LT1158
(Refer to Functional Diagram)
OPERATION
goeslowinPWMoperation,andismaintainedbythecharge
pump when the top MOSFET is on DC. A regulated boost
driver at pin 1 employs a source-referenced 15V clamp
that prevents the bootstrap capacitor from overcharging
comparator input pins 11 and 12 are normally connected
across a shunt in the source of the top power MOSFET
(or to a current-sensing MOSFET). When pin 11 is more
+
than 1.2V below V and V12 – V11 exceeds the 110mV
+
regardless of V or output transients.
offset, FAULT pin 5 begins to sink current. During a short
circuit, the feedback loop regulates V12 – V11 to 150mV,
thereby limiting the top MOSFET current.
TheLT1158providesacurrent-sensecomparatorandfault
output circuit for protection of the top power MOSFET. The
APPLICATIONS INFORMATION
Power MOSFET Selection
and the available heat sinking has a thermal resistance of
20°C/W, the MOSFET junction temperature will be 125°C,
and∂=0.007(125–25)=0.7.Thismeansthattherequired
Since the LT1158 inherently protects the top and bottom
MOSFETsfromsimultaneousconduction,therearenosize
or matching constraints. Therefore selection can be made
R
of the MOSFET will be 0.089Ω/1.7 = 0.0523Ω,
DS(ON)
which can be satisfied by an IRFZ34.
basedontheoperatingvoltageandR
requirements.
DS(ON)
should be at least 2 • V
The MOSFET BV
, and
Notethatthesecalculationsareforthecontinuousoperating
condition; power MOSFETs can sustain far higher dissipa-
DSS
SUPPLY
should be increased to 3 • V
with frequent fault conditions. For the LT1158 maximum
in harsh environments
SUPPLY
tionsduringtransients.AdditionalR
)constraintsare
DS(ON)
operating supply of 30V, the MOSFET BV
from 60V to 100V.
should be
discussed under Starting High In-Rush Current Loads.
DSS
TheMOSFETR
isspecifiedatT =25°Candisgener-
J
DS(ON)
ally chosen based on the operating efficiency required as
longasthemaximumMOSFETjunctiontemperatureisnot
exceeded. The dissipation in each MOSFET is given by:
GATE DR
LT1158
GATE FB
R
G
R
G
2
P=D I
1+ ∂ R
DS ON
(
)
(
)
DS
(
)
R : OPTIONAL 10Ω
G
1158 F01
where D is the duty cycle and ∂ is the increase in R
DS(ON)
Figure 1. Paralleling MOSFETs
attheanticipatedMOSFETjunctiontemperature.Fromthis
equation the required R
can be derived:
DS(ON)
Paralleling MOSFETs
MOSFETs can be paralleled. The MOSFETs will inherently
share the currents according to their R ratio. The
P
RDS ON
=
(
)
2
D I
1+ ∂
DS(ON)
(
)
)
(
DS
LT1158 top and bottom drivers can each drive four power
MOSFETs in parallel with only a small loss in switching
speeds (see Typical Performance Characteristics). Indi-
vidual gate resistors may be required to “decouple” each
MOSFET from its neighbors to prevent high frequency
oscillations—consult manufacturer’s recommendations.
For example, if the MOSFET loss is to be limited to 2W
when operating at 5A and a 90% duty cycle, the required
R
would be 0.089Ω/(1 + ∂). (1 + ∂) is given for
DS(ON)
each MOSFET in the form of a normalized R
vs
DS(ON)
temperature curve, but ∂ = 0.007/°C can be used as an
approximationforlowvoltageMOSFETs. ThusifT =85°C
A
1158fb
9
LINEAR_DIMENSIONS [ Linear Dimensions ]
