MIC3232YMM datasheet(15/20 Pages) MICREL

15 / 20 page

Micrel, Inc.

MIC3230/1/2

January 2009

M9999-011409-A

and for RSLC. If

is too high than use a higher

value for RCS. The calculated value of RCS for a 1.9A

current limit was 179mΩ.

In this example, we have

chosen a lower value which results in a higher current

limit. If we use a higher standard value the current limit

will have a lower value. The designer does not have the

same choices for small valued resistors as with larger

valued resistors. The choices differ from resistor

manufacturers. If too large a current sense resistor is

selected, the maximum output power may not be able to

be achieved at low input line voltage levels. Make sure

the inductor will not saturate at the actual current limit

Limit

actual

I _

()

511

Limit

actual

I _

250

Perform a check at IIN=2.34Apk.

150

Maximum Power dissipated in RCS is;

Eq. (17)

RMS

Eq. (18)

⎟

⎠

⎞

⎜

⎝

⎛

max

RMS

FET

RMS

max

AVE

rms

RMS

⎟

⎠

⎞

⎜

⎝

⎛

watt

Use a 1/2 Watt resistor for RCS.

Output Capacitor

In this LED driver application, the ILED ripple current is a

more important factor compared to that of the output

ripple voltage (although the two are directly related). To

find the COUT for a required ILED ripple use the following

calculation:

For an output ripple

20% of

ripple

ILED

nom

ILED

ILEDripple

Eq. (19)

)

(

_ total

LED

adj

ripple

nom

out

ILED

Find the equivalent ac resistance

from the

datasheet of the LED. This is the inverse slope of the

ILED vs. VF curve i.e.:

LED

Eq. (20)

ILED

LED

In this example use

for each LED.

= 1

_ ac

LED

If the LEDs are connected in series, multiply

s, we obtain the following:

= 1

_ ac

LED

by the total number of LEDs. In this

example of 6 LED

_ total

LED

ILED

total

LED

adj

ripple

nom

out

)

(

Use the next highest standard value, which is 4.7uF.

is shown in Figure 5. For superior

There is a trade off between the output ripple and th

rising edge of the PWMD pulse.

This is because

between PWM dimming pulses, the converter stops

pulsing and COUT will start to discharge. The amount that

COUT will discharge depends on the time between PWM

Dimming pluses. At the next PWMD pulse COUT has to

be charged up to the full output voltage VOUT before the

desired LED current flows.

Input Capacitor

The input current

performance, ceramic capacitors should be used

because of their low equivalent series resistance (ESR).

The input ripple current is equal to the ripple in the

inductor plus the ripple voltage across the input

capacitor, which is the ESR of CIN times the inductor

ripple. The input capacitor will also bypass the EMI

generated by the converter as well as any voltage spikes

generated by the inductance of the input line. For a

required VIN_RIPPLE:

Eq. (21)

()

kHz

RIPPLE

500

This is the minimum value that should be used. The

e, the FET has to hold off an output

input capacitor should also be rated for the maximum

RMS input current. To protect the IC from inductive

spikes or any overshoot, a larger value of input

capacitance may be required and it is recommended that

ceramic capacitors be used. In this design example a

value of 4.7µF ceramic capacitor was selected.

MOSFET Selection

In this design exampl

voltage maximum of 30V. It is recommended to use an

80% de-rating value on switching FETs, so a minimum

of a 38V FET should be selected.

In this design

example, a 75V FET has been selected.

The switching FET power losses are the sum of the

conduction loss and the switching loss:

Eq. (22)

FET

COND

FET

SWITCH

The conduction loss of the FET is when the FET is

, where

turned on. The conduction power loss of the FET is

found by the following equation:

Eq. (23)

FET

COND

FET

DSON

RMS

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