51
Rev. B
APPLICATIONS INFORMATION
V
IN
TO V
OUT
STEP-DOWN RATIOS
There are restrictions in the maximum V
IN
and V
OUT
step-
down ratio that can be achieved for a given input voltage.
Each output of the LTM4680 is capable of 95% duty cycle
at 500kHz, but the V
IN
to V
OUT
minimum dropout is still
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)
is another consideration in
operating at a specified duty cycle while operating at a
certain frequency due to the fact that t
ON(MIN)
< D/f
SW
,
where D is duty cycle and f
SW
is the switching frequency.
t
ON(MIN)
is specified in the electrical parameters as 60ns.
See Note 6 in the Electrical Characteristics section for
output current guideline.
INPUT CAPACITORS
The LTM4680 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:
D
n
=
V
OUT
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
CINn(RMS)
=
I
OUT
n(MAX)
η
%
• D
n
• 1
−
D
n
(
)
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 capacitor.
OUTPUT CAPACITORS
The LTM4680 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, 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
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 15A to 30A step,
15A/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 opera-
tion reduces effective output ripple as a function of the
number of phases. Application Note 77 discusses this
noise reduction versus output ripple current cancella-
tion, but the output capacitance should be considered
carefully as a function 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
OUT
n
to the V
OSNS0
+
pin to allow
for a bode plot analyzer to inject a signal into the control
loop and validate the regulator stability. The LTM4680’s
stability compensation can be adjusted using two external
capacitors, and the MFR_PWM_COMP commands.
LIGHT LOAD CURRENT OPERATION
The LTM4680 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_MODE
n
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
