15
LTC3729
sn3729 3729fas
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to
meet size or height requirements in the design. Always
consult the capacitor manufacturer if there is any
question.
The graph shows that the peak RMS input current is
reduced linearly, inversely proportional to the number, N
of stages used. It is important to note that the efficiency
loss is proportional to the input RMS current
squared and
therefore a 2-stage implementation results in 75% less
power loss when compared to a single phase design.
Battery/input protection fuse resistance (if used), PC
board trace and connector resistance losses are also
reduced by the reduction of the input ripple current in a
PolyPhase system. The required amount of input capaci-
tance is further reduced by the factor, N, due to the
effective increase in the frequency of the current pulses.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment has been met, the RMS current rating generally far
exceeds the I
RIPPLE(P-P)
requirements. The steady state
output ripple (
∆
V
OUT
) is determined by:
∆
∆
V
I
ESR
NfC
OUT
RIPPLE
OUT
≈
+
1
8
Where f = operating frequency of each stage, N is the
number of phases, C
OUT
= output capacitance, and
∆
I
RIPPLE
= combined inductor ripple currents.
The output ripple varies with input voltage since
∆
I
L
is a
function of input voltage. The output ripple will be less than
50mV at max V
IN
with
∆
I
L
= 0.4I
OUT(MAX)
/N assuming:
C
OUT
required ESR < 2N(R
SENSE
) and
C
OUT
> 1/(8Nf)(R
SENSE
)
The emergence of very low ESR capacitors in small,
surface mount packages makes very physically small
implementations possible. The ability to externally
compensate the switching regulator loop using the I
TH
pin(OPTI-LOOP compensation) allows a much wider
selection of output capacitor types. OPTI-LOOP compen-
sation effectively removes constraints on output capacitor
ESR. The impedance characteristics of each capacitor
type are significantly different than an ideal capacitor and
therefore require accurate modeling or bench evaluation
during design.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo and the Panasonic SP
surface mount types have the lowest (ESR)(size) product
of any aluminum electrolytic at a somewhat higher price.
An additional ceramic capacitor in parallel with OS-CON
type capacitors is recommended to reduce the inductance
effects.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the ESR or RMS current
handling requirements of the application. Aluminum
electrolytic and dry tantalum capacitors are both available
in surface mount configurations. New special polymer
surface 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
choices are the AVX TPS, AVX TPSV or the KEMET T510
series of surface mount tantalums, available in case heights
ranging from 2mm to 4mm. Other capacitor types include
Sanyo OS-CON, Nichicon PL series and Sprague 595D
series. Consult the manufacturer for other specific recom-
mendations. A combination of capacitors will often result
in maximizing performance and minimizing overall cost
and size.
INTV
CC
Regulator
An internal P-channel low dropout regulator produces 5V
at the INTV
CC
pin from the V
IN
supply pin. The INTV
CC
regulator powers the drivers and internal circuitry of the
LTC3729. The INTV
CC
pin regulator can supply up to 50mA
peak and must be bypassed to power ground with a
minimum of 4.7
µ
F tantalum or electrolytic capacitor. An
additional 1
µ
F ceramic capacitor placed very close to the
IC is recommended due to the extremely high instanta-
neous currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
APPLICATIO S I FOR ATIO
W
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