Application information – Diodes AL8400 /AL8400Q User Manual

Page 10

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AL8400/ AL8400Q

Document number: DS35115 Rev. 4 - 2

10 of 13

www.diodes.com

August 2012

© Diodes Incorporated

AL8400 /AL8400Q

Application Information

(cont.)

MOSFET Example Choosing R

B

and C

L

(cont.)

R

Bmax

The minimum recommended I

OUT

for AL8400 is 0.3mA.

The maximum V

GS

is not stated explicitly, but from the datasheet graphs (Figures 8 and 9) it is expected to be approximately 3.8V at -50°C.

=

=

min

OUT

FB

max

GS

min

CC

max

B

I

V

V

V

R

=

=

mA

3

.

0

V

2

.

0

V

8

.

3

V

4

.

11

24.7k


To ensure that the output capability of the AL8400 is not exceeded at maximum V

IN

and minimum V

GS

these values should be substituted back

into the R

B

equation to determine the minimum allowable value for R

B

.

R

Bmin

The maximum recommended I

OUT

for the AL8400 is 15mA. The minimum V

GS

is about 1V and assuming V

CCmax

= 8.4V:

max

OUT

FB

min

GS

max

CC

min

B

I

V

V

V

R

=

=

=

mA

15

V

2

.

0

V

1

V

6

.

12

= 480

this is less than 12k

Ω and so the AL8400 output current is within its ratings.

Figure 8 Typical Transfer Characteristics

Figure 9 Normalised Curves and Temperature

Assuming V

GS

~ 3V and choosing an R

B

= 8.2k

Ω satisfies the requirements for the AL8400 conformance and sets approximately 1mA in the OUT

pin. The required compensation capacitor can therefore be calculated from:

F

243

.

0

k

2

.

8

ms

2

C

L

μ

Ω

Æ 220nF

The value of R

SET

is V

REF

/I

LED

R

SET

= 0.2/0.2 = 1

Finally, the maximum power dissipation of the external MOSFET is:

P

TOT

= I

LED

x V

DSMAX

=

I

LED

x (V

CCmax

– V

LEDMIN

– V

FB

)

= 0.2 x( 12.6 – 9 -0.2)

= 0.68W


This determines the package choice (

θ

JA

) in order to keep the junction temperature below the maximum value allowed.

T

J

= T

A

+ P

TOT

x

θ

JA

= 60 + 0.68 x 62.5

= +102.5˚C

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