LT1375/LT1376
U
W U U
APPLICATIONS INFORMATION
surface mount solid tantalum capacitor, but the final
capacitor chosen must be looked at carefully for ESR
characteristics.
Peak diode current:
Continuous Mode =
V + V
V
V
(
)
( )(
)
IN
OUT
IN OUT
Ripple Current in the Input and Output Capacitors
IOUT
+
V
2 L f V + V
( )( )(
IN
)
IN
OUT
Positive-to-negativeconvertershavehighripplecurrentin
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.
2 I
V
(
OUT)(
)
OUT
Discontinuous Mode =
L f
( )( )
Keep in mind that during start-up and output overloads,
average diode current may be much higher than with
normalloads.Careshouldbeusedifdiodesratedlessthan
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.
VOUT
Capacitor IRMS = ff I
( )( OUT
)
V
IN
ff = Fudge factor1 (1.2 to 2.0)
Diode Current
Average diodecurrentisequaltoloadcurrent. Peak diode
current will be considerably higher.
D2
1N914
C2
0.1µF
BOOST
INPUT
OUTPUT
5V
V
V
SW
IN
6V TO 25V
L1*
10µH
BIAS
LT1376-5
SHDN
GND
SENSE
C
V
+
+
C1**
+
C3
22µF
100µF
R
C
10V TANT
35V TANT
470Ω
D1
1N5818
C
C
0.01µF
GND
+
C4**
C5**
* L1 IS A SINGLE CORE WITH TWO WINDINGS
COILTRONICS #CTX10-2P
L1*
D3
1N5818
100µF
100µF
10V TANT
10V TANT
** AVX TPSD107M010
OUTPUT
†
IF LOAD CAN GO TO ZERO, AN OPTIONAL
†
–5V
PRELOAD OF 1k TO 5k MAY BE USED TO
IMPROVE LOAD REGULATION
1375/76 F20
Figure 20. Dual Output SEPIC Converter
1Normally, Jamoca Almond
13756fd
26
Linear [ Linear ]
