Datasheet

LTM4628
12
4628fe
For more information www.linear.com/LTM4628
APPLICATIONS INFORMATION
Input Capacitors
The LTM4628 module should be connected to a low ac-
impedance DC source. For the regulator input three 22µF,
or four 10µF input ceramic capacitors are used for RMS
ripple current. 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 in
-
ductive 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 current ripple, 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 above equation, η% is the estimated efficiency of the
power module. The bulk capacitor can be a switcher-rated
aluminum electrolytic capacitor or a Polymer capacitor.
Output Capacitors
The LTM4628 is designed for low output voltage ripple
noise and good transient response. The bulk output
capacitors defined as C
OUT
are chosen with low enough
effective series resistance (ESR) to meet the output volt-
age ripple
and transient requirements. C
OUT
can be a low
ESR tantalum capacitor, the low ESR polymer capacitor
or ceramic capacitor. The typical output capacitance range
for each output is from 200µF to 470µF. Additional output
filtering may be required by the system designer, if further
reduction of output ripples or dynamic transient spikes
is required. Table 4 shows a matrix of different output
voltages and output capacitors to minimize the voltage
droop and overshoot during a 4A/µs transient. The table
optimizes total equivalent ESR and total bulk capacitance
to optimize the transient performance. Stability criteria
are considered in the Table 4 matrix, and LTpowerCAD™
is available 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. LTpowerCAD
can calculate the output ripple reduction as the number of
implemented phases increases by N times. A small value
10Ω to 50Ω resistor can be placed in series from V
OUT
to the V
OUTS
pin to allow for a bode plot analyzer to inject
a signal into the control loop and validate the regulator
stability. The same resistor could be placed in series from
V
OUT
to DIFFP and a bode plot analyzer could inject a signal
into the control loop and validate the regulator stability.
Burst Mode Operation
The LTM4628 is capable of Burst Mode operation on
each regulator 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. Burst Mode operation is enabled with
the MODE_PLLIN pin floating. During this operation, the
peak current of the inductor is set to approximately one
third of the maximum peak current value in normal opera
-
tion even though the voltage at the COMP pin indicates
a lower value. The voltage at the COMP pin drops when
the
inductor’s average 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 to about 450µA for each
output. The load current is now being supplied from the
output capacitors. When the output voltage drops, caus
-
ing COMP to rise above 0.5V, the internal sleep line goes
low, and the LTM4628 resumes normal operation. The
next oscillator cycle will turn on the top power MOSFET
and the switching cycle repeats. Either regulator can be
configured for Burst Mode operation.
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