MC3479 Ver la hoja de datos (PDF) - Motorola => Freescale
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MC3479 Datasheet PDF : 10 Pages
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MC3479
The value of RB (between this pin and ground) is then
determined by:
+ * RB
VM 0.7 V
IBS
b) When this pin is opened (raised to VM) such that IBS is
<5.0 µA, the internal logic is set to the Phase A condition, and
the four driver outputs are put into a high impedance state.
The Phase A output (Pin 11) goes active (low), and input
signals at the controls are ignored during this time. Upon
re–establishing IBS, the driver outputs become active, and
will be in the Phase A position (L1 = L3 = VOHD, L2 = L4 =
VOLD). The circuit will then respond to the inputs at the
controls.
The Set function (opening this pin) can be used as a
power–up reset while supply voltages are settling. A CMOS
logic gate (powered by VM) can be used to control this pin as
shown in Figure 11.
c) Whenever the motor is not being stepped, power
dissipation in the IC and in the motor may be lowered by
reducing IBS, so as to reduce the output (motor) current.
Setting IBS to 75 µA will reduce the motor current, but will not
reset the internal logic as described above. See Figure 12 for
a suggested circuit.
Power Dissipation
The power dissipated by the MC3479 must be such that
the junction temperature (TJ) does not exceed 150°C. The
power dissipated can be expressed as:
P = (VM IM) + (2 IOD) [(VM – VOHD) + VOLD]
where
VM = Supply voltage;
IM = Supply current other than IOD;
IOD = Output current to each motor coil;
VOHD = Driver output high voltage;
VOLD = Driver output low voltage.
The power supply current (IM) is obtained from Figure 8. After
the power dissipation is calculated, the junction temperature
can be calculated using:
TJ = (P RθJA) + TA
where RθJA = Junction–to–ambient thermal resistance
(52°C/W for the DIP, 72°C/W for the FN Package);
TA = Ambient Temperature.
Figure 8. Power Supply Current
For example, assume an application where VM = 12 V, the
motor requires 200 mA/coil, operating at room temperature
with no heatsink on the IC. IBS is calculated:
IBS = 200 0.86
IBS = 172 µA
RB is calculated:
RB = (12 – 0.7) V/172 µA
RB = 65.7 kΩ
From Figure 8, IM (max) is determined to be 40 mA. From
Figure 9, VOLD is 0.46 volts, and from Figure 10, (VM – VOHD)
is 1.4 volts.
P = (12 0.040) + (2 0.2) (1.4 + 0.46)
P = 1.22 W
TJ = (1.22 W 52°C/W) + 25°C
TJ = 88°C
This temperature is well below the maximum limit. If the
calculated TJ had been higher than 150°C, a heatsink such
as the Staver Co. V–7 Series, Aavid #5802, or Thermalloy
#6012 could be used to reduce RθJA. In extreme cases,
forced air cooling should be considered.
The above calculation, and RθJA, assumes that a ground
plane is provided under the MC3479 (either or both sides of
the PC board) to aid in the heat dissipation. Single nominal
width traces leading from the four ground pins should be
avoided as this will increase TJ, as well as provide potentially
disruptive ground noise and IR drops when switching the
motor current.
Figure 9. Maximum Saturation Voltage —
Driver Output Low
0.8
0.6
0.4
0.2
0
0
100
200
300
IOD (mA)
70
60
IOD = 0
50
40
Figure 10. Maximum Saturation Voltage —
Driver Output High
2.0
1.5
30
1.0
20
10
0
50 100 150 200 250 300 350
IBS (µA)
0.5
0
0
100
200
300
IOD (mA)
MOTOROLA ANALOG IC DEVICE DATA
7
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