Applications Information; Input Capacitors; Output Capacitors; Light Load Current Operation - Analog Devices Linear ADI Power LTM4681 Manual

LTM4681

APPLICATIONS INFORMATION

V TO V STEP-DOWN RATIOS
IN OUT
There are restrictions in the maximum V
down ratio that can be achieved for a given input voltage.
Each output of the LTM4681 is capable of 95% duty cycle
at 500kHz, but the V to V
IN
a function of its load current and will limit output current
capability related to high duty cycle on the topside switch.
Minimum on-time t
ON(MIN)
operating at a specified duty cycle while operating at a
certain frequency due to the fact that t
where D is duty cycle and f
t is specified in the electrical parameters as 60ns.
ON(MIN)
See Note 6 in the Electrical Characteristics section for
output current guideline.

INPUT CAPACITORS

The LTM4681 module should be connected to a low AC
impedance DC source. For the regulator input, four 22µF
input ceramic capacitors are used to handle the RMS
ripple current. A 47µF to 150µF 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:
V
OUT n
D =
n
V
IN n
Without considering the inductor current ripple, for each
output, the RMS current of the input capacitor can be
estimated as:
I
OUT n (MAX)
I =
CIN n (RMS)
η%
In the above equation, η% is the estimated efficiency of
the power module. The bulk capacitor can be a switcher-
rated electrolytic aluminum capacitor, or a polymer capac-
itor. Application Note 77
ripple current cancellation for multiphase applications.
56
and V
IN OUT
minimum dropout is still
OUT
is another consideration in
< D/f
ON(MIN)
is the switching frequency.
SW
( )
• D • 1− D
n n
can be utilized to help calculate
For more information

OUTPUT CAPACITORS

step-
The LTM4681 is designed for low output voltage ripple
noise and good transient response. The bulk output
capacitors defined as C
effective series resistance (ESR) to meet the output volt-
age ripple and transient requirements. C
ESR tantalum capacitor, a low ESR polymer capacitor or
ceramic capacitor. The typical output capacitance range
for each output is from 400µF to 1000µF. Additional out-
put filtering may be required by the system designer, if
,
SW
further reduction of output ripple or dynamic transient
spikes is required. Table 13 shows a matrix of different
output voltages and output capacitors to minimize the
voltage droop and overshoot during a 10A to 20A step,
10A/µs transient each channel. Table 13 optimizes total
equivalent ESR and total bulk capacitance to optimize the
transient performance. Stability criteria are considered in
the Table 13 matrix, and the LTPowerCAD Design Tool will
be provided for stability analysis. Multiphase operation
reduces effective output ripple as a function of the number
of phases. Application Note 77 discusses this noise reduc-
tion versus output ripple current cancellation, but the out-
put capacitance should be considered carefully as a func-
tion of stability and transient response. The LTPowerCAD
Design Tool can calculate the output ripple reduction as
the number of implemented phases increases by N times.
A small value 10Ω resistor can be placed in series from
V to the V
OUTn OSNS0
to inject a signal into the control loop and validate the
regulator stability. The LTM4681's stability compensation
can be adjusted using two external capacitors (COMPna ,
COMPnb), and the MFR_PWM_COMP commands.

LIGHT LOAD CURRENT OPERATION

The LTM4681 has two modes of operation including high
efficiency, discontinuous conduction mode or forced
continuous conduction mode. The mode of operation is
configured by bit 0 of the MFR_PWM_MODEn command
(discontinuous conduction is always the start-up mode,
forced continuous is the default running mode).
If a channel is enabled for discontinuous mode opera-
tion, the inductor current is not allowed to reverse. The
reverse current comparator, I
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are chosen with low enough
OUT
can be a low
OUT
+
pin to allow for a bode plot analyzer
, turns off the bottom
REV
Rev. A