33394 datasheet(30/44 Pages) FREESCALE

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33394

MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

5.2.1.4. Selecting the Power MOSFET Q2

The boost converter maximum output voltage plus the

voltage drop across the output schottky rectifier D2 gives the

MOSFET’s maximum drain–source voltage stress:

BVdsQ2>Vo+Vfwd2 = 6 V+0.5 V, as can be seen, the

breakdown voltage parameter is not critical.

The more important in our case is the Q2 current handling

capability. The external power MOSFET has to withstand

higher currents than the upper current limit of the 33394:

IDQ2>3A

In order to keep the power dissipation of the 33394 boost

converter to its minimum, a very low RDSon power MOSFET

has to be selected. Moreover, due to the fact that the 33394

external MOSFET gate driver is supplied from VPRE, in

order to assure proper switching of Q2 a logic level device

has to be selected.

Last but not least, the Q2 package has to suitable for the

harsh automotive environment with low thermal resistance.

These requirements are met, for example by the

MTD20N03HDL power MOSFET from ON Semiconductor.

5.2.1.5. Selecting the Boost Converter Output

Rectifier D2

Criteria similar to that of selecting the power MOSFET was

used to select the boost converter output rectifier. Its reverse

breakdown voltage is not a critical parameter:

VrD2>Vo=6 V

The D2 rectifier has to withstand higher peak current than

is the 33394 internal switch upper current limit Ilim(max).

The most important parameter is its forward voltage drop,

which has to be minimal. This parameter is also crucial for the

proper 33394 switcher functionality, and especially for proper

transition between the buck and boost modes.

Finally, its switching speed, forward and reverse recovery

parameters play a significant role when selecting the output

rectifier D2.

These requirements are met, for example by the HSM350

schottky rectifier from Microsemi, Inc.

5.2.2. Input Filter Selection

Since the switcher will work in the Boost mode only during

cold crank condition, the 33394 EMC (electromagnetic

compatibility) performance is not of concern during this mode

of operation. Therefore, only the Buck mode of operation is

important for selecting the appropriate input filter. For the

Buck converter topology (see Figure 13) the low impedance

3rd order filter (C3, L2, C4 and C26 in the Application

Schematic Diagram Figure 20) offers a good solution. It can

be seen from the Buck converter current waveforms that

comparatively high current pulses are drawn from the

converter’s input source. The filter inductance must be kept

minimal and the capacitor, which is placed right next to the

power switch, must be sized large enough to provide

sufficient energy reservoir for proper switcher operation.

The ESR of this input capacitor combination C4, C26 has

to be sufficiently low to reduce the switching ripple of the

switcher input node VBAT. There are three main reasons to

keep the voltage ripple of the VBAT pin at its minimum. First,

it is the EMC (electromagnetic compatibility) performance of

the switcher in the normal operating mode (buck mode).

Second, it allows a smooth transition between the boost and

buck mode of operation. Third, it helps to avoid entering an

undervoltage condition too early. A practical way to achieve

sufficiently low ESR of the switcher input capacitor, even at

low temperature extremes, is to use several high value

ceramic capacitors in parallel with a large electrolytic

capacitor. These capacitors should be physically placed as

close to the VBAT pins as possible.

5.2.3. Buck Converter Feedback Compensation

A typical control loop of the buck regulator is shown in

Figure 17. The loop consists of a power processing block —

the modulator in series with an error–detecting block — the

Error (Feedback) Amplifier. In principle, a portion of the

output voltage (VPRE of the 33394 switcher) is compared to

a reference voltage (Vbg) in the Error Amplifier and the

difference is amplified and inverted and used as a control

input for the modulator to keep the controlled variable (output

voltage VPRE) constant.

Figure 17. The Buck Converter Control Loop

–

Zin

Reference

Voltage

ERROR FEEDBACK AMPLIFIER

MODULATOR

Vout

To Load

Ramp

PWM

Signal

Vin

Gain Block

(Modulator)

Feedback

Block

Vout

Vin

–

Vout/Vin = G/(1 + GH)

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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