33981 datasheet(28/37 Pages) FREESCALE

28 / 37 page

Analog Integrated Circuit Device Data

Freescale Semiconductor

33981

TYPICAL APPLICATIONS

POWER DISSIPATION

Figure 36. Positive and Negative Slew Rate

vs. SR Capacitor

Figure 37. OUT switching vs. SR Capacitor

JUNCTION TEMPERATURE

The junction temperature of the 33981 can be calculated

knowing the power dissipation and the thermal

characteristics of the PC board with this formula:

TJ = TA + (Pon_state + Pswitching). RTHJA

where TJ is the junction temperature, TA the ambient

temperature, and RTHJA the thermal impedance junction to

ambient.

RECIRCULATION PHASE

In standard configuration, the motor current recirculation is

handled by an external freewheeling diode. With the 33981,

the freewheeling diode can be replaced by an external low-

side discrete MOSFET.

The power dissipation during the recirculation phase is

calculated as follows for the diode and the low-side MOSFET

respectively:

Pdiode = (1-a) . VF . IOUT

where ‘a’ is the duty cycle

Pmosfet_ls = (1-a) . RDS(ON)_ls . IOUT2

where RDS(ON)_ls is the on resistance of the low side.

APPLICATIONS EXAMPLES

EXCEL TOOL

An excel tool has been created with all the above formulas

to calculate the dissipated power and the junction

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.

Figure 38. Excel Tool

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.

The listed conditions in Figure 38 are the ones chosen for

the entire document.

100

120

4.56

914

Vbat

2.2

3.3

4.7

6.8

4.5

Vbat

2.2

3.3

4.7

6.8

Vpw r

12 V

Imax

20 A

Frequency

20 KH z

R DSON

@150°C

6.8 mO hm

C apaci

tor

0 nF

R DSON

@150°C

20 mO hm

Rthja

15°C /W

T am biant

85°C

INPUTS

Low Si

de Characteristics

High Side

Device (HS)

Load

R ecirculati

Board

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