LTM4608 LINER [Linear Technology], LTM4608 Datasheet - Page 11

• Manufacturer Part Number: LTM4608
• Description: Low VIN, 8A DC/DC ?ModuleTM with Tracking, Margining, and Frequency Synchronization
• Manufacturer: LINER [Linear Technology]

Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
In the above equation, η% is the estimated effi ciency 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 LTM4608 is designed for low output voltage ripple
noise. The bulk output capacitors defi ned as C
chosen with low enough effective series resistance (ESR)
to meet the output voltage ripple and transient require-
ments. C
ESR polymer capacitor or ceramic capacitor. The typical
output capacitance range is from 47μF to 220μF . Additional
output fi ltering may be required by the system designer,
if further reduction of output ripple or dynamic transient
spikes is required. 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
μModule Power Design Tool will be provided 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 current cancellation, but the output capacitance
will be more a function of stability and transient response.
The Linear Technology μModule Power Design Tool will
calculate the output ripple reduction as the number phases
implemented increases by N times.
APPLICATIONS INFORMATION
D
I
CIN(RMS)
=
V
V
OUT
OUT
IN
=
can be a low ESR tantalum capacitor, a low
I
OUT(MAX)
η%
• D • 1– D
(
)
OUT
are
Burst Mode Operation
The LTM4608 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 effi ciency 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
is set to approximately 20% of the maximum peak current
value in normal operation even though the voltage at the
I
drops when the inductor’s average current is greater than
the load requirement. As the I
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, re-
ducing the quiescent current to about 450μA. The load cur-
rent is now being supplied from the output capacitor. When
the output voltage drops, causing I
the internal sleep line goes low, and the LTM4608 resumes
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 effi ciency
at intermediate currents are desired, pulse-skipping mode
should be used. Pulse-skipping operation allows the
LTM4608 to skip cycles at low output loads, thus increasing
effi ciency by reducing switching loss. Floating the MODE
pin or tying it to V
This allows discontinuous conduction mode (DCM) opera-
tion down to near the limit defi ned by the chip’s minimum
on-time (about 100ns). Below this output current level,
the converter will begin to skip cycles in order to main-
tain output regulation. Increasing the output load current
slightly, above the minimum required for discontinuous
conduction mode, allows constant frequency PWM.
Forced Continuous Operation
In applications where fi xed frequency operation is more
critical than low current effi ciency, and where the lowest
output ripple is desired, forced continuous operation should
be used. Forced continuous operation can be enabled by
TH
IN
. During this operation, the peak current of the inductor
pin indicates a lower value. The voltage at the I
IN
/2 enables pulse-skipping operation.
TH
voltage drops below 0.2V,
TH
to rise above 0.25V,
LTM4608
11
TH
4608f
pin