BD9422EFV Datasheet, PDF (18/28 Pages) Rohm – White LED Driver for large LCD panel

BD9422EFV
Datasheet
●3.5.10 Setting of the LED_LV voltage (LED_LV pin)
LED_LV pin is in the OPEN (High Impedance) state.
Please be sure to use an external seal of approval, carrying out by inputting REG9V output by resistance division. It cannot
use in the state of OPEN.
○Equation of Setting LED_LV voltage
When LED_LV voltage is set up by resistance division of R1 and R2 using REG9V, it becomes like the following formula.
V
LED
_
LV^
REG9V

R2
R1 R2
[V
]
*Also including the variation in IC, please also take the part variation in a set into consideration for an actual constant setup,
and inquire enough to it.
●3.6 Selecting of DCDC part
Selecting inductor L
V_IN
I_L
L
ΔIL
Figure 23.
V_OUT
R_CS
C_OUT
The value of inductor has a great influence on input ripple current. As
shown in Equation (1), as the inductor becomes large and switching
frequency becomes high, the ripple current of an inductor ⊿IL becomes low.
ΔIL  ^{(VOUT  VIN )  VIN} 　[ A]　　　　・・・・・　 　(1)
L  V_OUT  f _SW
When the efficiency is expressed by Equation (2), input peak current will be
given by Equation (3).
  ^{VOUT  I OUT} 　　　　　・・・・・　　(2)
V_IN  I _IN
IL_MAX
 I _IN
 ^ΔIL
2
 ^{VOUT  I OUT}
V_IN 
　　 　　Δ^IL 　・・・・・　(　3)
2
Here,
L: reactance value [H]
V_OUT: DC/DC output voltage [V]
V_IN: input voltage [V]
L_OUT: output load current (total of LED current) [A]
I_IN: input current [A]
F_SW: oscillation frequency [Hz]
Generally, ⊿IL is set at around 30 to 50 % of output load current.
Figure 24.
* Current exceeding the rated current value of inductor flown through the coil causes magnetic saturation, resulting in
decrease in efficiency. Inductor needs to be selected to have such adequate margin that peak current does not exceed
the rated current value of the inductor.
* To reduce inductor loss and improve efficiency, inductor with low resistance components (DCR, ACR) needs to be
selected.
Selecting output capacitor C_OUT
V_IN
I_L
L
V_OUT
Output capacitor needs to be selected in consideration of equivalent series
resistance required to even the stable area of output voltage or ripple voltage.
Be aware that set LED current may not be flown due to decrease in LED
terminal voltage if output ripple voltage is high.
Output ripple voltage ⊿V_OUT is determined by Equation (4):
ΔV_OUT
 ILMAX
 R_ESR

1
C_OUT

I _OUT


1
f _SW
　[V ]　・・・・・　　(4)
R_ESR: equivalent series resistance of C_OUT
* Rating of capacitor needs to be selected to have adequate margin against
R_ESR
output voltage.
* To use an electrolytic capacitor, adequate margin against allowable current
is also necessary. Be aware that current larger than set value flows
R_CS
C_OUT
transitionally in case that LED is provided with PWM dimming especially.
Figure 25.
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TSZ22111・15・001
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TSZ02201-0F1F0C100210-1-2
31.May.2013 Rev.003