BD9285F Datasheet, PDF (24/39 Pages) Rohm – Boost 1channel white LED driver For large LCDs

BD9285F
3.5.2. Inductor selection
V_IN
I_L
L
ΔIL
V_OUT
R_CS
C_OUT
Datasheet
The inductor value affects the input ripple current. The equality in the section
3.5.1 is as following.
ΔIL = ^{(VOUT [V ] − VIN [V ]) × VIN [V ]} [ A]
L[H ] × V_OUT [V ] × f _SW [Hz ]
I _IN
= ^{VOUT [V ]× IOUT [ A]}
V_IN [V ]×η[%]
[ A]　
Ipeak
=
I _IN [ A] +
∆IL[ A]
2
[ A]
Where
L: the coil inductance [H]
Vout: the DCDC output voltage [V]
Vin: the input voltage [V]
Iout: the output load current (the summation of LED current) [A]
Iin: the input current [A]
Fsw: the oscillation frequency [Hz]
If in the continuous current mode, Please set ⊿IL to 30% - 50% of the output
load current.
Figure 29. the waveform and the circuit of
inductor current
* The current exceeding the rated current value of inductor flown through the coil causes magnetic saturation, results
in decreasing 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
3.5.3. Output capacitance Cout selection
V_IN
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
I_L
L
aware that set LED current may not be flown due to decrease in LED terminal
voltage if output ripple component is high.
V_OUT
Output ripple voltage ⊿VOUT is determined by Equation (4):
R_ESR
ΔV_OUT
= ILMAX
× R_ESR
+
1
C_OUT
×
I _OUT
η
×
1
f _SW
　[V ]　・・・・・　(　4)
R_CS
C_OUT
where, RESR is the equivalent series resistance of Cout.
Figure 30. the output capacitor circuit
* Rating of capacitor needs to be selected to have adequate margin against output voltage.
* To use an electrolytic capacitor, adequate margin against allowable current is also necessary. Be aware that the
LED current is larger than the set value transitionally in case that LED is provided with PWM dimming especially.
3.5.4. MOSFET selection
Though there is no problem if the absolute maximum rating is larger than the rated current of the inductor L, or is
larger than the sum of the tolerance voltage of COUT and the rectifying diode VF. The product with small gate
capacitance (injected charge) needs to be selected to achieve high-speed switching.
* One with over current protection setting or higher is recommended.
* The selection of one with small on resistance results in high efficiency.
3.5.5. Rectifying diode selection
A schottky barrier diode which has current ability higher than the rated current of L, the reverse voltage larger than the
tolerance voltage of COUT, and the low forward voltage VF especially needs to be selected.
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TSZ22111・15・001
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TSZ02201-0F1F0C100030-1-2
11.Jul.2012 Rev.001