Datasheet

LT1336
12
1336fa
applicaTions inForMaTion
Using the components as shown in Figure 2 the flyback
regulator will run at around 800kHz. To lower the frequency
C
FILTER
can be increased and to increase the frequency
C
FILTER
can be decreased.
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
DS(ON)
requirements.
The MOSFET BV
DSS
should be at least equal to the LT1336
absolute maximum operating voltage. For a maximum
operating HV supply of 60V, the MOSFET BV
DSS
should
be from 60V to 100V.
The MOSFET R
DS(ON)
is specified at T
J
= 25°C and is gen-
erally chosen based on the operating efficiency required
as long as the maximum MOSFET junction temperature is
not exceeded. The dissipation in each MOSFET is given by:
P =D I
DS
( )
2
1+ ∂
( )
R
DS ON
( )
where D is the duty cycle and ∂ is the increase in R
DS(ON)
at the anticipated MOSFET junction temperature. From this
equation the required R
DS(ON)
can be derived:
R
DS ON
( )
=
P
D I
DS
( )
2
1+ ∂
( )
For example, if the MOSFET loss is to be limited to 2W
when operating at 5A and a 90% duty cycle, the required
R
DS(ON)
would be 0.089Ω/(1 + ∂). (1 + ∂) is given for
each MOSFET in the form of a normalized R
DS(ON)
vs
temperature curve, but ∂ = 0.007/°C can be used as an
approximation for low voltage MOSFETs. Thus, if T
A
= 85°C
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. This means that the required
R
DS(ON)
of the MOSFET will be 0.089Ω/1.7 = 0.0523Ω,
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)
2
and vary from insignificant to being a limiting factor on
operating frequency in some high voltage applications.
Figure 2. Using the Flyback Regulator
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
+
. However, no power
is transferred to V
BOOST
because the rectifier diode D2 is
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
+
, the secondary forward biases the rectifier diode
D2 and the transformer’s energy is transferred to V
BOOST
.
Meanwhile the primary inductor current goes to zero and
the voltage at I
SENSE
decays to the lower inductor current
threshold with a time constant of (R
SENSE
)(C
FILTER
), thus
completing the cycle.
SWITCH
SV
+
PV
+
R
SENSE
2Ω
1/4W
D2
1N4148
40V
1N4148
24V
1000pF
6.2k
S
HV =
60V MAX
1336 F02
LT1336
T1*
1:1
D1
1N4148
* COILTRONICS CTX100-1P
+
SWGND
I
SENSE
C
BOOST
1µF
C
FILTER
0.1µF
–
+
V
BOOST
+
TGATEDR
TGATEFB
BOOST
TSOURCE