LTC3730_15 Datasheet, PDF (16/28 Pages) Linear Technology – 3-Phase, 5-Bit Intel Mobile VID, 600kHz, Synchronous Buck Controller

LTC3730
APPLICATIO S I FOR ATIO
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, 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 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_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
16
has a typical 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 cur-
rent, 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:
R_SENSE
=
N
50mV
I_MAX
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 the internal circuits
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 1µF immediately 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_B, D_B)
External bootstrap capacitors, C_B, connected to the BOOST
pins, supply the gate drive voltages for the topside
MOSFETs. Capacitor C_B in the Functional Diagram is
charged though diode D_B from V_CC when the SW pin is
low. When one of the topside MOSFETs turns on, the
driver places the C_B voltage across the gate-source of the
desired MOSFET. This enhances the MOSFET and turns on
the topside switch. The switch node voltage, SW, rises to
V_IN and the BOOST pin follows. With the topside MOSFET
on, the boost voltage is above the input supply (V_BOOST =
V_CC + V_IN). The value of the boost capacitor C_B needs to be
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