LT3999
10
3999fa
For more information www.linear.com/LT3999
APPLICATIONS INFORMATION
Turns Ratio
The turns ratio of the transformer determines the output
voltage. The following equation is used as a first pass to
calculate the turns ratio:
S
N
P
=
OUT
F
2 V
IN
– V
SW
( )
DC
where V
F
is the forward voltage of the output diode, V
SW
is the voltage drop across the internal switches (see the
Typical Performance curves) and DC is the duty cycle.
Sufficient margin should be added to the turns ratio to
account for voltage drops due to transformer winding
resistance.
Magnetizing Current
The magnetizing inductance of the transformer causes
a ripple current that is independent of load
current. This
ripple current is calculated by:
∆I=
IN
f
•L
where ∆I and L
M
are primary ripple current and magnetizing
inductance referred to the primary side of the transformer,
respectively. Increasing the transformer magnetizing in-
ductance, L
M
, reduces the ripple current. The ripple current
formula shows the effect of the switching frequency on
the magnetizing inductance. Setting the LT3999 at high
switching frequency reduces the ripple current for the
same magnetizing inductance. Therefore, it is
possible to
reduce the transformer turns and still achieve low ripple
current. This helps to reduce the power converter footprint
as well. The transformer magnetizing inductance should
be designed for the worst-case duty cycle and input line
voltage combination.
A good rule of thumb is to set the primary current ripple
amplitude 10% to 30% of the average primary current, I
P
:
I
P
=
OUT
V
•eff
where P
OUT
is the output power of the converter and eff
is the converter efficiency, typically around 85%.
Winding Resistance
Resistance in either the primary or secondary winding
reduces overall efficiency and degrades load regulation.
If efficiency or load regulation is unsatisfactory, verify
that the voltage drops in the transformer windings are
not excessive.
Capacitors
In applications with full duty cycle operation, the input
supply current is approximately constant
. Therefore, large
input “hold-up type” capacitors are not necessary. A low
value (>4.7µF), low ESR ceramic will be adequate to filter
high frequency noise at the input. The output capacitors
supply energy to the output load only during switch
transitions. Therefore, large capacitance values are not
necessary on the output.
Transformer winding capacitance between the isolated
primary and secondary has parasitic currents that can
cause noise
on the grounds. Providing a high frequency,
low impedance path between the primary and secondary
gives the parasitic currents a local return path. A 2.2nF,
1kV ceramic capacitor is recommended.
Optional LC Filter
An optional LC filter, as shown on the Typical Application
on the first page of this data sheet, should be included if
ultralow noise and ripple are required. It is recommended
that the corner
frequency of the filter should be set a
decade below the switching frequency so that the switch
noise is attenuated by a factor of 100. For example, if the
f
OSC
= 100kHz, then f
CORNER
= 10kHz where:
f
CORNER
=
2•π LC
Switching Diode Selection
A fast recovery, surface mount diode such as a Schottky
is recommended. The proximity of the diodes to the
transformer outputs is important and should be as close
as possible with short, wide traces connecting them.
