Applications Information; Input Capacitors; Output Capacitors; Light Load Current Operation - Analog Devices LTM4683 Manual

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 LTM4683 is capable of a 95% duty
cycle at 500kHz, but the V
is still a function of its load current and will limit output
current capability related to the 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 since t
ON(MIN)
duty cycle and f is the switching frequency. t
SW
specified in the electrical parameters as 85ns. See Note 6
in the Electrical Characteristics section for output current
guideline.

INPUT CAPACITORS

The LTM4683 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 esti-
mated with Equation 2.
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 with Equation 3.
I
OUT n (MAX)
I =
CIN n (RMS)
η%
In Equation 3, η% is the estimated efficiency of the module.
The bulk capacitor can be a switcher-rated electrolytic alu-
minum capacitor or a polymer capacitor. Analog Devices,
and V step-
IN OUT
to V minimum dropout
IN OUT
is another consideration in
< D/f , where D is the
SW
ON(MIN)
( )
• D • 1− D
n n
For more information
Inc's Application Note 77
ripple current cancellation for multiphase applications.

OUTPUT CAPACITORS

The LTM4683 is designed for low output voltage ripple
noise and good transient response. The bulk output capaci-
tors defined as C
OUT
series resistance (ESR) to meet the output voltage ripple
and transient requirements. The C
tantalum capacitor, a low ESR polymer, or a ceramic capaci-
tor. The typical output capacitance range for each output
is from 400µF to 1000µF. Additional output filtering may
is
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 over-
shoot during a 10A to 20A step, with a 10A/µs transient
in each channel. Table 13 optimizes total equivalent ESR
and total bulk capacitance to optimize the transient per-
formance. Stability criteria are considered in the Table 13
matrix, and the LTpowerCAD design tool will be provided
for stability analysis. Multiphase operation reduces effec-
tive output ripple as a function of the number of phases.
Application Note 77
output ripple current cancellation, but the output capaci-
tance should be considered carefully as a function of sta-
bility 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
+
V pin to allow for a bode plot analyzer to inject a
OSNS0
signal into the control loop and validate the regulator stabil-
(2)
ity. The LTM4683's stability compensation can be adjusted
using two external capacitors (COMPna, COMPnb) and the
MFR_PWM_COMP commands.

LIGHT LOAD CURRENT OPERATION

The LTM4683 has two modes of operation, including
(3)
high efficiency, discontinuous-conduction mode (DCM)
or forced continuous mode (FCM). The mode of operation
is configured by bit 0 of the MFR_PWM_MODEn com-
mand (discontinuous-conduction mode is always the
www.analog.com
LTM4683
can be utilized to help calculate
are chosen with low enough effective
can be a low ESR
OUT
discusses this noise reduction versus
to the
OUTn
Rev. 0
55