LTC3728 Datasheet, PDF (16/32 Pages) Linear Technology – Dual, 550kHz, 2-Phase Synchronous Step-Down Switching Regulator

LTC3728
APPLICATIO S I FOR ATIO
The benefit of the LTC3728 multiphase can be calculated
by using the equation above for the higher power control-
ler and then calculating the loss that would have resulted
if both controller channels switch on at the same time. The
total RMS power lost is lower when both controllers are
operating due to the interleaving of current pulses through
the input capacitor’s ESR. This is why the input capacitor’s
requirement calculated above for the worst-case control-
ler is adequate for the dual controller design. Remember
that input protection fuse resistance, battery resistance
and PC board trace resistance losses are also reduced due
to the reduced peak currents in a multiphase system. The
overall benefit of a multiphase design will only be fully
realized when the source impedance of the power supply/
battery is included in the efficiency testing. The drains of
the two top MOSFETS should be placed within 1cm of each
other and share a common C_IN(s). Separating the drains
and C_IN may produce undesirable voltage and current
resonances at V_IN.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically once the ESR require-
ment is satisfied the capacitance is adequate for filtering.
The output ripple (∆V_OUT) is determined by:
∆V_OUT
≈
⎛
∆I_{L⎝⎜}ESR +
1^⎞
8fC_OUT ⎠⎟
Where f = operating frequency, C_OUT = output capacitance,
and ∆I_L= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ∆I_L increases
with input voltage. With ∆I_L = 0.3I_OUT(MAX) the output
ripple will typically be less than 50mV at max V_IN assum-
ing:
C_OUT Recommended ESR < 2 R_SENSE
and C_OUT > 1/(8fR_SENSE)
The first condition relates to the ripple current into the
ESR of the output capacitance while the second term
guarantees that the output capacitance does not signifi-
cantly discharge during the operating frequency period
due to ripple current. The choice of using smaller output
capacitance increases the ripple voltage due to the dis-
charging term but can be compensated for by using
capacitors of very low ESR to maintain the ripple voltage
at or below 50mV. The I_TH pin OPTI-LOOP compensation
components can be optimized to provide stable, high
performance transient response regardless of the output
capacitors selected.
Manufacturers such as Nichicon, United Chemicon and
Sanyo can be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest (ESR)(size)
product of any aluminum electrolytic at a somewhat
higher price. An additional ceramic capacitor in parallel
with OS-CON capacitors is recommended to reduce the
inductance effects.
In surface mount applications multiple capacitors may
need to be used in parallel to meet the ESR, RMS current
handling and load step requirements of the application.
Aluminum electrolytic, dry tantalum and special polymer
capacitors are available in surface mount packages. Spe-
cial polymer surface mount capacitors offer very low ESR
but have lower storage capacity per unit volume than other
capacitor types. These capacitors offer a very cost-effec-
tive output capacitor solution and are an ideal choice when
combined with a controller having high loop bandwidth.
Tantalum capacitors offer the highest capacitance density
and are often used as output capacitors for switching
regulators having controlled soft-start. Several excellent
surge-tested 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. Aluminum
electrolytic capacitors can be used in cost-driven applica-
tions providing that consideration is given to ripple current
ratings, temperature and long term reliability. A typical
application will require several to many aluminum electro-
lytic capacitors in parallel. A combination of the above
mentioned capacitors will often result in maximizing per-
formance and minimizing overall cost. Other capacitor
types include Nichicon PL series, NEC Neocap, Cornell
Dubilier ESRE and Sprague 595D series. Consult manu-
facturers for other specific recommendations.
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