LT1375/ LT1376
U
W
U U
APPLICATIONS INFORMATION
of thumb here to make a final decision. If modest ripple is
needed and the larger inductor does the trick, go for it. If
ripple is noncritical use the smaller inductor. If ripple is
extremely critical, a second filter may have to be added in
any case, and the lower value of inductance can be used.
Keep in mind that the output capacitor is the other critical
factor in determining output ripple voltage. Ripple shown
on the graph (Figure 19) is with a capacitor ESR of 0.1Ω.
This is reasonable for an AVX type TPS “D” or “E” size
surface mount solid tantalum capacitor, but the final
capacitor chosen must be looked at carefully for ESR
characteristics.
Diode Current
Average diodecurrentis equaltoloadcurrent. Peak diode
current will be considerably higher.
Peak diode current:
Continuous Mode =
V + V
V V
(
)
(
IN)(
)
IN
OUT
OUT
IOUT
+
V
2 L f V + V
IN
( )( )(
)
IN
OUT
2 I
V
(
OUT)(
)
OUT
Discontinuous Mode =
L f
( )( )
Ripple Current in the Input and Output Capacitors
Positive-to-negativeconverters havehighripplecurrentin
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
mode and large inductor value. Small inductors will give
somewhat higher ripple current, especially in discontinu-
ous mode. The exact formulas are very complex and
appear in Application Note 44, pages 30 and 31. For our
purposes here I have simply added a fudge factor (ff). The
value for ff is about 1.2 for higher load currents and
L ≥10µH. It increases to about 2.0 for smaller inductors at
lower load currents.
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with
normalloads.Careshouldbeusedifdiodes ratedless than
1A are used, especially if continuous overload conditions
must be tolerated.
Dual Output SEPIC Converter
The circuit in Figure 20 generates both positive and
negative 5V outputs with a single piece of magnetics. The
two inductors shown are actually just two windings on a
standard Coiltronics inductor. The topology for the 5V
output is a standard buck converter. The –5V topology
would be a simple flyback winding coupled to the buck
converter if C4 were not present. C4 creates the SEPIC
(Single-Ended Primary Inductance Converter) topology
which improves regulation and reduces ripple current in
L1. For details on this circuit see Design Note 100.
V
OUT
Capacitor IRMS = ff I
( )( OUT
)
V
IN
ff = Fudge factor1 (1.2 to 2.0)
1Normally, Jamoca Almond
26
Linear [ Linear ]
