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LTM4608A
APPLICATIONS INFORMATION
V OUT
? D ? ( 1 – D )
I CIN(RMS) =
Iflowimpedancepowerplanesareused,thenthis47μF
capacitor is not needed.
For a buck converter, the switching duty-cycle can be
estimated as:
D =
V IN
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
I OUT(MAX)
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, polymer capacitor
for bulk input capacitance due to high inductance traces
or leads. If a low inductance plane is used to power the
device, then only one 10μF ceramic is required. The three
internal 10μF ceramics are typically rated for 2A of RMS
ripple current, so the ripple current at the worse case for
8A maximum current is 4A or less.
Output Capacitors
The LTM4608A is designed for low output voltage ripple
noise. The bulk output capacitors defined as C OUT are
chosen with low enough effective series resistance (ESR)
to meet the output voltage ripple and transient require-
ments. C OUT can be a low ESR tantalum capacitor, a low
ESR polymer capacitor or ceramic capacitor. The typical
output capacitance range is from 47μF to 220μF. Additional
output filtering may be required by the system designer,
if further reduction of output ripple or dynamic transient
spikes is desired. Table 3 shows a matrix of different output
voltages and output capacitors to minimize the voltage
droop and overshoot during a 3A/μs transient. The table
optimizes total equivalent ESR and total bulk capacitance
to optimize the transient performance. Stability criteria are
considered in the Table 3 matrix, and the Linear Technology
LTpowerCAD? Design Tool is available for stability analysis.
Multiphase operation will reduce effective output ripple as
a function of the number of phases. Application Note 77
discusses this noise reduction versus output ripple cur-
rent cancellation, but the output capacitance will be more
a function of stability and transient response. The Linear
Technology LTpowerCAD Design Tool will calculate the
output ripple reduction as the number phases implemented
increases by N times.
Burst Mode Operation
The LTM4608A is capable of Burst Mode operation 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. To
enable Burst Mode operation, simply tie the MODE pin to
V IN . During this operation, the peak current of the inductor
is set to approximately 20% of the maximum peak current
value in normal operation even though the voltage at the
I TH pin indicates a lower value. The voltage at the I TH pin
drops when the inductor’s average current is greater than
the load requirement. As the I TH voltage drops below 0.2V,
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. The load
current is now being supplied from the output capacitor.
When the output voltage drops, causing I TH to rise above
0.25V, the internal sleep line goes low, and the LTM4608A re-
sumes normal operation. The next oscillator cycle will turn
on the top power MOSFET and the switching cycle repeats.
Pulse-Skipping Mode Operation
In applications where low output ripple and high efficiency
at intermediate currents are desired, pulse-skipping mode
should be used. Pulse-skipping operation allows the
LTM4608A to skip cycles at low output loads, thus increasing
efficiency by reducing switching loss. Floating the MODE
pin or tying it to V IN /2 enables pulse-skipping operation.
This allows discontinuous conduction mode (DCM) opera-
tion down to near the limit defined by the chip’s minimum
on-time (about 100ns). Below this output current level,
the converter will begin to skip cycles in order to maintain
output regulation. Increasing the output load current slightly,
above the minimum required for discontinuous conduction
mode, allows constant frequency PWM.
4608afd
11
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