MIC3263 datasheet(19/32 Pages) MICREL

19 / 32 page

Micrel, Inc.

MIC3263

January 2010

M9999-012110

Figure 11. Boost Stage

The operating duty cycle can be calculated using the

equation provided below:

OUT

- eff × V )

and D

′ = 1 − D

Find L using the following equation:

L_PP

I×F

V ×D

IL_PP is the inductor peak-to-peak ripple current.

Use a I

SW is

Outp

given VOUT

L_PP of 20% to 40% of the total load current. F

the boost switching frequency.

ut Capacitor

In a boost converter, to find the COUT for a

ripple use the following calculation:

total

LED

OUT

RIPPLE

I×D

V× sw

VRIPPLE can usually be kept below 50mV:

ILED_TOTAL = 6 × 30mA = 180mA

In the MIC3263, the LED current in each channel is

individually regulated by that channels current amplifier

(linear current regulator). These current regulators are fast

enough to follow the boost output voltage ripple and to

keep the LED ripple currents much lower than COUT can

filter the output ripple voltage.

ode and requires

ensation is required to

st stage across all duty

table oscillations.

The

internal

slope

compensation that is

. The ability to set the

proper slope compensation through the use of a single

external component results in design flexibility. This slope

compensation resistor, RSLP, can be calculated as

follows:

Slope Compensation

The boost stage uses peak current m

slope compensation. Slope comp

maintain internal stability of the boo

cycles and to prevent any uns

MIC3263

uses

combinatio

compensation and a additional slope

set by an external resistor, RSLP

OUT(MAX)

V- L×F

RSLP =

-6

8.64×10 × V

IN(MIN)

IN(MAX)

OUT(MAX) can be selected to system

ficatio

value of RSLP should be 15kΩ.

e lowest VIN and maximum VOUT

will operate.

Example: For these operating conditions:

2V, L = 22μH,

FSW = 1MHz

where V

and V

speci

ns. The lowest

lculate RSLP using th

the system

VIN(MIN) = 12V, VOUT(MAX) = 3

32V - 22μH×1Mhz

RSLP =

= 96.5kΩ

-6

8.64×10 ×12V

Use the next highest standard value.

Table 3 compiles and lists RSLP values for one set of

operating conditions. Select RSLP for VIN_MIN and VO_MAX.

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