LTC3727A-1 Datasheet, PDF (16/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

LTC3727A-1
APPLICATIO S I FOR ATIO
The benefit of the LTC3727A-1 multiphase can be calcu-
lated by using the equation above for the higher power
controller 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 controller 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 imped-
ance of the power supply/battery is included in the effi-
ciency 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 unde-
sirable 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 capaci-
tance, 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
assuming:
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
discharging 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, Nippon Chemi-Con 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 TPS Series III
or the KEMET T510 series of surface mount tantalums,
available in case heights ranging from 1.2mm to 4.1mm.
Aluminum electrolytic capacitors can be used in cost-
driven applications providing that consideration is given
to ripple current ratings, temperature and long term reli-
ability. A typical application will require several to many
aluminum electrolytic capacitors in parallel. A combina-
tion of the above mentioned capacitors will often result in
maximizing performance and minimizing overall cost. Other
capacitor types include Nichicon PL series, NEC Neocap,
Cornell Dubilier ESRE and Sprague 595D series. Consult
manufacturers for other specific recommendations.
3727a1f
16