LT1158
APPLICATIONS INFORMATION
–
Notethatforthefirsteventonly,t
isapproximately
SENSE pin is within 1.2V of supply. Under these condi-
tions the current is limited only by the R in series
SHUTDOWN
twice the above value since C is being discharged all
EN
DS(ON)
the way from its quiescent voltage. Allowable values for
R are from zero to 10k.
F
with R
. For a 5-lead MOSFET the current is limited
SENSE
DS(ON)
by R
alone, since R
is not in the output path
SENSE
(see Figure 7). Again adjusting R
the worst-case start currents are:
for temperature,
DS(ON)
7.5V
1.15V
1.2V
25μA
7.5V
ISTART
=
=
3-Lead MOSFET
5-Lead MOSFET
allows inductive
ENABLE
1+ ∂ R
)+ RSENSE
(
(
)
DS ON
(
+
C
EN
1μF
1.2V
R
F
ISTART
1k
LT1158
1+ ∂ R
)
DS ON
(
)
FAULT
Properly sizing the MOSFET for I
START
loads with long time constants, such as motors with high
mechanical inertia, to be started.
OPTIONAL THERMOSTAT
CLOSE ON RISE
AIRPAX #67FXXX
1158 F09
Returning to the example used in Power MOSFET Selec-
Figure 9. Self-Protection with Auto Restart
tion, an IRFZ34 (R
= 0.05Ω max) was selected for
DS(ON)
operationat5A.Iftheshort-circuitcurrentisalsosetat5A,
what start current can be supported? From the equation
t
becomes more difficult to analyze when the
SHUTDOWN
output is shorted with a PWM input. This is because the
FAULT pin only conducts when fault currents are actually
present in the MOSFET. FAULT does not conduct while the
for R
, a 0.03Ω shunt would be required, allowing
SENSE
the worst-case start current to be calculated:
1.2V
input is low in Figures 6 and 7 or during the interval I
0 in Figure 8. Thus t
the duty cycle of the current in the top MOSFET is low,
maintaining the average MOSFET current at a relatively
constant level.
=
DS
ISTART
=
=10A
will safely increase when
1.7 0.05Ω+0.03Ω
(
)
SHUTDOWN
This calculation gives the minimum current which could
be delivered with the IRFZ34 at T = 125°C without activat-
J
ing the FAULT pin on the LT1158. If more start current is
The length of time following shutdown before restart is
attempted is given by:
required, using an IRFZ44 (R
= 0.028Ω max) would
DS(ON)
increase I
to over 15A at T = 110°C, even though
START
J
⎛
⎜
⎞
⎟
the short-circuit current remains at 5A.
1.5V
⎝ 25μA⎠
tRESTART
=
C = 6×104 C
(
)
EN
EN
In order for the V sensing function to work properly, the
DS
supply pins for the LT1158 must be connected at the drain
of the top MOSFET, which must be properly decoupled
(see Ugly Transient Issues).
In Figure 9, the top MOSFET would shut down after being
in DC current limit for 0.9ms and try to restart at 60ms
intervals, thus producing a duty cyle of 1.5% in short
circuit. The resulting average top MOSFET dissipation
during a short is easily measured by taking the product of
the supply voltage and the average supply current.
Driving Lamps
Incandescent lamps represent a challenging load because
they have much in common with a short circuit when cold.
The top gate driver in the LT1158 can be configured to turn
on large lamps while still protecting the power MOSFET
Starting High In-Rush Current Loads
TheLT1158hasaV sensingfunctionwhichallowsmore
than I to flow in the top MOSFET providing that the
DS
SC
1158fb
15
LINEAR_DIMENSIONS [ Linear Dimensions ]
