33981 datasheet(27/37 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33981

TYPICAL APPLICATIONS

POWER DISSIPATION

Figure 33 illustrates the typical application diagram.

Figure 33. Typical Application Diagram

APPLICATION

Engine cooling, air conditioning, and fuel pump are the

targeted automotive applications for the 33981. Conventional

solutions are designed with discrete components that are not

optimized in terms of component board size, protection, and

diagnostics. The 33981 is the right candidate to develop

lighter and more compact units.

The adjustment of the DC motor speed allows optimizing

of energy consumption. It is realized by chopping the supply

voltage, hence the mean voltage, applied to the motor. The

commonly used control technique is pulse wide modulation

(PWM) where the average voltage is proportional to the duty

cycle. Most applications require a PWM frequency of at least

20 kHz to avoid audible noise. Figure 34 illustrates typical

waveforms when switching the 33981 at 20 kHz with a duty

cycle of 80%. The output voltage (OUT) and current in the

motor (IMOTOR) waveforms are represented.

Figure 34. Current and Voltage waveforms

POWER DISSIPATION

The 33981 power dissipation is the sum of two kinds of

losses:

• On-State losses when device is fully ON,

• Switching losses when the device switches ON and

OFF.

The analysis that follows assumes an inductive load and

assumes that the current is constant in the load.

The case being considered in this paper is inductive load

and the hypothesis is that the current is constant in the load.

ON-STATE LOSSES

The mean on-state loss periods in the 33981 can be

calculated as follows:

Pon_state = a · RDS(ON) · IOUT2 where ‘a’ is the duty cycle.

The critical parameter is the on resistance (RDS(ON)) that

increases with temperature. The 33981 has a maximum

RDS(ON) at 25ºC of 4.0 mΩ and its deviation with temperature

is only 1.7 as shown in Figure 35.

Figure 35. RDS(ON) vs. Temperature

SWITCHING LOSSES

The mean switching losses in the 33981 can be calculated

as follows:

Pswitching = (tON . FREQ . VPWR . IOUT) / 2 + (tOFF . FREQ .

VPWR . IOUT) / 2

where tON/tOFF is the turn on/off time.

The switching time is a critical parameter. The 33981

provides adjustable slew rates through an external capacitor

(SR) that slow down the rise and fall times to reduce the

electromagnetic emissions. However, this adjustment will

have an impact on power dissipation. Figure 36 gives the

positive (SRR) and negative (SRF) slew rate versus different

values of SR. This is illustrated in Figure 37.

Imotor (10A/di

OU T

M C 33981 ON

MC 33981 OFF

-50

100

150

200

Tem perature (°C )

33981 Datasheet(PDF) 27 Page - Freescale Semiconductor, Inc

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