Micrel, Inc.

MIC3230/1/2

November 2009

13

M9999-113009-B

L

47µH

D1

R8

100k

R9

4.33k

COUT

4.7µF

100V

R2

100k

RADJ

1/4W

RFS

16.5k

CCOMP

10nF

CIN

4.7µF/50v

RSLC

51

RCS

1/2W

Analog ground

Power ground

VOUT

VIN

PWMD

ENABLE

Synch to other MIC3230

ILED Return

LED 1

LED X

Q1

COMP

PWMD

VDD

AGND

PGND

EPAD

IS

OVP

DRV

VIN

EN

IADJ

MIC3230/31

SYNC

FS

C3

10µF

10V

Figure 7. Design Example Schematic

Design Example

In this example, we will be designing a boost LED driver

operating off a 12V input. This design has been created

to drive six LEDs at 350mA with a ripple of about 12%.

We are designing for 80% efficiency at a switching

frequency of 500kHz.

resistor must be chosen using Equation 3.

() ()

Ω

=

=

Ω

k

k

6

.

16

500

.

1

Use the closest standard value resistor of 16.5kΩ.

Having chosen the LED drive current to be 350mA in this

example, the current can be set by choosing the RADJ

resistor from Equation 1:

Ω

=

=

71

.

0

35

.

0

25

.

0

A

V

The power dissipation in this resistor is:

()

mW

R

I

R

P

ADJ

ADJ

87

*

2

=

=

Use a resistor rated at ¼ watt or higher. Choose the

closest value from a resistor manufacture.

Operating Duty Cycle

The operating duty cycle can be calculated using

Equation 12 provided below:

Eq. (12)

diode

diode

V

Vout

V

Vin

eff

Vout

D

+

+

×

−

=

)

(

These can be calculated for the nominal (typical) operating

conditions, but should also be understood for the minimum

and maximum system conditions as listed below.

schottky

nom

schottky

nom

nom

nom

V

Vout

V

Vin

eff

Vout

D

+

+

×

−

=

)

(

schottky

schottky

V

Vout

V

Vin

eff

Vout

D

+

+

×

−

=

max

min

max

max

)

(

schottky

schottky

V

Vout

V

Vin

eff

Vout

D

+

+

×

−

=

min

max

min

min

)

(

Inductor Selection

First, it is necessary to calculate the RMS input current

(nominal, min and max) for the system given the operating

conditions listed in the design example table. This minimum

value of the RMS input current is necessary to ensure proper

operation. Using Equation 9, the following values have been

calculated:

rms

A

V

eff

I

V

I

IN

OUT

OUT

RMS

IN

_

64

.

1

min

_

max

_

max

_

max

_

_

=

×

×

=

rms

A

V

eff

I

V

I

nom

IN

nom

OUT

nom

OUT

nom

RMS

IN

_

78

.

0

_

_

_

_

_

=

×

×

=

rms

A

V

eff

I

V

I

IN

OUT

OUT

RMS

IN

_

48

.

0

max

_

min

_

min

_

min

_

_

=

×

×

=

Iout is the same as ILED

obtain:

P

P

nom

rms

in

nom

PP

in

A

I

I

−

=

=

=

31

.

0

78

.

0

*

4

.

0

4

.

0

_

_

_

_