16
LTC3732
3732f
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 I ESR
NfC
OUT RIPPLE
OUT
≈+
1
8
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 imple-
mentations 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 sig-
nificantly different than an ideal capacitor and therefore
requires 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 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 elec-
trolytic 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 avail-
able in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POS-CAP, Sanyo OS-CON,
Nichicon PL series and Sprague 595D series. Consult the
manufacturer for other specific recommendations.
R
SENSE
Selection for Output Current
Once the frequency and inductor have been chosen,
R
SENSE1,
R
SENSE2,
R
SENSE3
are determined based on the
required peak inductor current. The current comparator
has a maximum threshold of 75mV/R
SENSE
and an input
common mode range of SGND to (1.1) • V
CC
. The current
comparator threshold sets the peak inductor current,
yielding a maximum average output current I
MAX
equal to
the peak value less half the peak-to-peak ripple current,
∆I
L
.
Allowing a margin for variations in the IC and external
component values yields:
RN
mV
I
SENSE
MAX
=
50
The IC works well with values of R
SENSE
from 0.002Ω to
0.02Ω.
V
CC
Decoupling
The V
CC
pin supplies power not only to the internal circuits
of the controller but also to the top and bottom gate
drivers on the IC and therefore must be bypassed
very carefully to ground with a ceramic capacitor, type
X7R or X5R (depending upon the operating temperature
environment) of
at least 1
µ
F imme
diately next to the IC
and preferably an additional 10µF placed very close to
the IC due to the extremely high instantaneous currents
involved. The total capacitance, taking into account the
voltage coefficient of ceramic capacitors, should be
100 times as large as the total combined gate charge
capacitance of ALL of the MOSFETs being driven. Good
APPLICATIO S I FOR ATIO
WUUU
