Obsolete Product(s) - Obsolete Product(s)
THERMAL PROTECTION
A thermalprotection circuit has been included that
will disable the device if the junction temperature
reaches 150°C. When the temperature has fallen
to a safe level the device restarts under the con-
trol of the input and enable signals.
APPLICATION INFORMATION
RECIRCULATION
During recirculationwith the ENALBE input high,
the voltage drop across the transistor is R
DS(ON)
.
for voltages less than 0.6V and is clamped at a
voltages depending on the characteristics of the
source-drain diode for greater voltages. Although
the device is protected against cross conduction.
POWER DISSIPATION
each bridge
In order to achieve the high performance provided
by the L9925 some attention must be paid t en-
sure that it has an adequate PCB area to dissi-
pate the heat. The forst stage of any thermal de-
sign is to calculate the dissipated power in the
application, for this example the half step opera-
tion shown in Fig. 6 is considered.
RISE TIME T
R
When an arm of the half bridge is turned on cur-
rent begins to flow in the inductive load until the
maximum current I
L
is reached after a time T
R
,
The dissipated energy E
OFF/ON
.
E
OFF/ON
= [R
DS(ON)
⋅
I
L
2
⋅
T
R
]
⋅
2
3
ON TIME T
ON
During this time the energy dissipated is due to
the ON resistance of the transistors E
ON
and the
commutation E
COM
. As two of the POWER DMOS
transistors are ON E
ON
is given by:
E
ON
= I
L
2
⋅
R
DS(ON)
⋅
2
⋅
T
ON
In the commutation the energy dissipated is:
E
CON
= V
S
⋅
I
L
⋅
T
COM
⋅
f
SWITCH
⋅
T
ON
Where:
T
COM
= Communication Time and it is assumed that:;
T
COM
= t
rise
= t
fall
≤
20
µ
s
T
SWITCH
= Chopper frequency
FALL TIME T
F
For this example it is assumed that the energy
dissipated in this part of the cycle takes the same
form as that shown for the rise time:
E
OFF/ON
= [R
DS(ON)
⋅
I
L
2
⋅
T
F
]
⋅
2
3
QUIESCENT ENERGY
The last contribution of the energy dissipation is
due to the quiescrent supply current and is given
by:
E
QUIESCENT
= I
QUIESCENT
⋅
V
S
⋅
T
TOTAL ENERGY PER CYCLE
E
TOT
= (2
⋅
E
OFF/ON
+ E
ON
+ E
COM
)
bridge1
+
+ (2
⋅
E
OFF/ON
+ E
ON
+ E
COM
)
bridg2
+ E
QUIESCENT
The total power dissipation P
DIS
is simply:
P
DIS
=
E
tot
T
T
R
= Rise time
T
ON
= ON time
T
F
= Fall time
T
OFF
= OFF time
T = Period
T = T
R
+ T
ON
+ T
F
+ T
OFF
T
switch
T
R
T
ON
T
OFF
T
F
I
L
D99AT435
Figure 6.
L9925
7/9
Obsolete Product(s) - Obsolete Product(s)
