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LTM4648EY#PBF

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LTM4648
10
4648f
For more information www.linear.com/LTM4648
applicaTions inForMaTion
The typical LTM4648 application circuit is shown in
Figure18. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 3 for specific external capacitor
requirements for particular applications.
V
IN
to V
OUT
Step-Down Ratios
There are restrictions in the V
IN
to V
OUT
step-down ratio
that can be achieved for a given input voltage. The V
IN
to
V
OUT
minimum dropout is a function of load current and
at very low input voltage and high duty cycle applications
output power may be limited as the internal top power
MOSFET is not rated for 10A operation at higher ambient
temperatures. At very low duty cycles the minimum 90ns
on-time must be maintained. See the Frequency Adjust
-
ment section and temperature derating curves.
Output Voltage Programming
The PWM controller has an internal 0.6V ±1% reference
voltage. As shown in the Block Diagram, a 10k 0.5%
internal feedback resistor connects the V
OUT_LCL
and
V
FB
pins together. When the remote sense amplifier is
used, then DIFFOUT is connected to the V
OUT_LCL
pin.
If the remote sense amplifier is not used, then V
OUT_LCL
connects to V
OUT
. The output voltage will default to 0.6V
with no feedback resistor. Adding a resistor R
FB
from V
FB
to ground programs the output voltage:
V
OUT
= 0.6V •
10k
+
R
FB
R
FB
Table 1. V
FB
Resistor Table vs Various Output Voltages
V
OUT
(V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
R
FB
(k) OPEN 15 10 6.65 4.99 3.09 2.21 1.37
For parallel operation of N LTM4648, the following equa-
tion can be used to solve for R
FB
:
R
FB
=
10k
N
V
OUT
0.6
–1
In parallel operation the V
FB
pins have an I
FB
current of
20nA maximum each channel. To reduce output voltage
error due to this current, an additional V
OUT_LCL
pin can
be tied to V
OUT
, and an additional R
FB
resistor can be used
to lower the total Thevenin equivalent resistance seen by
this current.
Input Capacitors
The LTM4648 module should be connected to a low AC
impedance DC source. Additional input capacitors are
needed for the RMS input ripple current rating. The I
CIN(RMS)
equation which follows can be used to calculate the input
capacitor requirement. Typically 22µF X7R ceramics are a
good choice with RMS ripple current ratings of ~2A each.
A 47µF to 100µF surface mount aluminum electrolytic bulk
capacitor can be used for more input bulk capacitance.
This bulk input capacitor is only needed if the input source
impedance is compromised by long inductive leads, traces
or not enough source capacitance. If low impedance power
planes are used, then this bulk capacitor is not needed.
For a buck converter, the switching duty cycle can be
estimated as:
D =
V
OUT
V
IN
Without considering the inductor ripple current, for each
output, the RMS current of the input capacitor can be
estimated as:
I
CIN(RMS)
=
I
OUT(MAX)
η%
• D • 1− D
( )
In the previous equation, η% is the estimated efficiency of
the power module. The bulk capacitor can be a switcher-
rated electrolytic aluminum capacitor or a Polymer
capacitor.
Output Capacitors
The LTM4648 is designed for low output voltage ripple
noise. 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 require
-
ments. C
OUT
can be a low ESR tantalum capacitor, low
ESR Polymer capacitor or ceramic capacitors. The typical
output capacitance range is from 200µF to 470µF. Additional
LTM4648
11
4648f
For more information www.linear.com/LTM4648
applicaTions inForMaTion
output filtering may be required by the system designer
if further reduction of output ripple or dynamic transient
spikes is 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. Stabil
-
ity 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 num
-
ber 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 LTM4648 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 LTM4648 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
LTM4648 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 LTM4648’s output
voltage is in regulation.
Frequency Selection
The LTM4648 device is internally programmed to 450kHz
switching frequency to improve power conversion ef
-
ficiency. It is recommended for all of the application.
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 650kHz. 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.
LTM4648
12
4648f
For more information www.linear.com/LTM4648
applicaTions inForMaTion
PLL and Frequency Synchronization
The LTM4648 device operates over a range of frequen-
cies to improve power conversion efficiency. The nominal
switching frequency is
450kHz
. It can also be synchronized
from 350kHz to 650kHz with an input clock that has a high
level above 2V and a low level below 0.8V at the CLKIN pin.
Once the LTM4648 is synchronizing to an external clock
frequency, it will always running in Forced Continuous
Operation. The 350kHz low end operation frequency limit
is put in place to limit inductor ripple current.
Multiphase Operation
For outputs that demand more than 10A of load current,
multiple LTM4648 devices can be paralleled to provide
more output current and reduced input and output volt
-
age ripple.
The CLKOUT signal together with CLKIN pin can be used
to cascade additional power stages to achieve the multi
-
phase power supply solution. Tying the PHMODE pin to
INTV
CC
, GND, or (floating) generates a phase difference
(between MODE/PLLIN and CLKOUT) of 180°, 120°, or
90° respectively as shown in Table 2. A total of 4 phases
can be cascaded to run simultaneously with respect to
each other by programming the PHMODE pin of each
LTM4648 channel to different levels. Figure 3 shows a
3-phase design and 4-phase design example for clock
phasing with the PHASMD table.
Table 2. PHASEMD and CLKOUT Signal Relationship
PHASEMD GND FLOAT INTV
CC
CLKOUT 120° 90° 180°
The LTM4648 device is an inherently current mode con-
trolled device, so parallel modules will have good current
sharing.
This will balance the thermals in the design. Tie
the COMP, V
FB
, TRACK/SS and RUN pins of each LTM4648
together to share the current evenly. Figures 20 and 21
show a schematic of the parallel design.
4648 F03
4-PHASE DESIGN
3-PHASE DESIGN
90 DEGREE90 DEGREE 90 DEGREE
0 PHASE
FLOAT
FLOAT FLOAT FLOAT
CLKIN
V
OUT
PHASMD
CLKOUT
90 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
180 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
270 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
120 DEGREE
120 DEGREE
0 PHASE
GND GND GND
CLKIN
V
OUT
PHASMD
CLKOUT
120 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
240 PHASE
CLKIN
V
OUT
PHASMD
CLKOUT
Figure 3. Examples of 3-Phase, 4-Phase Operation with PHASMD Table
FREQ PIN VOLTAGE (V)
0
FREQUENCY (kHz)
900
800
600
400
100
200
700
500
300
0
2
4648 F02
2.51 1.50.5
Figure 2. Operating Frequency vs FREQ Pin Voltage
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LTM4648EY#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Low VIN, 10A Step-Down DC/DC Module Regulator
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