LTC3802 Datasheet, PDF (24/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used as
input capacitors, but each has drawbacks: ceramics have
high voltage coefficients of capacitance and may have
audible piezoelectric effects; tantalums need to be surge-
rated; OS-CONs suffer from higher inductance, larger case
size and limited surface mount applicability; and
electrolytics’ higher ESR and dryout possibility require
several to be used. Sanyo OS-CON SVP, SVPD series;
Sanyo POSCAP TQC series or aluminum electrolytic ca-
pacitors from Panasonic WA series or Cornel Dublilier
SPV series, in parallel with a couple of high performance
ceramic capacitors, can be used as an effective means of
achieving low ESR and its big bulk capacitance goal for the
input bypass.
C_OUT Selection
The selection of C_OUT is primarily determined by the ESR
required to minimize voltage ripple and load step tran-
sients. The output ripple ∆V_OUT is approximately bounded
by:
∆V_OUT
≤
∆I_L
_ESR
+
8
•
1
f_SW •
C_OUT


where ∆I_L is the inductor ripple current.
∆I_L may be calculated using the equation:
∆I_L
=
V_OUT
L • f_SW
_1–
V_OUT
V_IN


Since ∆I_L increases with input voltage, the output ripple
voltage is highest at maximum input voltage. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering and has the necessary RMS current
rating.
Manufacturers such as Sanyo, Panasonic and Cornell
Dublilier should be considered for high performance
through-hole capacitors. The OS-CON semiconductor elec-
trolyte capacitor available from Sanyo has a good
(ESR)(size) product. An additional ceramic capacitor in
parallel with OS-CON capacitors is recommended to offset
the effect of lead inductance.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or transient current
handling requirements of the application. Aluminum elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer sur-
face mount capacitors offer very low ESR also but have
much lower capacitive density per unit volume. In the case
of tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. Several excellent
output capacitor choices are the Sanyo POSCAP TPD,
POSCAP TPB, AVX TPS, AVX TPSV, the Kemet T510 series
of surface mount tantalums,Kemet AO-CAPs or the Pana-
sonic SP series of surface mount special polymer capaci-
tors available in case heights ranging from 2mm to 4mm.
Other capacitor types include Nichicon PL series and
Sprague 595D series. Consult the manufacturer for other
specific recommendations.
Inductor Selection
The inductor in a typical LTC3802 circuit is chosen prima-
rily for inductance value and saturation current. The induc-
tor should not saturate below the hard current limit
threshold.
The inductor value sets the ripple current, which is
commonly chosen at around 40% of the anticipated full
load current. Lower ripple current reduces core losses in
the inductor, ESR losses in the output capacitors and
output voltage ripple. Highest efficiency is obtained at low
frequency with small ripple current. However, achieving
high efficiency requires a large inductor and generates
higher output voltage excursion during load transients.
There is a tradeoff between component size, efficiency
and operating frequency. Given a specified limit for ripple
current, the inductor value can be obtained using the
following equation:
L
=
f_SW
V_OUT
• ∆I_L(MAX)
•

^1–
V_OUT ^
V_{IN(MAX) }
Once the value for L is known, the type of inductor must be
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
3802f
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