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LTM4700
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 LTM4700 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 LTM4700 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 of each channel. 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 capaci-
tance. 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 capacitor.
50
and V
step-
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
For more information
OUTPUT CAPACITORS
The LTM4700 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
output filtering may be required by the system designer,
,
SW
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 0% to 25% 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 opera-
tion reduces effective output ripple as a function of the
number of phases.
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
for a bode plot analyzer to inject a signal into the control
loop and validate the regulator stability. The LTM4700's
stability compensation can be adjusted using two external
capacitors, and the MFR_PWM_COMP commands.
LIGHT LOAD CURRENT OPERATION
The LTM4700 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 operation,
the inductor current is not allowed to reverse. The reverse
www.analog.com
are chosen with low enough
OUT
can be a low
OUT
Application Note 77
discusses this
+
to the V
pin to allow
OUTn
OSNSn
Rev. B
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