Technical Note
12/20
www.rohm.com
2011.08 - Rev.B
© 2011 ROHM Co., Ltd. All rights reserved.
BD8119FM-M
1. Computation of the Input Peak Current and IL_MAX
①Calculation 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 + ΔVF) × N + 1.0V ΔVF: VF Variation N: Number of LED connection in series
②Calculation of the output current Iout
Iout = ILED × 1.05 × M Number of LED connection in parallel
③Calculation of the input peak current IL_MAX
IL_MAX = IL_AVG + 1/2ΔIL
IL_AVG = (VIN + Vout) × Iout / (n × VIN)
ΔIL= × × 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%
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:
0.05 [V/µS] < < 0.3 [V/µS]
The smaller 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 > IL_MAX ―
Diode D1 > Iocp > VIN_MAX
Diode D2 > Iocp > Vout
MOSFET M1 > Iocp > VIN_MAX
MOSFET M2 > Iocp > 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.
VIN
L
1
Fosc
Vout
V
IN+Vout
V
IN
Rcs
D1
L
D2
M2
M1
Co
Vout
I
L
CS
Vout×Rcs
L
Vout×Rcs
L
