TYPICAL APPLICATIONS
POWER DISSIPATION
Figure 33 illustrates the typical application diagram.
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.
Figure 33. Typical Application Diagram
APPLICATION
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.
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.
7
6
5
4
3
2
1
0
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.
-50
0
50
100
150
200
Temperature (°C)
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 OFF is the turn on/off time.
The switching time is a critical parameter. The 33981
OUT
.
Imotor (10A/div)
/t
MC33981 OFF
MC33981 ON
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.
Figure 34. Current and Voltage waveforms
33981
Analog Integrated Circuit Device Data
Freescale Semiconductor
27
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