BD9285F Datasheet, PDF (22/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
Datasheet
●3.5. DCDC parts selection
3.5.1. how to set OCP / the calculation method for the current rating of DCDC parts
BD9285 stops the switching by the OCP detect, when the CS pin voltage is more than 0.5V. The resistor value of CS pin,
RCS 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.
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]
VIN
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
I _IN
= ^{VOUT [V ]× IOUT [ 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.
L
IL
fsw
GATE
CS
Rcs
GND
VOUT
Ipeak
=
I _IN [ A] +
∆IL[ A]
2
[ A]
… (1)
Therefore, the bottom of the ripple current Imin is
Im
in
=
I _IN
[ A]
−
∆IL[ A]
2
　
or 0
As Imin>0, that operation mode is CCM (Continuous Current Mode),
otherwise another mode is DCM (Discontinuous Current Mode).
(the selection method of Rcs)
Ipeak flows into Rcs and that cause the voltage signal to CS pin. (Please
refer the right timing chart)
That peak voltage VCSpeak is as following.
VCS _peak = Rcs × Ipeak [V ]
As this VCSpeak reaches to 0.5V, the DCDC output stops the switching.
Therefore, Rcs value is necessary to meet the under condition.
Rcs × Ipeak [V ] << 0.5[V ]
(the current rating of the external DCDC parts)
The peak current as the CS voltage reaches to OCP level (0.5V) is
defined as Ipeak_det.
I _{peak _ det}
=
0.5[V ]
Rcs[Ω]
[ A]
… (2)
Figure 28. Coil current waveform
The relation among Ipeak (equality (1)), Ipeak_det (equality (2)) and the current rating of parts is required to meet the
following
I _peak << I _{peak _ det} << ^{The current rating of parts}
Please make the selection of the external parts to meet the above condition such as FET, Inductor, diode.
[setting example]
The output voltage = VOUT [V] = 40V
LED total current = IOUT [A] = 0.48V
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11.Jul.2012 Rev.001