Datasheet

LTM4649
11
4649f
For more information www.linear.com/LTM4649
APPLICATIONS INFORMATION
required. Table 3 shows a matrix of different output voltages
and output capacitors to minimize the voltage droop and
overshoot during a 5A/µs transient. The table optimizes
total equivalent ESR and total bulk capacitance to optimize
the transient performance. Stability criteria are considered
in the Table 3 matrix, and the Linear Technology µModule
Power Design Tool will be provided for stability analysis.
Multiphase operation will reduce effective output ripple
as a function of the number of phases. Application Note
77 discusses this noise reduction versus output ripple
current cancellation, but the output capacitance should be
considered carefully as a function of stability and transient
response. The Linear Technology µModule Power Design
Tool can calculate the output ripple reduction as the number
of implemented phase’s increases by N times.
Burst Mode Operation
The LTM4649 is capable of Burst Mode operation in which
the power MOSFETs operate intermittently based on load
demand, thus saving quiescent current. For applications
where maximizing the efficiency at very light loads is a
high priority, Burst Mode operation should be applied. To
enable Burst Mode operation, simply tie the MODE pin to
INTV
CC
. During Burst Mode operation, the peak current
of the inductor is set to approximately 30% of the maxi-
mum peak current value in normal operation even though
the voltage at the COMP pin indicates a lower value. The
voltage at the COMP pin drops when the inductor’s aver-
age current is greater than the load requirement. As the
COMP voltage drops below 0.5V, the burst comparator
trips, causing the internal sleep line to go high and turn
off both power MOSFETs.
In sleep mode, the internal circuitry is partially turned
off, reducing the quiescent current. The load current is
now being supplied from the output capacitors. When the
output voltage drops, causing COMP to rise, the internal
sleep line goes low, and the LTM4649 resumes normal
operation. The next oscillator cycle will turn on the top
power MOSFET and the switching cycle repeats.
Pulse-Skipping Mode Operation
In applications where low output ripple and high efficiency
at intermediate currents are desired, pulse-skipping mode
should be used. Pulse-skipping operation allows the
LTM4649 to skip cycles at low output loads, thus increasing
efficiency by reducing switching loss. Floating the MODE
pin enables pulse-skipping operation. With pulse-skipping
mode at light load, the internal current comparator may
remain tripped for several cycles, thus skipping opera-
tion cycles. This mode has lower ripple than Burst Mode
operation and maintains a higher frequency operation than
Burst Mode operation.
Forced Continuous Operation
In applications where fixed frequency operation is more
critical than low current efficiency, and where the lowest
output ripple is desired, forced continuous operation
should be used. Forced continuous operation can be
enabled by tying the MODE pin to ground. In this mode,
inductor current is allowed to reverse during low output
loads, the COMP voltage is in control of the current
comparator threshold throughout, and the top MOSFET
always turns on with each oscillator pulse. During start-up,
forced continuous mode is disabled and inductor current
is prevented from reversing until the LTM4649’s output
voltage is in regulation.
Frequency Selection
The LTM4649 device is internally programmed to 450kHz
switching frequency to improve power conversion effi-
ciency. It is recommended for all of the application of low
V
IN
or low V
OUT
. For the application with high V
IN
(V
IN
>
= 12V) and high V
OUT
(V
OUT
> = 1.8V), a higher 750kHz
frequency is recommended to limit inductor ripple cur-
rent by simply tie FREQ to INTV
CC
. Table 3 listed different
frequency and FREQ pin recommendations for different
V
IN
, V
OUT
conditions.
If desired, a resistor can be connected from the FREQ pin
to INTV
CC
to adjust the FREQ pin DC voltage to increase
the switching frequency between default 450kHz and
maximum 750kHz by. Figure 2 shows a graph of frequency
setting verses FREQ pin DC voltage. Figure 18 shows an
example of frequency programmed to 650kHz. Please be
aware FREQ pin has an accurate 10µA current sourced
from this pin when calculate the resistor value.