ltc3730 Linear Technology Corporation, ltc3730 Datasheet - Page 16

• Manufacturer Part Number: ltc3730
• Description: 3-phase, 5-bit Intel Mobile Vid, 600khz, Synchronous Buck Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3730
and
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
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, 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
RMS current rating generally far exceeds the I
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 sur-
face mount special polymer capacitors available 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 manufacturers for
other specific recommendations.
R
Once the frequency and inductor have been chosen,
R
required peak inductor current. The current comparator
16
C
C
SENSE
SENSE1,
OUT
OUT
required ESR < N • R
> 1/(8Nf)(R
Selection for Output Current
R
SENSE2,
SENSE
R
U
SENSE3
)
U
are determined based on the
SENSE
OUT
W
has been met, the
TH
pin allows a
RIPPLE(P-P)
U
has a typical maximum threshold of 75mV/R
input common mode range of SGND to (1.1) • V
current comparator threshold sets the peak inductor cur-
rent, yielding a maximum average output current I
equal to the peak value less half the peak-to-peak ripple
current, ∆I
Allowing a margin for variations in the IC and external
component values yields:
The IC works well with values of R
0.02Ω.
V
The V
of the controller but also 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 environ-
ment) 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 in-
volved. The total capacitance, taking into account the
voltage coefficient of ceramic capacitors, should be 100
times as large as the total combined gate charge capaci-
tance of ALL of the MOSFETs being driven. Good bypass-
ing close to the IC is necessary to supply the high transient
currents required by the MOSFET gate drivers while keep-
ing the 5V supply quiet enough so as not to disturb the very
small-signal high bandwidth of the current comparators.
Topside MOSFET Driver Supply (C
External bootstrap capacitors, C
pins, supply the gate drive voltages for the topside
MOSFETs. Capacitor C
charged though diode D
low. When one of the topside MOSFETs turns on, the
driver places the C
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V
on, the boost voltage is above the input supply (V
V
CC
IN
CC
R
and the BOOST pin follows. With the topside MOSFET
+ V
Decoupling
SENSE
CC
IN
pin supplies power not only the internal circuits
). The value of the boost capacitor C
L
=
.
N
50
I
MAX
mV
B
voltage across the gate-source of the
B
B
in the Functional Diagram is
from V
B
, connected to the BOOST
CC
SENSE
B
, D
when the SW pin is
B
)
from 0.002Ω to
B
SENSE
needs to be
BOOST
CC
and an
. The
3730fa
MAX
=