LTC3729_15 Datasheet, PDF (15/30 Pages) Linear Technology – 550kHz, PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3729
APPLICATIONS INFORMATION
This makes it advisable to further derate the capacitor, or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design. Always consult
the capacitor manufacturer if there is any question.
The graph shows that the peak RMS input current is
reduced linearly, inversely proportional to the number, N
of stages used. It is important to note that the efficiency
loss is proportional to the input RMS current squared
and therefore a 2-stage implementation results in 75%
less power loss when compared to a single phase design.
Battery/input protection fuse resistance (if used), PC board
trace and connector resistance losses are also reduced by
the reduction of the input ripple current in a PolyPhase
system. The required amount of input capacitance is further
reduced by the factor, N, due to the effective increase in
the frequency of the current pulses.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically once the ESR require‑
ment has been met, the RMS current rating generally far
exceeds the I_RIPPLE(P-P) requirements. 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 is the
number of phases, C_OUT = output capacitance, and
∆I_RIPPLE = combined inductor ripple currents.
The output ripple varies with input voltage since ∆I_L is a
function of input voltage. The output ripple will be less than
50mV at max V_IN with ∆I_L = 0.4I_OUT(MAX)/N assuming:
C_OUT required ESR < 2N(R_SENSE) and
C_OUT > 1/(8Nf)(R_SENSE)
The emergence of very low ESR capacitors in small,
surface mount packages makes very physically small
implementations possible. The ability to externally
compensate the switching regulator loop using the
I_TH pin(OPTI-LOOP compensation) allows a much
wider selection of output capacitor types. OPTI-LOOP
compensation effectively removes constraints on output
capacitor ESR. The impedance characteristics of each
capacitor type are significantly different than an ideal
capacitor and therefore require 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 the lowest (ESR)(size) product
of any aluminum electrolytic at a somewhat higher price.
An additional ceramic capacitor in parallel with OS-CON
type capacitors is recommended to reduce the inductance
effects.
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 or the KEMET
T510 series of surface mount tantalums, available in case
heights ranging from 2mm to 4mm. Other capacitor types
include Sanyo OS-CON, Nichicon PL series and Sprague
595D series. Consult the manufacturer for other specific
recommendations. A combination of capacitors will often
result in maximizing performance and minimizing overall
cost and size.
INTV_CC Regulator
An internal P-channel low dropout regulator produces
5V at the INTV_CC pin from the V_IN supply pin. The INTV_CC
regulator powers the drivers and internal circuitry of the
LTC3729. The INTV_CC pin regulator can supply up to
50mA peak and must be bypassed to power ground with
a minimum of 4.7µF tantalum or electrolytic capacitor. An
additional 1µF ceramic capacitor placed very close to the IC
is recommended due to the extremely high instantaneous
currents required by the MOSFET gate drivers.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the
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