LT1158ISW#PBF Datasheet LT1158CSW#PBF, LT1158ISW#PBF, LT1158CSW#TRPBF | P13-P15

LT1158

1158fb

APPLICATIONS INFORMATION

Low Current Shutdown

The LT1158 may be shutdown to a current level of 2mA by

pulling the enable pin 4 low. In this state both the top and

bottom MOSFETs are actively held off against any transients

which might occur on the output during shutdown. This

is important in applications such as 3-phase DC motor

control when one of the phases is disabled while the other

two are switching.

If zero standby current is required and the load returns to

ground, then a switch can be inserted into the supply path

of the LT1158 as shown in Figure 5. Resistor R

ensures

that the top MOSFET gate discharges, while the voltage

across the bottom MOSFET goes to zero. The voltage drop

across the P-channel supply switch must be less than

300mV, and R

must be 330k or greater for DC operation.

This technique is not recommended for applications which

require the LT1158 V

sensing function.

Figure 5. Adding Zero Current Shutdown

Figure 7. Short-Circuit Protection with Current-Sensing MOSFET

Figure 6. Short-Circuit Protection with Standard MOSFET

Short-Circuit Protection in Bridge Applications

The LT1158 protects the top power MOSFET from output

shorts to ground, or in a full bridge application, shorts

across the load. Both standard 3-lead MOSFETs and cur-

rent-sensing 5-lead MOSFETs can be protected. The bottom

MOSFET is not protected from shorts to supply.

Current is sensed by measuring the voltage across a cur-

rent shunt in the source lead of a standard 3-lead MOSFET

(Figure 6). For the current-sensing MOSFET shown in

Figure 7, the sense resistor is inserted between the sense

and Kelvin leads.

The SENSE

and SENSE

–

PC traces must be routed together

at minimum spacing to prevent stray pickup, and a Kelvin

connection must be used at the current shunt for the 3-lead

MOSFET. Using a twisted pair is the safest approach and

is recommended for sense runs of several inches.

When the voltage across R

SENSE

exceeds 110mV, the

LT1158 FAULT pin begins to conduct, signaling a fault

condition. The current in a short circuit ramps very rapidly,

limited only by the series inductance and ultimately the

MOSFET and shunt resistance. Due to the response time

100k

LT1158

VP0300

1158 F05

LOAD

GND

TO OTHER

CONTROL

CIRCUITS

CMOS

ON/OFF

100k

T GATE DR

T GATE FB

T SOURCE

B GATE DR

B GATE FB

2N2222

T GATE DR

T GATE FB

T SOURCE

SENSE

–

FAULT

LT1158

SENSE

1158 F06

10k

T GATE DR

T GATE FB

T SOURCE

SENSE

–

FAULT

LT1158

1158 F07

10k

SENSE

KELVIN

SENSE

OUTPUT

LT1158

1158fb

APPLICATIONS INFORMATION

the value of R

SENSE

for the 5-lead MOSFET increases by

the current sensing ratio (typically 1000 – 3000), thus

eliminating the need for a low valued shunt. ΔV is in the

range of 1V to 3V in most applications.

Assuming a dead short, the MOSFET dissipation will rise

to V

SUPPLY

• I

. For example, with a 24V supply and I

= 10A, the dissipation would be 240W. To determine how

long the MOSFET can remain at this dissipation level before

it must be shut down, refer to the SOA curves given in

the MOSFET data sheet. For example, an IRFZ34 would

be safe if shut down within 10ms.

A Tektronix A6303 current probe is highly recommended

for viewing output fault currents.

If Short-Circuit Protection is Not Required

In applications which do not require the current sense

capability of the LT1158, the sense pins 11 and 12 should

both be connected to pin 13, and the FAULT pin 5 left

open. The enable pin 4 may still be used to shut down

the device. Note, however, that when unprotected the top

MOSFET can be easily (and often dramatically) destroyed

by even a momentary short.

Self-Protection with Automatic Restart

When using the current sense circuits of Figures 6 and 7,

local shutdown can be achieved by connecting the FAULT

pin through resistor R

to the enable pin as shown in

Figure 9. An optional thermostat mounted to the load or

MOSFET heatsink can also be used to pull enable low.

An internal 25μA current source normally keeps the enable

capacitor CEN charged to the 7.5V clamp voltage (or to V

for V

< 7.5V). When a fault occurs, CEN is discharged to

below the enable low threshold (1.15V typ) which shuts

down both MOSFETs. When the FAULT pin or thermostat

releases, CEN recharges to the upper enable threshold

where restart is attempted. In a sustained short circuit,

FAULT will again pull low and the cycle will repeat until the

short is removed. The time to shut down for a DC input

or thermal fault is given by:

SHUTDOWN

= (100 + 0.8R

) C

DC input

of the LT1158 current limit loop, an initial current spike of

from 2 to 5 times the ﬁ nal value will be present for a few

μs, followed by an interval in which I

= 0. The current

spike is normally well within the safe operating area (SOA)

of the MOSFET, but can be further reduced with a small

(0.5μH) inductor in series with the output.

Figure 8. Top MOSFET Short-Circuit Turn-On current

5μs/DIV

LT1158 F08

5A/DIV

If neither the enable nor input pins are pulled low in

response to the fault indication, the top MOSFET current

will recover to a steady-state value I

regulated by the

LT1158 as shown in Figure 8:

()

150mV

r 150mV

SENSE

SSENSE

SENSE

150mV

r 150mV

−

⎛

⎝

⎜

⎞

⎠

⎟

()

−

1150mV

sense ratio, V = V

⎛

⎝

⎜

⎞

⎠

⎟

−

r current

GGS

=−VV

GS T

The time for the current to recover to I

following the

initial current spike is approximately Q

/0.5mA, where

is the MOSFET gate-to-source charge. I

need not

be set higher than the required start-up current for mo-

tors (see Starting High In-Rush Current Loads). Note that

LT1158

1158fb

APPLICATIONS INFORMATION

Figure 9. Self-Protection with Auto Restart

SHUTDOWN

becomes more difﬁ cult 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 I

0 in Figure 8. Thus t

SHUTDOWN

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:

RESTART EN EN

⎛

⎝

⎜

⎞

⎠

⎟

=×

()

610

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 V

sensing function which allows more

than I

to ﬂ ow in the top MOSFET providing that the

Note that for the ﬁ rst event only, t

SHUTDOWN

is approximately

twice the above value since C

is being discharged all

the way from its quiescent voltage. Allowable values for

are from zero to 10k.

SENSE

–

pin is within 1.2V of supply. Under these condi-

tions the current is limited only by the R

DS(ON)

in series

with R

SENSE

. For a 5-lead MOSFET the current is limited

by R

DS(ON)

alone, since R

SENSE

is not in the output path

(see Figure 7). Again adjusting R

DS(ON)

for temperature,

the worst-case start currents are:

START

DS ON

SENSE

()

∂

3-Lead MOSFET

START

DS ON

()

∂

5-Lead MOSFET

Properly sizing the MOSFET for I

START

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 (R

DS(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 R

SENSE

, a 0.03Ω shunt would be required, allowing

the worst-case start current to be calculated:

START

()

17 005 003

.. .ΩΩ

This calculation gives the minimum current which could

be delivered with the IRFZ34 at T

= 125°C without activat-

ing the FAULT pin on the LT1158. If more start current is

required, using an IRFZ44 (R

DS(ON)

= 0.028Ω max) would

increase I

START

to over 15A at T

= 110°C, even though

the short-circuit current remains at 5A.

In order for the V

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 conﬁ gured to turn

on large lamps while still protecting the power MOSFET

FAULT

LT1158

1μF

1158 F09

7.5V

1.15V

ENABLE

OPTIONAL THERMOSTAT

CLOSE ON RISE

AIRPAX #67FXXX

25μA

7.5V

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LT1158ISW#PBF

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Description:

Gate Drivers Half Bridge N-Ch Pwr MOSFET Drvr

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