Datasheet

LTM4649
13
4649f
For more information www.linear.com/LTM4649
APPLICATIONS INFORMATION
Input RMS Ripple Current Cancellation
Application Note 77 provides a detailed explanation of
multiphase operation. The input RMS ripple current can-
cellation mathematical derivations are presented, and a
graph is displayed representing the RMS ripple current
reduction as a function of the number of interleaved phases
(see Figure 4).
Minimum On-Time
Minimum on-time t
ON
is the smallest time duration that
the LTM4649 is capable of turning on the top MOSFET.
It is determined by internal timing delays, and the gate
charge required turning on the top MOSFET. Low duty
cycle applications may approach this minimum on-time
limit and care should be taken to ensure that:
V
OUT
V
IN
• FREQ
> t
ON(MIN)
If the duty cycle falls below what can be accommodated
by the minimum on-time, the controller will begin to skip
cycles. The output voltage will continue to be regulated, but
the output ripple and current will increase. The minimum
on-time can be increased by lowering the switching fre-
quency. A good rule of thumb is to use an 110ns on-time.
Soft-Start
The TRACK/SS pin of the master can be controlled by a
capacitor placed on the master regulator TRACK/SS pin
to ground. A 1.2µA current source will charge the TRACK/
SS pin up to the reference voltage and then proceed up
to INTV
CC
. After the 0.6V ramp, the TRACK/SS pin will no
longer be in control, and the internal voltage reference
will control output regulation from the feedback divider.
Foldback current limit is disabled during this sequence of
turn-on during tracking or soft-starting. The TRACK/SS
pins are pulled low when the RUN pin is below 1.2V. The
total soft-start time can be calculated as:
t
SS
=
C
SS
1.2µA






• 0.6
DUTY FACTOR (V
O
/V
IN
)
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
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
4649 F04
RMS INPUT RIPPLE CURRENT
DC LOAD CURRENT
1-PHASE
2-PHASE
3-PHASE
4-PHASE
6-PHASE
Figure 4. Input RMS Current Ratios to DC Load Current as a Function of Duty Cycle