Linear Technology EN55022B, Руководство пользователя

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LTM4612
14
4612fc
For more information www.linear.com/LTM4612
APPLICATIONS INFORMATION
Output Capacitors
The LTM4612 is designed for low output voltage ripple.
The bulk output capacitors defined as C
OUT
are chosen
with low enough effective series resistance (ESR) to meet
the output voltage ripple and transient requirements. C
OUT
can be low ESR tantalum capacitor, low ESR polymer
capacitor or ceramic capacitor. The typical capacitance is
150µF if all ceramic output capacitors are used. Additional
output filtering may be required by the system designer,
if further reduction of output ripple or dynamic transient
spike is required. Table 2 shows a matrix of different output
voltages and output capacitors to minimize the voltage
droop and overshoot during a 2A/µs transient. The table
optimizes total equivalent ESR and total bulk capacitance
to maximize transient performance.
Multiphase operation with multiple LTM4612 devices in
parallel will also lower the effective output ripple current
due to the phase interleaving operation. Refer to Figure 5
for the normalized output ripple current versus the duty
cycle. Figure 5 provides a ratio of peak-to-peak output
ripple current to the inductor ripple current as functions
of duty cycle and the number of paralleled phases. Pick
the corresponding duty cycle and the number of phases
to get the correct output ripple current value. For example,
each phase’s inductor ripple current DIr at zero duty cycle
is ~4.3A for a 36V to 12V design. The duty cycle is about
0.33. The 2-phase curve has a ratio of ~0.33 for a duty
cycle of 0.33. This 0.33 ratio of output ripple current to
the inductor ripple current DIr at 4.3A equals 1.4A of the
output ripple current (
D
I
L
).
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.
The equation is:
D
V
OUT(P-P)
≈
D
I
L
8 • f • N • C
OUT
⎛
⎝
⎜
⎞
⎠
⎟ +
ESR •
D
I
L
where f is the frequency and N is the number of paralleled
phases.
DUTY CYCLE (V
O
/V
IN
)
0.1 0.15 0.2 0.25 0.35 0.3 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9
1.00
0.95
0.90
0.85
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
4612 F05
6-PHASE
4-PHASE
3-PHASE
2-PHASE
1-PHASE
PEAK-TO-PEAK OUTPUT RIPPLE CURRENT
DIr
RA
TIO =
Figure 5. Normalized Output Ripple Current vs Duty Cycle, Dlr = V
O
T/L
I