Datasheet

26
LT1375/LT1376
13756fd
APPLICATIONS INFORMATION
WUU
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surface mount solid tantalum capacitor, but the final
capacitor chosen must be looked at carefully for ESR
characteristics.
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
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.
Capacitor ff I
V
V
OUT
OUT
IN
I
RMS
=
()( )
ff = Fudge factor
1
(1.2 to 2.0)
Diode Current
Average diode current is equal to load current. Peak diode
current will be considerably higher.
1
Normally, Jamoca Almond
Peak diode current:
Continuous
I
VV
V
VV
LfV V
Discontinuous
V
Lf
OUT
IN OUT
IN
IN OUT
IN OUT
OUT
Mode
Mode =
2I
OUT
=
+
()
+
()( )
()()
+
()
()( )
()()
2
Keep in mind that during start-up and output overloads,
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.
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.
BOOST
LT1376-5
V
IN
OUTPUT
5V
OUTPUT
–5V
†
* L1 IS A SINGLE CORE WITH TWO WINDINGS
COILTRONICS #CTX10-2P
** AVX TPSD107M010
†
IF LOAD CAN GO TO ZERO, AN OPTIONAL
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
INPUT
6V
TO 25V
GND
1375/76 F20
C2
0.1µF
C
C
0.01µF
R
C
470Ω
D1
1N5818
C1**
100µF
10V TANT
C5**
100µF
10V TANT
C3
22µF
35V TANT
C4**
100µF
10V TANT
D2
1N914
D3
1N5818
L1*
10µH
L1*
V
SW
SENSE
BIAS
GND
SHDN
V
C
+
+
+ +
Figure 20. Dual Output SEPIC Converter