Micrel, Inc.

MIC3201

January, 2010

9

M9999-010710-B

Application Information

The MIC3201 is a hysteretic step-down constant-current

High-Brightness LED (HB LED) driver. The internal block

diagram is shown in Figure 1. The MIC3201 is

composed of a current sense comparator, voltage and

current reference, 5V regulator, MOSFET driver, and a

MOSFET. Hysteretic mode control, also called bang-

bang control, is the topology that does not employ an

error amplifier, and instead uses an error comparator.

The inductor current is controlled within a hysteretic

window. If the inductor current is too small, the power

MOSFET is turned on; if the inductor current is large

enough, the power MOSFET is turned off. It is a simple

control scheme with no oscillator and no loop

compensation. Since the control scheme does not need

loop compensation, it makes a design easy, and avoids

problems of instability.

Transient response to load and line variation is very fast

and only depends on propagation delay. This makes the

control scheme very popular for certain applications.

The main feature in MIC3201 is to control the LED

current accurately within ± 5% of set current. Choosing a

current irrespective of wide input voltage range. The

)

(

2

1

)

(

)

(

LED

MIN

CS

MAX

CS

CS

I

V

V

R

+

=

ILED (A)

I

Size (SMD)

2.00

0.1

0.0200

0402

1.00

0.2

0.0400

0402

0.63

0.3

0.0567

0402

0.56

0.35

0.0691

0603

0.50

0.4

0.0800

0603

0.40

0.5

0.1000

0805

0.33

0.6

0.1188

0805

0.28

0.7

0.1372

0805

0.24

0.8

0.1536

0805

0.22

0.9

0.1782

0805

0.20

1.0

0.2000

1206

table.

Frequency of Operation

To calculate the frequency spread across input supply:

dt

dI

L

V

L is the inductance, dI is fixed (the value of the hysteresis)

CS

MIN

CS

MAX

CS

R

V

V

dI

)

(

)

(

−

=

For current rising (MOSFET is ON):

RISE

L

r

V

dI

L

t

_

=

where:

For current falling (MOSFET is OFF):

FALL

L

f

V

dI

L

t

_

=

where:

f

r

t

t

T

+

=

,

T

F

SW

1

=

)

V

V

(

dI

L

)

V

R

I

V

(

)

V

R

I

V

(

F

IN

D

LED

CS

LED

IN

LED

CS

LED

D

SW

+

⋅

⋅

−

⋅

−

•

+

⋅

+

=

where

According to the above equation, choose the inductor to make

the operating frequency not beyond 1MHz.

Free Wheeling Diode

The free wheeling diode should have the reverse voltage

rating to accommodate the maximum input voltage. The

forward voltage drop should be small to get the lowest

conduction dissipation for high efficiency. The forward current

rating has to be at least equal to LED current. A Schottky

diode is recommended.

LED Ripple Current

The LED current is the same as inductor current. If LED ripple

current needs to be reduced then place a 10µF capacitor

across LED.