33394 datasheet(25/44 Pages) FREESCALE

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33394

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

5. APPLICATION INFORMATION

This section provides information on external components

that are required by the 33394. The IC is designed to operate

in an automotive environment. Conducted immunity and

radiated emissions requirements have been addressed

during the design. However, the IC requires some external

protection.

Protection is required for all pins connected directly to

battery. The module designer should use an MOV or another

transient voltage suppressor in all cases, when the load

dump transition exceeds + 45 volts with respect to ground.

Protection should also include a reverse battery protection

diode (or relay) and input filter. This is required to protect the

33394 from ESD and +/– 300V ignition transients. Typical

configurations are shown in Figure 1. Outputs and inputs

connected directly to connector pins require module level

ESD protection.

5.1. Selecting Components for Linear Regulators

The output capacitor of the linear regulator serves two

different purposes. It maintains the linear regulator loop

stability, and it provides an energy reservoir to supply current

during very fast load transients. This is especially true when

supplying highly modulated loads like microcontrollers and

other high–speed digital circuits. Due to the limited

bandwidth of the linear regulators, the output capacitor is

selected to limit the ripple voltage caused by these abrupt

changes in the load current. During the fast load current

transients, the linear regulator output capacitor alone

controls the initial output voltage deviation. Hence, the output

capacitor’s equivalent series resistance (ESR) is the most

critical parameter.

The outputs, which do not experience such severe

conditions (the VREF e.g.), use the output capacitor mainly

for stability purpose, and therefore its capacitance value can

be significantly smaller. The typical output capacitor

parameters are: C = 1.0

µF; ESR = 2.0 ohms. When a

ceramic 1

µF capacitor is used, the ESR can be provided by

a discrete serial resistor (see Figure 20).

The following example shows how to determine the output

capacitance for a heavily loaded output supplying digital

circuits.

5.1.1. Selecting the Output Capacitor Example:

The output capacitance must be selected to provide

sufficiently low ESR. The selected capacitor must have an

adequate voltage, temperature and ripple current rating for

the particular application.

In order to calculate the proper output capacitor

parameters, several assumptions will be made.

1) During the very fast load current transients, the linear

regulator can not supply the required current fast enough,

and therefore for a certain time the entire load current is

supplied by the output capacitor. 2) The capacitor’s

equivalent series inductance (ESL) is neglected. These

assumptions can greatly simplify the calculations, and are

reasonable for most of practical applications. Then the ESR

of the output capacitor has to satisfy the following condition:

ESR

DVo

DIo

Where:

∆Vo is the maximum allowed linear regulator voltage drop

caused by the load current transient.

∆Io is the maximum current transient, which can occur due to

the abrupt step in the linear regulator load current.

In this example the VDDH output with the 400 mA load step

is considered with the maximum voltage drop of 100mV. This

gives the output capacitor’s maximum ESR value of:

ESR

+ 100 mV

400 mA

+ 250 mW

This level of ESR requires a relatively large capacitance. In

order to maintain the linear regulator stability and to satisfy

large load current steps requirements the solid tantalum

capacitor 100

µF/10V with ESR = 200 mΩ. One device that

meets these requirements is the TPSC107K010S020

tantalum capacitor from the AVX Corporation.

DVESR + ESR

DIo + 200 mW

400 mA

+ 80 mV

In the next step, the voltage drop associated with the

capacitance can be calculated:

DVC +

DIo

0.4 A

100

+ 20 mV

Where:

C is the output capacitance.

Dt is the linear regulator response time.

∆Io is the maximum current transient, which can occur due to

the abrupt step in the linear regulator load current.

Assuming that the capacitor ESL is negligible, the total

voltage drop in the voltage regulator output caused by the

current fast transient can be calculated as:

DVtotal + DVESR ) DVC + 80 mV ) 20 mV + 100 mV

A ceramic capacitor with capacitance value 10nF should

be placed in parallel to provide filtering for the high frequency

transients caused by the switching regulator.

Properly sized decoupling ceramic capacitor close to the

microprocessor supply pin should be used as well. Table 1

shows the suggested output capacitors for the 33394 IC

linear regulator outputs.

Other factors to consider when selecting output capacitors

include key off timing for memory retention. Though the

VKAM is not a heavily loaded output, the VKAM output

capacitor has to have a sufficiently large capacitance value to

supply current to the microcontroller for a certain time after

battery voltage is disconnected.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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