33394 datasheet(27/44 Pages) FREESCALE

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33394

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

Figure 13. Basic Buck Converter Operation and its Waveforms

Vin

RLOAD

–

Vin

RLOAD

–

POWER SWITCH ON

ILOAD

Vout

ILO

VD(fwd)

RLOAD

–

POWER SWITCH OFF

ILOAD

Vout

ILO

–ID

VCo

DIL

ton toff

Vfwd

Boost Mode

The operation of the boost converter also consists of two

parts, when the power switch is on and off. When the power

switch turns on, the input voltage source is placed directly

across the inductor, and the current ramps up linearly

through the inductor as described by:

iL(on) +

Vin

ton

Where:

ton is the on–time of the power switch.

Vin is the input voltage.

iL(on) is the inductor current during the on–time.

L is the inductance of the inductor L.

The current ramping across the inductor stores energy

within the core material. In order to maintain steady–state

operation, the amount of energy stored during each switching

cycle, times the frequency of operation must be higher (to

cover the losses) than the power demands of the load:

Psto +

LI2pk

u Pout

When the power switch turns off again, the inductor voltage

flies back above the input voltage and is clamped by the

forward biased rectifier at the output voltage.

The current ramps down through the inductor to the output

until the new on time begins or, in case of discontinuous

mode of operation, until the energy stored in the inductor core

drops to zero.

iL(off) +

(Vo * Vin)

toff

Where:

toff is the off–time of the power switch.

Vo is the output voltage.

During the steady state operation iL(on) = iL(off) = ∆IL, and

Vo * Vin

Where:

d is the duty cycle, and d = ton/T.

T is switching period, T = 1/f.

f is the frequency of operation.

The ripple voltage of the boost converter can be described

as:

VppCo +

(Vo * Vin)

Where:

VppCo is the ripple caused by output current.

The portion of the output ripple voltage caused by the ESR

of the output capacitor is:

VppESR + (Io

Vin

) 1

DIL)

RESR

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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