LTC3829 Datasheet, PDF (25/40 Pages) Linear Technology – 3-Phase, Single Output Synchronous Step-Down DC/DC Controller with Diffamp

LTC3829
Applications Information
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
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,
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 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 LTC3829 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
error amplifier.
Active Voltage Positioning (AVP)
In an application, the AVP scheme modifies the regu-
lated output voltage depending its current loading. AVP
can improve overall transient response and save power
consumption.
The LTC3829 senses inductor current information through
monitoring voltage drops on the sense resistor R_SENSE or
DCR sensing network of all three channels. The voltage
drops are added together and applied as V_PRE-AVP between
the AVP and DIFFP pins, which are connected through
resistor R_{PRE-AVP }. Then V_PRE-AVP is scaled through R_AVP
and added to output voltage as the compensation for the
load voltage drop.
Let:
∆V = V_SENSE1+ – V_SENSE1–
∆V = V_SENSE2+ – V_SENSE2–
∆V = V_SENSE3+ – V_SENSE3–
then:
∆V_DIFFP,VOUT
=
3
•
∆V


R_AVP
R_PRE-AVP


3829f
25