STA6940M datasheet(13/19 Pages) ALLEGRO

13 / 19 page

Brushed DC Motor Driver IC With PWM Control

STA6940M

Thermal Considerations

To accurately calculate the losses incurred by the STA6940M,

detailed knowledge of the motor characteristics, input waveforms,

and dynamic properties of the circuit must all be considered. The

formulas below are simplified approximations using worst-case

conditions:

For constant voltage drive (DC current):

× RDS(on) × 2 ,

(5)

and, for constant-current drive (PWM):

tON

tON + tOFF

tOFF

tON + tOFF

× RDS(on) × 2

( IO

× RDS(on) + VF × IO ) ×

(6)

where:

PD is the device power dissipation,

IO is the motor current (

≈ IO),

RDS(on) is the internal MOSFET on-resistance,

RS is the external sensing resistor,

VF is the internal MOSFET body diode forward voltage,

tON is the PWM on-time, and

tOFF is the PWM off-time.

Using the above calculated power dissipation, it is possible to

estimate the junction temperature using the curves in figure 15. In

the worst conditions (with high ambient temperatures), as long as

the junction temperature does not exceed 150ºC the device will

not be damaged. However, the actual temperature of the device

should be measured along with the junction temperature calcula-

tion.

If a heatsink is being used, the ΔTJA (junction to ambient) as

calculated previously will be different because the thermal resis-

tance, RθJA, is different. The new value can be calculated using

the heatsink heat resistance RθFIN:

R JA R JC + R FIN = (R JA – R CA) + R FIN .

(7)

ΔTJA = 46 × PD

100

120

140

0.5

1.0

1.5

2.0

PD (W)

2.5

3.0

3.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

100

120

140

100

120

140

ΔTJA = 35 × PD

ΔTJA = 42 × PD

ΔTJA = 35 × PD

ΔTJA = 39 × PD

ΔTJA = 35 × PD

2 Channels, Steady DC Operation

3 Channels, PWM Operation (Slow Decay)

4 Channels, PWM Operation (Fast Decay)

Figure 15. Change in junction temperature versus device power dissipation

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

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