LT3999
APPLICATIONS INFORMATION
Turns Ratio
Winding Resistance
The turns ratio of the transformer determines the output
voltage. The following equation is used as a first pass to
calculate the turns ratio:
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.
NS
VOUT +VF
=
NP 2 V – V
DC
(
)
IN
SW
Capacitors
where V is the forward voltage of the output diode, V
F
SW
In applications with full duty cycle operation, the input
supplycurrentisapproximatelyconstant. 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.
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:
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.
V •DC
fSW •LM
IN
∆I=
where∆IandL areprimaryripplecurrentandmagnetizing
M
Optional LC Filter
inductancereferredtotheprimarysideofthetransformer,
respectively. Increasing the transformer magnetizing in-
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
ductance,L ,reducestheripplecurrent.Theripplecurrent
M
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.Thishelpstoreducethepowerconverterfootprint
as well. The transformer magnetizing inductance should
be designed for the worst-case duty cycle and input line
voltage combination.
f
= 100kHz, then f
= 10kHz where:
OSC
CORNER
1
fCORNER
=
2•π LC
Switching Diode Selection
A good rule of thumb is to set the primary current ripple
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.
amplitude 10% to 30% of the average primary current, I :
P
POUT
IP =
V •eff
IN
where P
is the output power of the converter and eff
OUT
is the converter efficiency, typically around 85%.
3999fa
10
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