L294(1991) Просмотр технического описания (PDF) - STMicroelectronics
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L294 Datasheet PDF : 8 Pages
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L294
Figure 5. Safe Operating Areas.
Figure 6. Turn-off Phase.
CALCULATION OF THE SWITCHING TIMES
Referring to the block diagram and to the waveforms of fig. 1, it is possible to calculate the switching times
by means of the following relationships.
tr
=
−
L
RL
In
(1
−
RL
V1
•
Ip )
where : V1 = Vs - Vsat L - Vsat H _ VR sens
tf = − L In
V2
RL V2 + RL • Io
where : V2 = Vs + VD1 + VD2
IK ≤ Io ≤ Ip
Io is the value of the load current at the end of ton.
t1 = t3 = t5 = ...
t2 = t4 = t6 = ...
=
a) − L In
RL
0.9 Ip • RL + V3
Ip RL + V3
b) − R1 C1 In 0.9 ≅ 1 R1 C1
10
=
−
L
RL
In
V1
(
V1
−
−
Ip
IK
RL
RL
)
where
V3 = Vsat L + VR sens + VD1
Note that the time interval t1 = t3 = t5 = ... takes the longer value between case a) and case b). The switching
frequency is always :
fswitching =
1
t1 + t2
In the case a) the main regulation loop is always closed and it forces :
IK = (0.9 ± S) Ip
where : S = 3 % @ Vi = 1 V
S = 1.5 % @ Vi = 4 V
In the case b), the same loop is open in the recirculation phase and IK, which is always lower than 0.9 Ip,
is obtained by means of the following relationship.
IK
=
Ip
e
−
t1 RL
L
−
V3
RL
(1
−
e
−
t1
RL
L
)
With the typical application circuit, in the conditions Vs = 40V, Ip = 4A, the following switching times result:
tr = 255 µs
tf = 174 µs @ Io = Ip
t1
=
a)
b)
70
16
µs
µs
t2 = 29 µs
f = 10.2 KHz
6/8
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