11
DEMO MANUAL DC233
SOT-23 SWITCHING REGULATORS
OPERATIO
U
Diodes D1, D2 and D3 (Motorola MBR0520LT1) are one-
half amp, 20V Schottky diodes. This is a good choice for
nearly any LT1611/LT1613 application, unless the out-
put voltage or the circuit topology requires a diode rated
for higher reverse voltages. Motorola also offers 30V and
40V versions. Most one-half amp and one amp Schottky
diodes are suitable; these are available from many manu-
facturers. If you use a silicon diode, it must be an ultrafast
recovery type. Efficiency will be lower due to the silicon
diode’s higher forward voltage drop.
Inductors used with the LT1611 and LT1613 should be
rated for approximately 0.5A. The value of the inductor
should be matched to the power requirements and oper-
ating voltages of the application. In most cases a value of
4.7µH or 10µH is suitable. The Murata inductors used on
the DC233 are small and inexpensive and are a good fit
for the LT1611 and LT1613. Alternatives are the CD43
series from Sumida and the DO1608 series from Coil-
craft. These inductors are slightly larger but will result in
slightly higher circuit efficiency.
The voltage rating of the input capacitor limits the input
voltage range of the circuits. The input range to the SEPIC
and inverting circuit can be raised to 10V by replacing the
input capacitor (C2 or C13) with a 16V capacitor and (in
the case of the inverter) the coupling capacitor (C14) with
a 16V part. Note that, in power supply applications, most
tantalum capacitor manufacturers recommend using a
capacitor with a voltage rating higher than the operating
voltage.
The coupling capacitor in the SEPIC and inverting circuits
(C3 or C14) should have a low ESR to ensure good
efficiency and must have an adequate ripple current
rating. It also must have a suitable voltage rating. In the
case of the SEPIC circuit, it should be rated for the
maximum input voltage or higher; in the inverter, its
voltage rating must be higher than the sum of the
magnitudes of the input and output voltages. If a coupled
inductor is used, the value of this ceramic capacitor can
be reduced to 0.22µF from the 1µF used here.
Lower Ripple
The quality of the output capacitor is the greatest deter-
minant of the output voltage ripple. The output capacitor
performs two major functions: it must have enough
capacitance to satisfy the load under transient conditions
and it must shunt the AC component of the current
coming through the diode from the inductor. The ripple
on the output results when this AC current passes
through the finite impedance of the output capacitor. The
capacitor should have low impedance at the 1.4MHz
switching frequency of the LT1611/LT1613. At this fre-
quency, the impedance is usually dominated by the
capacitor’s equivalent series resistance (ESR). Choosing
a capacitor with lower ESR will result in lower output
ripple.
The DC233 uses a combination of two capacitors to
achieve these ends. The 15µF tantalum output capacitor
(C4, C9 or C15) provides the bulk capacitance for good
transient response. A 1µF ceramic capacitor (C5, C10 or
C16) in parallel with the tantalum capacitor provides a
low impedance bypass at the switching frequency. This
results in low output ripple and helps to maintain good
efficiency at high loads by eliminating AC losses in the
main output capacitor.
This combination output capacitor provides good perfor-
mance at low cost. Both capacitors are quite small.
However, low ESR and the required bulk output capaci-
tance can be obtained using a single larger output capaci-
tor. Larger tantalum capacitors, newer capacitor tech-
nologies (for example the POSCAP from Sanyo and
SPCAP from Panasonic) or large value ceramic capaci-
tors will reduce the output ripple. Note, however, that the
stability of the circuit depends on both the value of the
output capacitor and its ESR. When using low value
capacitors or capacitors with very low ESR, circuit stabil-
ity should be evaluated carefully, as described below.