Micrel

MIC2297

Mach 2008

9

M9999-032708

Over-Voltage Protection

The MIC2297 has an over-voltage protection function. If an

LED is disconnected from the circuit or the feedback pin is

shorted to ground, the feedback pin will fall to ground

potential. This will cause the MIC2297 to switch at full duty

cycle in an attempt to maintain the feedback voltage. As a

result, the output voltage will climb out of control. This may

cause the switch node voltage to exceed its maximum

voltage rating, possibly damaging the IC and the external

components. To ensure the highest level of protection, the

MIC2297 OVP pin will shut the switch off when an over-

voltage condition is detected, saving itself and the output

capacitor.

Brightness Control

In the MIC2297, the reference to the voltage error amplifier

is pinned out. The BRT pin and REF pin form a voltage

divider off the internal 1.245V reference. The voltage is

such that with nothing connected to the BRT pin, the REF

voltage is 0.2V and the BRT voltage is 1V. The REF

voltage is 1/5 the BRT voltage.

The minimum REF voltage with BRT pulled to ground is

typically 10mV.

With a 10

Ω sense resistor, the LED

current is typically 1mA with the BRT pin pulled to ground.

An analog DC voltage can be connected to the BRT pin.

The MIC2297 will create an LED current proportional to

the BRT voltage according to the following equation:

sense

LED

R

BRT

I

•

=

5

Where BRT is the voltage applied to the BRT pin, and

Rsense is the sense resistor used in the LED string. It’s

important to use a 1uF ceramic capacitor on the REF pin

to filter any noise.

An external PWM signal can be applied to the BRT for

dimming. The 1uF REF capacitor and internal BRT 124k

Ω

resistor form an RC that filters the voltage to the REF pin.

The LED current is proportional the PWM duty cycle

according to the following equation:

sense

peak

LED

R

D

V

I

•

•

=

5

Where Vpeak is the peak PWM voltage and D is the duty

cycle of the PWM signal.

Component Selection

Inductor

Inductor selection is a balance between efficiency,

stability, cost, size, and rated current. For most

applications a 22

µH is the recommended inductor value. It

is usually a good balance between these considerations.

Larger inductance values reduce the peak-to-peak ripple

current, affecting efficiency. This has the effect of reducing

both the DC losses and the transition losses. There is also

a secondary effect of an inductor’s DC resistance (DCR).

The DCR of an inductor will be higher for more inductance

in the same package size. This is due to the longer

windings required for an increase in inductance. Since the

majority of input current (minus the MIC2297 operating

current) is passed through the inductor, higher DCR

inductors will reduce efficiency. To maintain stability,

increasing inductor size will have to be met with an

increase in output capacitance. This is due to the

unavoidable “right half plane zero” effect for the continuous

current boost converter topology. The frequency at which

the right half plane zero occurs can be calculated as

follows:

π

2

2

⋅

⋅

⋅

=

out

out

in

rhpz

I

L

V

V

f

The right half plane zero has the undesirable effect of

increasing gain, while decreasing phase. This requires that

the loop gain is rolled off before this has significant effect

on the total loop response. This can be accomplished by

either reducing inductance (increasing RHPZ frequency) or

increasing the output capacitor value (decreasing loop

gain).

Output Capacitor

Output capacitor selection is also a trade-off between

performance,

size,

and

cost.

Increasing

output

capacitance will lead to an improved transient response,

but also an increase in size and cost. X5R or X7R

dielectric ceramic capacitors are recommended for

designs with the MIC2297.

The output capacitor sets the frequency of the pole and

zero in the power stage. The zero is given by:

π

2

1

⋅

⋅

=

esr

z

R

C

f

For ceramic capacitors, the ESR is very small. This puts

the zero at a very high frequency where it can be ignored.

The frequency of the pole caused by the output capacitor

is given by.

π

⋅

⋅

=

out

out

p

V

C

I

f