LTC3729L-6 Datasheet, PDF (15/28 Pages) Linear Technology – PolyPhase, Synchronous Step-Down Switching Regulator

LTC3729L-6
APPLICATIO S I FOR ATIO
0.6
0.5
1-PHASE
0.4
2-PHASE
3-PHASE
0.3
4-PHASE
6-PHASE
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)
3729L-6 F04
Figure 4. Normalized Input RMS Ripple Current vs
Duty Factor for 1 to 6 Output Stages
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 ques-
tion.
The graph shows that the peak RMS input current is
reduced linearly, inversely proportional to the number, N
of stages used. It is important to note that the efficiency
loss is proportional to the input RMS current squared and
therefore a 2-stage 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
PolyPhase system. The required amount of input capaci-
tance is further reduced by the factor, N, 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
8NfC_OUT


Where f = operating frequency of each stage, N is the
number of phases, 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)/N assuming:
C_OUT required ESR < 2N(R_SENSE) and
C_OUT > 1/(8Nf)(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 selection of
output capacitor types. OPTI-LOOP compensation effec-
tively removes constraints on output capacitor ESR. The
impedance characteristics of each capacitor type are sig-
nificantly 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, Nichicon PL series and Sprague 595D
series. Consult the manufacturer for other specific recom-
mendations. A combination of capacitors will often result
in maximizing performance and minimizing overall cost
and size.
sn3729l6 3729l6fs
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