LTC3856 Datasheet, PDF (24/40 Pages) Linear Technology – 2-Phase Synchronous Step-Down DC/DC Controller with Diffamp

LTC3856
Applications Information
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. 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 applied. Physically,
if the capacitance value changes due to applied voltage
change, there is a concomitant 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 voltage can increase
at a considerably higher rate than the constant current being
supplied because the capacitance value is decreasing as
the voltage is increasing! Nevertheless, 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 requirement
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 = 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
maximum V_IN with ∆I_L = 0.4I_OUT(MAX) assuming:
C_OUT required ESR < N • R_SENSE
and
( ) ^COUT
>
(8Nf)
1
R_SENSE
The emergence of very low ESR capacitors in small, surface
mount packages makes very small physical implementa-
24
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
characteristic 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 capacitors 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
and Murata 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 surface mount tantalums or the Panasonic
SP series of surface mount special polymer capacitors
available in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo POSCAP, Sanyo OS-CON,
Nichicon PL series and Sprague 595D series. Consult the
manufacturers for other specific recommendations.
Differential Amplifier
The LTC3856 has a true remote voltage sense capability.
The sensing connections should be returned from the
load, back to the differential amplifier’s inputs through a
common, tightly coupled pair of PC traces. The differential
amplifier rejects common mode signals capacitively or
inductively radiated into the feedback PC traces as well as
ground loop disturbances. The differential amplifier output
signal is divided by a pair of resistors and is compared
with the internal, precision 0.6V voltage reference by the
3856f