LTC3415 Datasheet, PDF (11/28 Pages) Linear Technology – 7A, PolyPhase Synchronous Step-Down Regulator

LTC3415
OPERATION
of phases used times the output voltage). The output ripple
amplitude is also reduced by the number of phases used.
Figure 3 graphically illustrates the principle.
The worst-case RMS ripple current for a single stage design
peaks at an input voltage of twice the output voltage. The
worst case RMS ripple current for a two stage design re-
sults in peak outputs of 1/4 and 3/4 of input voltage. When
the RMS current is calculated, higher effective duty factor
results and the peak current levels are divided as long as
the current in each stage is balanced. Refer to Application
Note 19 for a detailed description of how to calculate RMS
current for the single stage switching regulator. Figures 4 and
5 illustrate how the input and output currents are reduced
by using an additional phase. For a 2-phase converter,
the input current peaks drop in half and the frequency is
doubled. The input capacitor requirement is thus reduced
theoretically by a factor of four! Just imagine the possibility
of capacitor savings with even higher number of phases!
Output Current Sharing
When multiple LTC3415s are cascaded to drive a com-
mon load, accurate output current sharing is essential to
achieve optimal performance and efficiency. Otherwise,
if one stage is delivering more current than another, then
the temperature between the two stages will be different,
and that could translate into higher switch R_DS(ON), lower
efficiency, and higher RMS ripple. Each LTC3415 is trimmed
such that when the I_TH pins of multiple LTC3415s are tied
SW1 V
I_CIN
SINGLE PHASE
I_COUT
SW1 V
SW2 V
I_L1
I_L2
I_CIN
I_COUT
DUAL PHASE
3415 F03
RIPPLE
Figure 3. Single and 2-Phase Current Waveforms
together, the amount of output current delivered from each
LTC3415 is nearly the same.
Different ground potentials among LTC3415 stages, caused
by physical distances and ground noises, could cause an
offset to the absolute I_TH value seen by each stage. To
ensure that the ground level doesn’t affect the I_TH value,
the LTC3415 uses a differential driver that takes as input
not just the I_TH pin, but also the I_THM pin. The I_THM pins
of all the LTC3415 stages should be tied together and then
connected to the SGND at only one point.
1.0
0.9
0.8
1 PHASE
0.7
0.6
0.5
0.4
0.3
2 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)
3415 F04
Figure 4. Normalized Output Ripple Current vs Duty Factor
[I_RMSʺ 0.3 (DI_C(PP))]
0.6
1 PHASE
0.5
0.4
0.3
2 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)
3415 F05
Figure 5. Normalized RMS Input Ripple Current vs Duty Factor
for 1 and 2 Output Stages
3415fa
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