BD9470AEFV Datasheet, PDF (28/39 Pages) Rohm – White LED driver for medium sized and large sized LCD back light

BD9470AFM・BD9470AEFV
3.2 Selection of DCDC components
●OCP setting procedure/DCDC component current tolerance selection procedure
The OCP detection function that is one of the functions of the CS pin will stop the DC/DC converter operating if the CS pin
voltage becomes greater than 0.4V. Consequently, it is needed to calculate a peak current flowing through the coilL and
then review the resistance of RCS. Furthermore, a current tolerance for DC/DC components should be larger than that for
peak current flowing through the coil L.
The following section describes the peak coil current calculation procedure, CS pin connection resistor RCS selection
procedure, and DC/DC component current tolerance selection procedure
○Calculation of coil current Ipeak
Ripple voltage generated at the CS pin is determined by conditions for DC/DC application components. Assuming the
conditions:
output voltage=VOUT [V]
L
VOUT
LED total current=IOUT [A]
DCDC input voltage=VIN [V]
VIN
IL
DCDC efficiency=η [%]
mean input current IIN required for the whole system is given by the following
fsw
equation
I _IN
= ^{VOUT [V ]× IOUT [ A]}
V_IN [V ]×η[%]
[ A]　
Further, according to drive operation with the DC/DC converter switching
frequency fsw [Hz], inductor ripple current ΔIL [A] generated at the inductor L
is given by the following equation.
N
CS
Rcs
DCDC_GND
ΔIL = ^{(VOUT [V ] − VIN [V ]) ×VIN [V ]} [ A]
L[H ] ×V_OUT [V ] × f _SW [Hz]
As a result, the peak current Ipeak of IL is given by the following equation.
Ipeak
=
I _IN
[ A] +
∆IL[ A]
2
[ A](1)
(V)
○CS pin connection resistor RCS selection procedure
The current Ipeak flows into RCS to generate voltage.(See timing chart
shown to the right.)
The voltage VCSpeak is given by the following equation.
VCS _peak = Rcs × Ipeak [V ]
（A)
If this VCSpeak voltage reaches 0.4V, DC/DC output will stop.
IIN
Consequently, to select RCS resistance, the following condition should be
met.
(t)
Ipeak
ΔIL
Imin
Rcs[Ω]× Ipeak[ A] < 0.4[V ]
(t)
（V)
○DCDC component current tolerance selection procedure
0.5V
Iocp current needed for OCP detection voltage CS to reach 0.4V is given
VCSpeak
by the following equation
I _ocp
=
0.4[V ]
Rcs[Ω]
[ A](2)
(t)
Figure32.
The relation among Ipeak current (Equation (1)), Iocp current (Equation (2)), ^{DCDCapplication diagram and coil current}
I _peak < I _ocp < Max. current tolerance for component
DC/DC application components including FETs, inductors, and diodes should be selected so that the Equation shown
above will be met.
Furthermore, it is recommended to normally use DC/DC application components in continuous mode. Assuming that the
lower limit value of coil ripple current is Imin, the following equation should be met
Im in
=
I
IN
[ A]
−
∆IL[ A][ _A]
2
>
0　
A failure to meet this condition is referred to as discontinuous mode.
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19.Oct.2013 Rev.003