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BD8119FM-M
Technical Note
1. Computation of the Input Peak Current and IL_MAX
① Calculation of the maximum output voltage (Vout_max)
To calculate the Vout_max, it is necessary to take into account of the VF variation and the number of LED connection in
series.
Vout_max = (VF + Δ VF) × N + 1.0V
② Calculation of the output current Iout
Iout = ILED × 1.05 × M
③ Calculation of the input peak current IL_MAX
IL_MAX = IL_AVG + 1/2 Δ IL
IL_AVG = (VIN + Vout) × Iout / (n × VIN)
Δ VF: VF Variation N: Number of LED connection in series
Number of LED connection in parallel
Δ IL=
V IN
L
×
1
Fosc
×
Vout
V IN +Vout
n: efficiency
Fosc: switching frequency
? The worst case scenario for VIN is when it is at the minimum, and thus the minimum value should be applied in the
equation.
? The L value of 10μF ? 47μF is recommended. The current-mode type of DC/DC conversion is adopted for BD8119FM-M,
which is optimized with the use of the recommended L value in the design stage. This recommendation is based upon
the efficiency as well as the stability. The L values outside this recommended range may cause irregular switching
waveform and hence deteriorate stable operation.
? n (efficiency) is approximately 80%
V IN
I L
Rcs
CS
M1
L
D2
Vout
M2
D1
Co
External Application Circuit
2. The setting of over-current protection
Choose Rcs with the use of the equation Vocp_min (=0.54V) / Rcs > IL_MAX
When investigating the margin, it is worth noting that the L value may vary by approximately ±30%.
3.
The selection of the L
In order to achieve stable operation of the current-mode DC/DC converter, we recommend selecting the L value in the
range indicated below:
Vout × Rcs
0.05 [V/μS] < < 0.3 [V/μS]
L
The smaller
Vout × Rcs
L
allows stability improvement but slows down the response time.
4. Selection of coil L, diode D1 and D2, MOSFET M1 and M2, and Rcs
Current rating Voltage rating
Heat loss
Coil L
Diode D1
Diode D2
MOSFET M1
MOSFET M2
> IL_MAX
> Iocp
> Iocp
> Iocp
> Iocp
―
> VIN_MAX
> Vout
> VIN_MAX
> Vout
Rcs
―
―
> Iocp 2 × Rcs
※ Allow some margin, such as the tolerance of the external components, when selecting.
※ In order to achieve fast switching, choose the MOSFETs with the smaller gate-capacitance.
www.rohm.com
? 2011 ROHM Co., Ltd. All rights reserved.
12/20
2011.08 - Rev.B
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