33394 datasheet(28/44 Pages) FREESCALE

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33394

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

Where Io is the average output current.

The inductor peak current is given by the following

equation:

IpkL + Io

Vin

) 1

DIL

Figure 14. Basic Boost Converter Operation and its Waveforms

Vin

RLOAD

–

Vin

RLOAD

–

POWER SWITCH ON

ILOAD

Vout

VIN

RLOAD

–

POWER SWITCH OFF

IOFF

Vout

VCo

DIL

ton

toff

ION

ILOAD

5.2.1. Switching Regulator Component Selection

The selection of the external inductor L2 and capacitor C2

values (see Figure 15) is a compromise between the two

modes of operation of the switching regulator, the pre

regulated voltage VPRE and the dropout voltage of the linear

regulators. Ideal equations describing the peak—peak

inductor current ripple, peak—peak output voltage ripple and

peak inductor current are shown below. Since the switching

regulator will work mostly in the buck mode, the inductor and

the switcher input and output capacitor were selected for

optimum buck controller performance, but also taking into

account the restriction placed by adopting the boost

converter as well.

Figure 15. 33394 Switcher Topology

VRL

Vfwd1

Vin

RLOAD

–

VRDS(on)

RDS(on)

ESR

Vfwd2

The following example shows a procedure for determining

the component values. The VPRE output is set to regulate to

5.6 V and the linear regulators require a minimum of 0.4 V

dropout voltage. This leaves a

±0.2 V window for the

peak—to–peak output voltage ripple. Assuming the following

conditions:

Vin(typ) = 13.5 V

Io = 1.2

VPRE = 5.6 V (+6 V in the boost mode)

f = 200 kHz

Vfwd1 = Vfwd2 = 0.5 V

Maximum allowed output voltage ripple in the buck mode

Vpp(max) = 0.2 V/2 = 0.1 V (to allow for process and

temperature variations).

5.2.1.1. Selecting the Inductor

In order to select the proper inductance value, the inductor

ripple current

∆IL has to be determined. The usual ratio of ∆IL

to output current Io is:

∆IL = 0.3 Io

As described in the previous section, and taking into

account the 33394 switcher topology (see Figure 15), the

inductor ripple current can be estimated as:

DIL +

(Vin * Vo * Vfwd2)

Vo ) Vfwd2

Vin

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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