN
It is shown below that the calculation method of the coil peak current, the
selection method of Rcs (the resistor value of CS pin) and the current rating of
the external DCDC parts.
(the calculation method of the coil peak current, Ipeak)
At first, since the ripple voltage at CS pin depend on the application condition of
DCDC, those put onto the equality to calculate as following.
The output voltage = VOUT [V]
LED total current = IOUT [A]
The DCDC input voltage of the power stage = VIN [V]
The efficiency of DCDC =η[%]
And then, the averaged input current IIN is calculated by the following
equality
(V)
I _IN

V_OUT[V] I_OUT[A]
V_IN[V] η[%]
[A]
And the ripple current of the inductor L (ΔIL[A]) can be calculated by using
DCDC the switching frequency, fsw, as following.
ΔIL  ^{(VOUT [V]  VIN [V])  VIN [V]} [A]
L[H]  V_OUT [V]  f _SW [Hz]
On the other hand, the peak current of the inductor Ipeak can be expressed
as the following equality.
Ipeak

I _IN [A]

ΔIL[A]
2
[A]
… (1)
（A)
IIN
（V)
Therefore, the bottom of the ripple current Imin is
0.4V
Imin

I _IN [A]

ΔIL[A]
2
　or
0
As Imin>0, that operation mode is CCM (Continuous Current Mode),
otherwise another mode is DCM (Discontinuous Current Mode).
L
IL
fsw
GATE
CS
Rcs
GND
Figure 27.
VOUT
(t)
Ipeak
ΔIL
Imin
(t)
Figure 28.
VCSpeak
(t)
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