Micrel

MIC2297

Mach 2008

10

M9999-032708

Reference Capacitor

A 1uF ceramic should be used on the reference pin to

prevent noise from getting into this node. A 1uF ceramic is

needed when a PWM signal is connected to the BRT pin.

Diode Selection

The MIC2297 requires an external diode for operation. A

Schottky diode is recommended for most applications due

to their lower forward voltage drop and reverse recovery

time. Ensure the diode selected can deliver the peak

inductor current and the maximum reverse voltage is rated

greater than the output voltage.

Input capacitor

A minimum 1F ceramic capacitor with an X5R or X7R

dielectric is recommended for designing with the MIC2297.

Increasing input capacitance will improve performance and

greater noise immunity on the source. The input capacitor

should be as close as possible to the inductor and the

MIC2297, with short traces for good noise performance.

The MIC2297 utilizes a feedback pin to compare the LED

current to an internal reference. The LED current is

adjusted by selecting the appropriate feedback resistor

value. The desired output current can be calculated as

follows:

R

V

I

LED

2

.

0

=

Compensation

The comp pin is connected to the output of the voltage

error

amplifier.

The

voltage

error amplifier is

a

transconductance amplifier. Adding a series RC to ground

adds a zero at:

1

1

2

1

C

R

f

zero

π

=

The resistor typically ranges from 10kOhm to 50kOhm.

The capacitor typically ranges from 1nF to 100nF.

Adding a capacitor from comp to ground adds a pole at

2

1

2

1

C

R

f

pole

π

=

This capacitor typically ranges from 100pF to 10nF.

Generally an RC to ground is all that is needed. The RC

should be placed as close as possible to the comp pin.

The capacitor should be a ceramic with a X5R, X7R, or

COG dielectric.

Grounding

Both the AGND and PGND must be connected to the

exposed backside pad. The exposed backside pad also

improves thermal performance. A large ground plane

decreases thermal resistance to ambient air.