BD9489F Datasheet, PDF (23/39 Pages) Rohm – DCDC converter with current mode

BD9489F
3.3.2. Inductor Selection
The inductor value affects the input ripple current, as shown the previous section 3.3.1.
ΔIL  ^{(VOUT [V]  VIN[V])  VIN[V]} [A]
L[H]  V_OUT [V]  f_SW [Hz]
ΔIL
I_IN

V_OUT[V]  I_OUT[A]
V_IN[V]  [%]
[A]
Ipeak
 I_IN[A ] 
IL[ A ]
2
[A]
Where
L: coil inductance [H]
V_OUT: DCDC output voltage [V]
V_IN: input voltage [V]
I_OUT: output load current (the summation of LED current) [A]
I_IN: input current [A]
f_SW: oscillation frequency [Hz]
Datasheet
Figure 33. Inductor current waveform and diagram
In continuous current mode, ⊿IL is set to 30% to 50% of the output load current in many cases.
In using smaller inductor, the boost is operated by the discontinuous current mode in which the coil current returns to
zero at every period.
*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
3.3.3. 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 aware that set
I_L
L
LED current may not be flown due to decrease in LED terminal voltage if output ripple
V_OUT
component is high.
Output ripple voltage V_OUT is determined by Equation (4):
R_ESR
ΔVout ΔIL  R_ESR　[V]　・・・・・　(　4)
R_CS
C_OUT
When the coil current is charged to the output capacitor as MOS turns off, much output
ripple is caused. Much ripple voltage of the output capacitor may cause the LED current
Figure 34. Output capacitor diagram
ripple.
* 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.
3.3.4. MOSFET Selection
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 C_OUT and the rectifying diode V_F. 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.
3.3.5. Rectifying Diode Selection
A schottky barrier diode which has current ability higher than the rated current of L, reverse voltage larger than the
tolerance voltage of C_OUT, and low forward voltage VF especially needs to be selected.
www.rohm.com
© 2013 ROHM Co., Ltd. All rights reserved.
TSZ22111・15・001
23/36
TSZ02201-0F1F0C100250-1-2
29.Oct 2014 Rev.002