BD9420F Datasheet, PDF (16/31 Pages) Rohm – White LED Driver for large LCD Panels (DCDC Converter Type)

BD9420F
Datasheet
●Selection of DC/DC Components
OCP setting / DCDC component current tolerance selection guide
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 larger than 0.4V.Consequently, it is needed to calculate a peak current flowing through the coil L 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 describes the peak coil current calculation procedure, CS pin
connection resistor RCS selection procedure, and DC/DC component current tolerance selection procedure .
○Peak coil current Ipeak calculation
Ripple voltage generated at the CS pin is determined by conditions
L
VOUT
for DC/DC application components first, Assuming the conditions as
below:
VIN
IL
「output voltage=VOUT[V] 」
「LED total current=IOUT[A] 」
fsw
「DCDC input voltage=VIN[V] 」
「DCDC efficiency=η[%] 」
N
And then mean input current IIN required for the whole system is given
by the following equation.
I _IN
 ^{VOUT [V ] IOUT [ A]}
V_IN [V ][%]
[ A]　
CS
Rcs
PGND
Further, according to drive operation with the DC/DC converter switching
frequency fsw [Hz], inductor ripple current ΔIL [A] generated at the inductor L (or
H) is given by the following equation.
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)
○CS pin connection resistor RCS selection procedure
The current Ipeak flows into RCS to generate voltage.（See the timing
chart shown to the right）The voltage VCSpeak is given by the following
equation.
VCS _peak  Rcs  Ipeak [V ]
If VCSpeak voltage reaches 0.4V, DC/DC output will stop.Consequently,
to select RCS resistance, the following condition should be met.
Rcs  Ipeak [V ]  0.4[V ]
○DC/DC component current tolerance selection procedure
Iocp current needed for OCP detection voltage CS to reach 0.4V is given
by the following equation:
I _OCP

0.4[V ]
Rcs[]
[ A](2)
The relation among Ipeak current (Equation (1)), Iocp current (Equation (2)), and Maximum current tolerance for
component should meet the following equation.
I _peak  I _OCP  MAX current tolerance
DC/DC application components including FETs, inductors, and diodes should be selected so that the Equation
shown above will be met.
In addition, it is recommended to use continuous mode in DCDC application. And the lower limit value of coil ripples
current Imin so as to meet the following equation:
I min
 I _IN [ A] 
IL[ A] _{[ A]}
2
0
A failure to meet this condition is referred to as discontinuous mode and this failure may result in an inadequate rise in
output voltage.
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TSZ22111 • 15 • 001
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TSZ02201-0F1F0C100300-1-2
31.Jan.2014 Rev.002