33394 datasheet(26/44 Pages) FREESCALE

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33394

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

Table 1. Linear Regulator Output Capacitor Examples

Output

SMD tantalum

Value/Rating

Part n. (AVX Corp.)

VDDH

100uF/10V

TPSC107K010S0200

VPP

33uF/10V

TPSB336K010S0650

VDD3_3

68uF/6.3V

TPSC686K006S0200

VDDL

100uF/6.3V

TPSC107K006S0150

VREFx

10uF/16V

THJB106K016S

VKAM*

100uF/6.3V

TPSC107K006S0150

5.2. Switching Regulator Operation

The 33394 switching regulator circuit consists of two basic

switching converter topologies. One is the typical voltage

mode PWM step–down or buck regulator, which provides

pre–regulated VPRE voltage (+5.6 V) during normal

operating conditions.

During cold start–up, when the car battery is weak, the

input voltage for the 33394 can fall below the lower operating

limit of the step–down converter. Under such conditions, the

step–up or boost converter provides the required value of the

VPRE voltage. The following paragraphs describe the basic

principles of the two converters operation.

Buck Mode

One switching cycle of the step–down converter operation

has two distinct parts: the power switch on state and the off

state. When the power switch is on, one inductor terminal is

connected to the input voltage Vin, and the other inductor

terminal is the output voltage Vo. During this part of the

switching period the rectifier (catch diode) is back biased,

and the current ramps up through the inductor to the output:

iL(on) +

(Vin * Vo)

ton

Where:

ton is the on–time of the power switch.

Vin is the input voltage.

Vo is the output voltage.

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

L is the inductance of the inductor L.

During the on time, current ramping through the inductor

stores energy in the inductor core.

During the off time of the power switch, the input voltage

source Vin is disconnected from the circuit. The energy

stored in the core forces current to continue to flow in the

same direction, the rectifier is forward biased and the

inductor input voltage is clamped one forward diode drop

below ground. The inductor current during the off time is:

iL(off) +

(Vo * Vfwd)

toff

Where:

toff is the off–time of the power switch.

iL(off) is the inductor current during the off time.

Vfwrd is forward voltage drop across the rectifier.

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

Vin/Vo = d

Where:

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

T is switching period, T = 1/f.

f is the frequency of operation.

Two relations give the ripple voltage in the output capacitor

Co. The first describes ripple voltage caused by current

variation upon the output capacitance Co:

VppCo +

DIL

8Co

The other is caused by current variations over the output

capacitor equivalent series resistance ESR:

VppESR + DIL

RESR

Practically, the ESR contributes predominantly to the buck

converter ripple voltage:

VppESR >>VppCo

The inductor peak current can be calculated as follows:

IpkL + Io )

DIL

Where:

Io is the average output current.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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