LTC3716 Datasheet, PDF (15/28 Pages) Linear Technology – 2-Phase, 5-Bit VID, Current Mode, High Efficiency, Synchronous Step-Down Switching Regulator

LTC3716
APPLICATIO S I FOR ATIO
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 capacitor manufacturer if there is any
question.
0.6
0.5
0.4
1-PHASE
2-PHASE
0.3
0.2
0.1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY FACTOR (V_OUT/V_IN)
3716 F04
Figure 4. Normalized RMS Input Ripple Current
vs Duty Factor for 1 and 2 Output Stages
It is important to note that the efficiency loss is propor-
tional to the input RMS current squared and therefore a
2-phase 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 2-phase system.
The required amount of input capacitance is further
reduced by the factor, 2, 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
16fC_OUT


Where f = operating frequency of each stage, 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)/2 assuming:
C_OUT required ESR < 4(R_SENSE) and
C_OUT > 1/(16f)(R_SENSE)
The emergence of very low ESR capacitors in small,
surface mount packages makes very physically small
implementations possible. The ability to externally com-
pensate the switching regulator loop using the I_TH
pin(OPTI-LOOP compensation) allows a much wider se-
lection of output capacitor types. OPTI-LOOP compensa-
tion 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 elec-
trolytic and dry tantalum capacitors are both available in
surface mount configurations. New special polymer sur-
face 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, POSCAPs, Panasonic SP caps, Nichicon
PL series and Sprague 595D series. Consult the manufac-
turer for other specific recommendations. A combination
of capacitors will often result in maximizing performance
and minimizing overall cost and size.
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