33981 datasheet(29/37 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33981

TYPICAL APPLICATIONS

POWER DISSIPATION

DC MOTOR 200W

A concrete example is the 33981. A 200 W DC motor, a

frequency of 20 kHz, and an ambient temperature of 85ºC

are chosen. The 33981 is evaluated using the following

board. The thermal impedance of the board is in the range of

15ºC/W.

Figure 39. 33981 Evaluation Board

POWER DISSIPATION

Figure 40 illustrates the power dissipation in the 33981.

The conditions are listed in Figure 38. Maximum power

dissipation of 3.1 W is obtained with a duty of 95%.

Figure 40. Power Dissipation (Pon and Pswitching) vs.

Duty Cycle

INFLUENCE OF SR CAPACITOR

The SR capacitor value has an impact on these switching

losses. Figure 41 illustrates the percentage of the switching

losses versus the total power dissipation for the same load

conditions as Figure 38. The higher the SR capacitor value,

the higher the switching losses. They can be more than 50%

of the total power dissipation in the 33981 with a 4.7 nF

capacitor and is a basic applications trade-off. A compromise

should be found between the power dissipation and the

electromagnetic capability (EMC) performance.

Figure 41. Power Switching vs. SR Capacitor

RECIRCULATION PHASE

Figure 42 illustrates the power dissipation for the two

recirculation approaches, diode or low side MOSFET. The

power dissipation gain for the entire system when using the

low side instead of the diode can reach up to 1.5 W with a

duty cycle of 50%.

Figure 42. Total Board Power Dissipation

JUNCTION TEMPERATURE

The junction temperature of the 33981 versus duty cycle

for the condition listed in Figure 38, is given in Figure 43. The

maximum obtained junction temperature is 132ºC with a duty

0.5

1.5

2.5

3.5

100

Pon_st

ate

P switching

Ptotal

M C 33981 Pow erDissipati

3.0

2.0

1.0

Duty Cycle (%)

02 .2

3 .3

4 .7

Pswitchin g

Pon

C s r (nF)

02 .2

3 .3

4 .7

Pswitchin g

Pon

C s r (nF)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

100

Pow erHS

Pow erDiode

Pow erTotalBoard with Diode

Pow erLS

Pow erTotalBoard with LS

Total Board Power Dissipation

Ratio PWM %

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