L294 datasheet(6/8 Pages) STMICROELECTRONICS

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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.

= −

(1 −

• Ip

)

where : V1 = Vs - Vsat L - Vsat H _ VR sens

= −

+ RL • Io

where : V2 = Vs + VD1 + VD2

≤ Io ≤ Ip

Io is the value of the load current at the end of ton.

t1 = t3 = t5 = ...

)

−

0.9 Ip • RL

+ V3

Ip RL

+ V3

where

V3 = Vsat L + VR sens + VD1

)

− R1 C1 In 0.9 ≅

R1 C1

t2 = t4 = t6 = ...

= −

(

− Ip RL

− IK RL

)

Note that the time interval t1 = t3 = t5 = ... takes the longer value between case a) and case b). The switching

frequency is always :

fswitching

+ 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.

= Ip e −

t1 RL

−

(1 − e −

t1 RL

)

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) 70

µs

b) 16

µs

t2 = 29

µs

f = 10.2 KHz

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L294

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