LTM4601A View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4601A' PDF : 28 Pages View PDF
LTM4601A/LTM4601A-1
APPLICATIONS INFORMATION
Multiphase operation with multiple LTM4601A devices
in parallel will lower the effective output ripple current
due to the interleaving operation of the regulators. For
example, each LTM4601A’s inductor current of a 12V to
2.5V multiphase design can be read from the Inductor
Ripple Current vs Duty Cycle graph (Figure 3). The large
ripple current at low duty cycle and high output voltage
12
2.5V OUTPUT
10
5V OUTPUT
1.8V OUTPUT
8
1.5V OUTPUT
1.2V OUTPUT
6
3.3V OUTPUT WITH
130k ADDED FROM
4
VOUT TO fSET
5V OUTPUT WITH
100k ADDED FROM
2
fSET TO GND
0
0
20
40
60
80
DUTY CYCLE (VOUT/VIN)
4601A F03
Figure 3. Inductor Ripple Current vs Duty Cycle
can be reduced by adding an external resistor from fSET to
ground which increases the frequency. If the duty cycle is
DC = 2.5V/12V = 0.21, the inductor ripple current for 2.5V
output at 21% duty cycle is ~6A in Figure 3.
Figure 4 provides a ratio of peak-to-peak output ripple cur-
rent to the inductor current as a function of duty cycle and
the number of paralleled phases. Pick the corresponding
duty cycle and the number of phases to arrive at the correct
output ripple current ratio value. If a 2-phase operation is
chosen at a duty cycle of 21%, then 0.6 is the ratio. This
0.6 ratio of output ripple current to inductor ripple of 6A
equals 3.6A of effective output ripple current. Refer to Ap-
plication Note 77 for a detailed explanation of output ripple
current reduction as a function of paralleled phases.
The output voltage ripple has two components that are
related to the amount of bulk capacitance and effective
series resistance (ESR) of the output bulk capacitance.
Therefore, the output voltage ripple can be calculated with
the known effective output ripple current. The equation:
ΔVOUT(P-P) ≈ (ΔIL/(8 • f • m • COUT) + ESR • ΔIL), where f
1.00
0.95
1-PHASE
0.90
2-PHASE
3-PHASE
0.85
4-PHASE
6-PHASE
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
DUTY CYCLE (VO/VIN)
4601A F04
Figure 4. Normalized Output Ripple Current vs Duty Cycle, Dlr = VOT/LI, Dlr = Each Phase’s Inductor Current
12
4601afb
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