LT1578C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1578C' PDF : 28 Pages View PDF
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
Ripple Current in the Input and Output Capacitors
Positive-to-negative converters have high ripple current in
both the input and output capacitors. For long capacitor
lifetime, the RMS value of this current must be less than
the high frequency ripple current rating of the capacitor.
The following formula will give an approximate value for
RMS ripple current. This formula assumes continuous
conduction mode and a large inductor value. Small induc-
tors will give somewhat higher ripple current, especially in
discontinuous mode. The exact formulas are very com-
plex and appear in Application Note 44, pages 30 and 31.
For our purposes here, a simple fudge factor (ff) is added.
The value for ff is about 1.2 for load currents above 0.38A
(in continuous conduction mode) and L ≥10µH. It in-
creases to about 2.0 for smaller inductors at lower load
currents (in discontinuous conduction mode).
( )( ) Capacitor IRMS = ff IOUT
VOUT
VIN
ff = Fudge factor (1.2 to 2.0)
Diode Current
Average diode current is equal to load current. Peak diode
current will be considerably higher.
Peak diode current:
Continuous Mode =
( ) ( )(( )()( ) ) IOUT
VIN + VOUT
VIN
+
2L
VIN VOUT
f VIN + VOUT
( )( ) Discontinuous Mode = 2 IOUT VOUT
( )( )L f
Keep in mind that during start-up and output overloads,
the average diode current may be much higher than with
normal loads. Care should be used if diodes rated less than
1A are used, especially if continuous overload conditions
must be tolerated.
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