Datasheet

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SLLS025A − JULY 1986
3−7
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
POST OFFICE BOX 1443
• HOUSTON, TEXAS 77251−1443
THERMAL INFORMATION
power dissipation precautions
Significant power may be dissipated in the SN75372 driver when charging and discharging high-capacitance
loads over a wide voltage range at high frequencies. Figure 5 shows the power dissipated in a typical SN75372
as a function of load capacitance and frequency. Average power dissipated by this driver is derived from the
equation
P
T(AV)
= P
DC(AV)
+ P
C(AV)
= P
S(AV)
where P
DC(AV)
is the steady-state power dissipation with the output high or low, P
C(AV)
is the power level during
charging or discharging of the load capacitance, and P
S(AV)
is the power dissipation during switching between
the low and high levels. None of these include energy transferred to the load, and all are averaged over a full
cycle.
The power components per driver channel are
P
C(AV)
[ CV
2
C
f
t
HL
t
LH
t
H
t
L
T = 1/f
Figure 12. Output Voltage Waveformwhere the times are as defined in Figure 14.
P
DC(AV)
=
P
H
t
H
+ P
L
t
L
T
P
S(AV)
=
P
LH
t
LH
+ P
HL
t
HL
T
P
L
, P
H
, P
LH
, and P
HL
are the respective instantaneous levels of power dissipation, C is the load capacitance.
V
C
is the voltage across the load capacitance during the charge cycle shown by the equation
V
C
= V
OH
− V
OL
P
S(AV)
may be ignored for power calculations at low frequencies.
In the following power calculation, both channels are operating under identical conditions:
V
OH
=19.2 V and V
OL
= 0.15 V with V
CC1
= 5 V, V
CC2
= 20 V, V
C
= 19.05 V, C = 1000 pF, and the
duty cycle = 60%. At 0.5 MHz, P
S(AV)
is negligible and can be ignored. When the output voltage is high, I
CC2
is negligible and can be ignored.
On a per-channel basis using data sheet values,
P
DC(AV)
+
ƪ
(5 V)
ǒ
2mA
2
Ǔ
) (20 V)
ǒ
0mA
2
Ǔ
ƫ
(0.6) )
ƪ
(5 V)
ǒ
16 mA
2
Ǔ
) (20 V)
ǒ
7mA
2
Ǔ
ƫ
(0.4)
P
DC(AV)
= 47 mW per channel
Power during the charging time of the load capacitance is
P
C(AV)
= (1000 pF) (19.05 V)
2
(0.5 MHz) = 182 mW per channel
Total power for each driver is
P
T(AV)
= 47 mW + 182 mW = 229 mW
and total package power is
P
T(AV)
= (229) (2) = 458 mW.