TYPICAL APPLICATIONS
POWER DISSIPATION
the freewheeling diode can be replaced by an external low-
side discrete MOSFET.
120
100
80
60
40
20
0
The power dissipation during the recirculation phase is
calculated as follows for the diode and the low-side MOSFET
respectively:
0
1
2.2
3.3
4.7
6.8
Pdiode = (1-a) . VF . IOUT
where ‘a’ is the duty cycle
2
Pmosfet_ls = (1-a) . RDS(ON)_ls . IOUT
where RDS(ON)_ls is the on resistance of the low side.
APPLICATIONS EXAMPLES
EXCEL TOOL
4.5
6
9
14
27
Vbat
90
80
70
60
50
40
30
20
10
0
0
An excel tool has been created with all the above formulas
to calculate the dissipated power and the junction
1
2.2
3.3
4.7
6.8
temperature knowing the application conditions. An example
of the interface is given in Figure 38. The parameters to enter
concern the load, the high-side device, the recirculation, and
the board. They are VPWR, DC current in the load (Imax for
100% of duty cycle), PWM frequency, 33981 RDS(ON) at
150ºC, SR capacitor, low-side RDS(ON) at 150ºC, ambient
temperature, and thermal impedance.
4.5
6
9
14
27
Vbat
Figure 36. Positive and Negative Slew Rate
vs. SR Capacitor
INPUTS
Vpwr
Imax
12
20
V
A
Load
Frequency
20 KHz
R
DSON
6.8mOhm
@150°C
High Side
Device (HS)
SR
Capacitor
0
nF
Low Side Characteristics
Recirculation
Board
R
DSON
@150°C
20 mOhm
Rthja
15°C/W
85°C
Figure 37. OUT switching vs. SR Capacitor
T ambiant
JUNCTION TEMPERATURE
The junction temperature of the 33981 can be calculated
knowing the power dissipation and the thermal
Figure 38. Excel Tool
characteristics of the PC board with this formula:
The calculations are done with the maximum RDS(ON) for
the 33981 and the low side. The current is also considered
constant in the load. The model taken for the VF of the diode
is (0.4 + 0.01 . IOUT) Volts.
TJ = TA + (Pon_state + Pswitching). RTHJA
where TJ is the junction temperature, TA the ambient
temperature, and RTHJA the thermal impedance junction to
ambient.
The listed conditions in Figure 38 are the ones chosen for
the entire document.
RECIRCULATION PHASE
In standard configuration, the motor current recirculation is
handled by an external freewheeling diode. With the 33981,
33981
Analog Integrated Circuit Device Data
Freescale Semiconductor
28
FREESCALE [ Freescale ]
