LTC1628_15 Datasheet, PDF (16/32 Pages) Linear Technology – High Efficiency, 2-Phase Synchronous Step-Down Switching Regulators

LTC1628/LTC1628-PG
APPLICATIO S I FOR ATIO
Medium voltage (20V to 35V) ceramic, tantalum, OS-CON
and switcher-rated electrolytic capacitors can be used as
input capacitors, but each has drawbacks: ceramic voltage
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:
[ ^{( )}]^1/2
V_OUT V_IN − V_OUT
C_INRequiredI_RMS ≈ I_MAX
V_IN
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 LTC1628 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
16
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 guaran-
tees that the output capacitance does not significantly
discharge during the operating frequency period due to
ripple current. The choice of using smaller output capaci-
tance 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 compo-
nents can be optimized to provide stable, high perfor-
mance transient response regardless of the output capaci-
tors selected.
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