Datasheet

Datasheet

19/34

TSZ02201-0F1F0C100240-1-2

© 2013 ROHM Co., Ltd. All rights reserved.

15.Feb.2016 Rev.006

www.rohm.com

TSZ22111・15・001

BD9486F

●3.3 DCDC Parts Selection

3.3.1. OCP Setting / Calculation Method for the Current Rating of DCDC Parts

be considered by the coil L current. And the current rating of DCDC external parts is required more than the peak current

of the coil.

Shown below are the calculation method of the coil peak current, the selection method of Rcs (the resistor value of CS

pin) and the current rating of the external DCDC parts at Continuous Current Mode.

（the calculation method of the coil peak current, Ipeak at Continuous Current Mode）

At first, since the ripple voltage at CS pin depends on the application

condition of DCDC, the following variables are used.

Vout voltage=VOUT[V]

LED total current=IOUT[A]

DCDC input voltage of the power stage =VIN[V]

Efficiency of DCDC =η[%]

And then, the average input current IIN is calculated by the following

equation.

And the ripple current of the inductor L (ΔIL[A]) can be calculated by using

DCDC the switching frequency, fsw, as follows.

On the other hand, the peak current of the inductor Ipeak can be expressed

as follows.

… (1)

Therefore, the bottom of the ripple current Imin is

or 0

If Imin>0, the operation mode is CCM (Continuous Current Mode),

otherwise the mode is DCM (Discontinuous Current Mode).

(the selection method of Rcs at Continuous Current Mode)

Ipeak flows into Rcs and that causes the voltage signal to CS pin. (Please

refer to the timing chart at the right)

Peak voltage VCSpeak is as follows.

As this VCSpeak reaches 0.4V, the DCDC output stops the switching.

Therefore, Rcs value is necessary to meet the condition below.

(the current rating of the external DCDC parts)

The peak current as the CS voltage reaches OCP level (0.4V) is defined as Ipeak_det.

… (2)

The relationship among Ipeak (equation (1)), Ipeak_det (equation (2)) and the current rating of parts is required to meet

the following

Please make the selection of the external parts such as FET, Inductor, diode meet the above condition.

　

]

A

[

[%]

]

V

[

V

]

A

[

I

]

V

[

V

I

IN

OUT

OUT

IN









]

A

[

]

Hz

[

f

]

V

[

V

]

H

[

L

]

V

[

V

])

V

[

V

]

V

[

V

(

IL

SW

OUT

IN

IN

OUT











Δ

]

A

[

]

A

[

IL

]

A

[

I

Ipeak

IN

2







]

V

[

Ipeak

Rcs

VCS

peak









det

_

peak

peak

I

I

　

2

]

A

[

IL

]

A

[

I

in

Im

IN







]

V

[

.

]

V

[

Ipeak

Rcs

4

0





]

A

[

]

[

Rcs

]

V

[

.

I

det

_

peak





4

0

The current rating of parts

Figure 28. Coil current waveform