LT1336 Linear Technology, LT1336 Datasheet - Page 11

• Manufacturer Part Number: LT1336
• Description: Half-Bridge N-Channel Power MOSFET Driver with Boost Regulator
• Manufacturer: Linear Technology

APPLICATIONS
Using the components as shown in Figure 2 the flyback
regulator will run at around 800kHz. To lower the fre-
quency C
frequency C
The flyback regulator works as follows: when switch S is
on, the primary current ramps up as the magnetic field
builds up. The magnetic field in the core induces a voltage
on the secondary winding equal to V
is transferred to V
reverse biased. The energy is stored in the transformer’s
magnetic field. When the primary inductor peak current is
reached, the switch is turned off. Energy is no longer
transferred to the transformer causing the magnetic field
to collapse. The collapsing magnetic field induces a change
in voltage across the transformer’s windings. During this
transition the Switch pin’s voltage flies to 10.6V plus a
diode above V
diode D2 and the transformer’s energy is transferred to
V
zero and the voltage at I
current threshold with a time constant of (R
thus completing the cycle.
C
FILTER
0.1 F
BOOST
* COILTRONICS CTX100-1P
. Meanwhile the primary inductor current goes to
FILTER
R
2
1/4W
FILTER
Figure 2. Using the Flyback Regulator
SENSE
+
, the secondary forward biases the rectifier
SV
PV
can be increased and to increase the
I
SENSE
+
+
BOOST
can be decreased.
U
LT1336
24V
SENSE
because the rectifier diode D2 is
INFORMATION
U
TSOURCE
1N4148
TGATEDR
TGATEFB
SWITCH
1000pF
SWGND
BOOST
decays to the lower inductor
S
1N4148
6.2k
+
V
–
D1
W
BOOST
+
. However, no power
1:1
T1*
SENSE
+
D2
1N4148
40V
C
1 F
)(C
U
BOOST
FILTER
60V MAX
HV =
1336 F02
+
),
Power MOSFET Selection
Since the LT1336 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
ments. The MOSFET BV
LT1336 absolute maximum operating voltage. For a maxi-
mum operating HV supply of 60V, the MOSFET BV
should be from 60V to 100V.
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,
the MOSFET junction temperature will be 125 C and
R
which can be satisfied by an IRFZ34 manufactured by
International Rectifier.
Transition losses result from the power dissipated in each
MOSFET during the time it is transitioning from off to on,
or from on to off. These losses are proportional to (f)(HV)
and vary from insignificant to being a limiting factor on
operating frequency in some high voltage applications.
DS(ON)
DS(ON)
= 0.007(125 – 25) = 0.7. This means that the required
P D I
R
DS ON
would be 0.089 /(1 + ). (1 + ) is given for each
of the MOSFET will be 0.089 /1.7 = 0.0523 ,
DS
2
D I
1
DS(ON)
DS
P
2
R
1
DS ON
is specified at T
DSS
should be at least equal to the
DS(ON)
can be derived:
DS(ON)
J
A
= 85 C and the
= 25 C and is
DS(ON)
vs tempera-
LT1336
require-
11
DS(ON)
DSS
2