LTC3727LX-1_15 Datasheet, PDF (15/28 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

LTC3727LX-1
APPLICATIO S I FOR ATIO
coefficients are very high and may have audible piezoelec-
tric effects; tantalums need to be surge-rated; OS-CONs
suffer from higher inductance, larger case size and limited
surface-mount applicability; electrolytics’ higher ESR and
dryout possibility require several to be used. Multiphase
systems allow the lowest amount of capacitance overall.
As little as one 22µF or two to three 10µF ceramic capaci-
tors are an ideal choice in a 20W to 35W power supply due
to their extremely low ESR. Even though the capacitance
at 20V is substantially below their rating at zero-bias, very
low ESR loss makes ceramics an ideal candidate for
highest efficiency battery operated systems. Also con-
sider parallel ceramic and high quality electrolytic capaci-
tors as an effective means of achieving ESR and bulk
capacitance goals.
In continuous mode, the source current of the top N-chan-
nel MOSFET is a square wave of duty cycle V_OUT/V_IN. To
prevent large voltage transients, a low ESR input capacitor
sized for the maximum RMS current of one channel must
be used. The maximum RMS capacitor current is given by:
( ) _{CIN Required IRMS ≅ IMAX} ⎡⎣V_OUT
V_IN − V_OUT
V_IN
⎤⎦ ^{1/ 2}
This formula has a maximum at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on only 2000 hours
of life. 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 manufacturer if there is any
question.
The benefit of the LTC3727LX-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
3727lx1fa
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