LT1158CS View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1158CS' PDF : 20 Pages View PDF
LT1158
APPLICATIONS INFORMATION
Note that for the first event only, tSHUTDOWN is approxi-
mately twice the above value since CEN is being discharged
all the way from its quiescent voltage. Allowable values for
RF are from zero to 10k.
7.5V
sense– pin is within 1.2V of supply. Under these condi-
tions the current is limited only by the RDS(ON) in series
with RSENSE. For a 5-lead MOSFET the current is limited by
RDS(ON) alone, since RSENSE is not in the output path (see
Figure 7). Again adjusting RDS(ON) for temperature, the
worst-case start currents are:
1.15V
CEN +
1µF
ENABLE
25µA
7.5V
RF
1k
LT1158
FAULT
( ) ( ) ISTART =
1.2V
1+ ∂ RDS ON + RSENSE
( ) ( ) ISTART =
1.2V
1+ ∂ RDS ON
3-Lead MOSFET
5-Lead MOSFET
OPTIONAL THERMOSTAT
CLOSE ON RISE
AIRPAX #67FXXX
1158 F09
Figure 9. Self-Protection with Auto Restart
tSHUTDOWN becomes more difficult to analyze when the
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
input is low in Figures 6 and 7 or during the interval IDS =
0 in Figure 8. Thus tSHUTDOWN will safely increase when
the duty cycle of the current in the top MOSFET is low,
maintaining the average MOSFET current at a relatively
constant level.
The length of time following shutdown before restart is
attempted is given by:
tRESTART
=
1.5V
25µA
CEN
=
6
×
104
CEN
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.
Starting High In-Rush Current Loads
The LT1158 has a VDS sensing function which allows more
than ISC to flow in the top MOSFET providing that the
Properly sizing the MOSFET for ISTART allows inductive
loads with long time constants, such as motors with high
mechanical inertia, to be started.
Returning to the example used in Power MOSFET Selec-
tion, an IRFZ34 (RDS(ON) = 0.05Ω max.) was selected for
operation at 5A. If the short-circuit current is also set at 5A,
what start current can be supported? From the equation
for RSENSE, a 0.03Ω shunt would be required, allowing the
worst-case start current to be calculated:
( ) ISTART
=
1.7
1.2V
0.05Ω +0.03Ω
= 10A
This calculation gives the minimum current which could
be delivered with the IRFZ34 at TJ = 125°C without activat-
ing the fault pin on the LT1158. If more start current is
required, using an IRFZ44 (RDS(ON) = 0.028Ω max.) would
increase ISTART to over 15A at TJ = 110°C, even though the
short-circuit current remains at 5A.
In order for the VDS sensing function to work properly, the
supply pins for the LT1158 must be connected at the drain
of the top MOSFET, which must be properly decoupled
(see Ugly Transient Issues).
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
14
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