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LTM4604
APPLICATIONS INFORMATION
A typical LTM4604 application circuit is shown in Figure 15.
External component selection is primarily determined by
Without considering the inductor current ripple, the RMS
current of the input capacitor can be estimated as:
the maximum load current and output voltage. Refer to
Table 4 for speci?c external capacitor requirements for a
particular application.
I CIN ( RMS ) =
I OUT(MAX )
η %
? D ? ( 1 – D )
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.
The LTM4604 is 100% duty cycle, but the V IN to V OUT
minimum dropout is a function of the load current. A typi-
cal 0.5V minimum is suf?cient (see Typical Performance
Characteristics).
Output Voltage Programming
The PWM controller has an internal 0.8V reference voltage.
As shown in the Block Diagram, a 4.99k, 0.5% internal
feedback resistor connects the V OUT and FB pins together.
The output voltage will default to 0.8V with no feedback
resistor. Adding a resistor R FB from the FB pin to GND
programs the output voltage:
In the above equation, η % is the estimated ef?ciency of
the power module. The bulk capacitor can be a switcher-
rated electrolytic aluminum capacitor, OS-CON capacitor
for bulk input capacitance due to high inductance traces
or leads. If a low inductance plane is used to power the
device, then no input capacitance is required. The two
internal 10μF ceramics are typically rated for 2A to 3A of
RMS ripple current. The worst-case ripple current for the
4A maximum current is 2A or less.
Output Capacitors
The LTM4604 is designed for low output voltage ripple.
The bulk output capacitors de?ned as C OUT are chosen
with low enough effective series resistance (ESR) to meet
the output voltage ripple and transient requirements. C OUT
can be a low ESR tantalum capacitor, a low ESR polymer
V OUT = 0 . 8 V ?
4 .99k + R FB
R FB
capacitor or an X5R/X7R ceramic capacitor. The typical
output capacitance range is 22μF to 100μF. Additional
output ?ltering may be required by the system designer
Table 1. FB Resistor vs Output Voltage
V OUT 0.8V 1.2V 1.5V
R FB Open 10k 5.76k
1.8V
4.02k
2.5V
2.37k
3.3V
1.62k
if further reduction of output ripple or dynamic transient
spike is required. Table 4 shows a matrix of different
output voltages and output capacitors to minimize the
voltage droop and overshoot during a 2A/μs transient.
Input Capacitors
The LTM4604 module should be connected to a low ac-
impedance DC source. Two 10μF ceramic capacitors are
included inside the module. Additional input capacitors
are only needed if a large load step is required up to a
full 4A level. An input 47μF bulk capacitor is only needed
if the input source impedance is compromised by long
inductive leads or traces.
For a buck converter, the switching duty cycle can be
estimated as:
The table optimizes the total equivalent ESR and total bulk
capacitance to maximize transient performance. The Linear
Technology μModule Power Design Tool can be provided
for further optimization.
Fault Conditions: Current Limit and Overcurrent
Foldback
The LTM4604 has current mode control, which inher-
ently limits the cycle-by-cycle inductor current not only
in steady-state operation, but also in transient.
To further limit current in the event of an overload condi-
D =
V OUT
V IN
tion, the LTM4604 provides foldback current limiting as
the output voltage falls. The LTM4604 device has over-
temperature shutdown protection that inhibits switching
operation around 150°C.
4604fa
9
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