LMD18400 Datasheet, PDF (13/18 Pages) National Semiconductor (TI) – Quad High Side Driver

Applications Information (Continued)
TL H 11026 – 23
FIGURE 13 Switching an Inductive Load
When switched ON the worst case power dissipation is
P_D(ON) e I_Peak2 c
R_ON
where
I_Peak
e
V_CC
R_ON a
R_S
The steady-state ON current of the inductor should be kept
less than 1A per power switch
The additional power dissipation during turn-off as the in-
ductor is de-energized and the voltage across the inductor
is clamped to b5V can be found by
P_D(OFF)
e
(V_CC
a
5V)
2
c
I_Peak
for the time interval t_Clamp which is the time required for
the inductor current to fall to zero
t_Clamp
e
I_Peak c
5V
L
The size of the inductor will determine the time duration for
this additional power dissipation interval Even though the
peak current is kept less than 1A the switch during this
interval will see a voltage across it of V_CC a 5V with no
power limit protection If the inductor is too large the time
interval may be long enough to heat the die temperature to
a170 C thereby shutting OFF all other loads on the pack-
age
The total average power dissipation during a full ON OFF
switching cycle of an inductive load will be
( ^{PD(tot) e}
I_Peak2
R_ON
t_ON
a
I_Peak2L
(V_CC
10
a
5V)
1
t_ON a t_OFF
Due to the common cut-off of all loads forced by thermal
shutdown the thermal time constants of the package be-
come a concern Figure 14 provides an indication of the
time it takes to heat the die to thermal shutdown with a step
increase in package power dissipation from an initial junc-
tion temperature of a25 C This data was measured using
a PC board layout providing a thermal resistance from junc-
tion to ambient of approximately 35 C W Less heatsinking
will of course result in faster thermal shutdown of the pow-
er switches
TL H 11026 – 24
FIGURE 14 Approximate time required for the die to
reach the 170 C thermal shutdown point from 25 C for
different total package power dissipation levels
13
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