LTC3731
17
3731fc
APPLICATIONS INFORMATION
height requirements in the design. Always consult the
capacitor manufacturer if there is any question.
The Figure 6 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 3-stage implementation results in 90% less
power loss when compared to a single phase design. Bat-
tery/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 capacitance is further
reduced by the factor, N, due to the effective increase in
the frequency of the current pulses.
Ceramic capacitors are becoming very popular for small
designs but several cautions should be observed. “X7R”,
“X5R” and “Y5V” are examples of a few of the ceramic
materials used as the dielectric layer, and these different
dielectrics have very different effect on the capacitance
value due to the voltage and temperature conditions ap-
plied. Physically, if the capacitance value changes due to
applied voltage change, there is a concommitant piezo
effect which results in radiating sound! A load that draws
varying current at an audible rate may cause an attendant
varying input voltage on a ceramic capacitor, resulting in
an audible signal. A secondary issue relates to the energy
flowing back into a ceramic capacitor whose capacitance
value is being reduced by the increasing charge. The volt-
age can increase at a considerably higher rate than the
constant current being supplied because the capacitance
value is decreasing as the voltage is increasing! Never-
theless, ceramic capacitors, when properly selected and
used, can provide the lowest overall loss due to their
extremely low ESR.
The selection of C
OUT
is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The steady-state output ripple (∆V
OUT
) is determined by:
∆V
OUT
≈ ∆I
RIPPLE
ESR+
1
8NfC
OUT
where f = operating frequency of each stage, N is the
number of output stages, C
OUT
= output capacitance and
∆I
L
= ripple current in each inductor. The output ripple is
highest at maximum input voltage since ∆I
L
increases with
input voltage. The output ripple will be less than 50mV at
max V
IN
with ∆I
L
= 0.4I
OUT(MAX)
assuming:
C
OUT
required ESR < N • R
SENSE
and
C
OUT
> 1/(8Nf)(R
SENSE
)
The emergence of very low ESR capacitors in small, surface
mount packages makes very small physical implementa-
tions possible. The ability to externally compensate the
switching regulator loop using the I
TH
pin allows a much
wider selection of output capacitor types. The impedance
characteristics of each capacitor type is significantly differ-
ent than an ideal capacitor and therefore requires accurate
modeling or bench evaluation during design.
Manufacturers such as Nichicon, Nippon Chemi-Con 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 a good (ESR)(size) product.
Once the ESR requirement for C
OUT
has been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement. Ceramic capacitors from AVX, Taiyo Yuden,
Murata and Tokin offer high capacitance value and very
low ESR, especially applicable for low output voltage
applications.
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, the KEMET
T510 series of sur
face-mount tantalums or the Panasonic
SP series of surface mount special polymer capacitors
