LT1158CS LINER [Linear Technology], LT1158CS Datasheet - Page 8

• Manufacturer Part Number: LT1158CS
• Description: Half Bridge N-Channel Power MOSFET Driver
• Manufacturer: LINER [Linear Technology]

OPERATIO
APPLICATIONS
Power MOSFET Selection
Since the LT1158 inherently protects the top and bottom
MOSFETs from simultaneous conduction, there are no
size or matching constraints. Therefore selection can be
made based on the operating voltage and R
requirements. The MOSFET BV
2 V
harsh environments with frequent fault conditions. For the
LT1158 maximum operating supply of 30V, the MOSFET
BV
The MOSFET R
generally chosen based on the operating efficiency re-
quired as long as the maximum MOSFET junction tem-
perature is not exceeded. The dissipation in each MOSFET
is given by:
where D is the duty cycle and is the increase in R
at the anticipated MOSFET junction temperature. From
this equation the required R
For example, if the MOSFET loss is to be limited to 2W
when operating at 5A and a 90% duty cycle, the required
R
MOSFET in the form of a normalized R
ture curve, but = 0.007/ C can be used as an approxima-
tion for low voltage MOSFETs. Thus if T
available heat sinking has a thermal resistance of 20 C/W,
LT1158
13 goes low in PWM operation, and is maintained by the
charge 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 over-
charging regardless of V
The LT1158 provides a current-sense comparator and
fault output circuit for protection of the top power MOSFET.
8
DS(ON)
DSS
P = D I
R
DS ON
SUPPLY
should be from 60V to 100V.
would be 0.089 /(1 + ). (1 + ) is given for each
DS
, and should be increased to 3 V
2
D I
1
DS(ON)
DS
U
P
2
U
R
1
(Refer to Functional Diagram)
DS ON
is specified at T
+
INFORMATION
U
or output transients.
DS(ON)
DSS
can be derived:
W
should be at least
DS(ON)
A
J
= 85 C and the
= 25 C and is
vs. tempera-
U
SUPPLY
DS(ON)
DS(ON)
in
The comparator input pins 11 and 12 are normally con-
nected 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
110mV 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.
the MOSFET junction temperature will be 125 C, and
R
which can be satisfied by an IRFZ34.
Note that these calculations are for the continuous oper-
ating condition; power MOSFETs can sustain far higher
dissipations during transients. Additional R
straints are discussed under Starting High In-Rush Cur-
rent Loads.
Paralleling MOSFETs
When the above calculations result in a lower R
is economically feasible with a single MOSFET, two or
more MOSFETs can be paralleled. The MOSFETs will
inherently share the currents according to their R
ratio. The LT1158 top and bottom drivers can each drive
four power MOSFETs in parallel with only a small loss in
switching speeds (see Typical Performance Characteris-
tics). Individual gate resistors may be required to
“decouple” each MOSFET from its neighbors to prevent
DS(ON)
= 0.007(125 – 25) = 0.7. This means that the required
of the MOSFET will be 0.089 /1.7 = 0.0523 ,
LT1158
Figure 1. Paralleling MOSFETs
GATE DR
GATE FB
R
G
: OPTIONAL 10
+
R
and V12 – V11 exceeds the
G
R
G
DS(ON)
1158 F01
DS(ON)
DS(ON)
con-
than