Technical Note

11/19

BD8113EFV

www.rohm.com

2011.07 - Rev.A

© 2011 ROHM Co., Ltd. All rights reserved.

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.

②

Calculation of the output current Iout

M:Number of LED connection in parallel

③

Δ

IL=

×

×

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

BD8113EFV, 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

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

―

Diode D1

> Iocp

> VIN_MAX

Diode D2

> Iocp

> Vout

MOSFET M1

> Iocp

> VIN_MAX

MOSFET M2

> Iocp

> Vout

Rcs

―

―

> Iocp

※

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.

L

1

Fosc

Vout

Vout×Rcs

L

Rcs

D1

L

D2

M2

M1

Co

Vout

CS

Vout×Rcs

L