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LTM4604A
APPLICATIONS INFORMATION
? D ? ( 1 – D )
I CIN(RMS) =
V OUT = 0.8V ?
V OUT
V IN
A typical LTM4604A application circuit is shown in Fig-
ure 15. External component selection is primarily deter-
mined by the maximum load current and output voltage.
Refer to Table 4 for specific external capacitor requirements
for a particular application.
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 LTM4604A is 100% duty cycle capable, but the V IN
to V OUT minimum dropout is a function of the load cur-
rent. A typical 0.5V minimum is sufficient (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 externally
applied feedback resistor. Adding a resistor R FB from the
FB pin to GND programs the output voltage:
4.99k + R FB
R FB
Table 1. FB Resistor vs Output Voltage
V OUT 0.8V 1V 1.2V 1.5V 1.8V 2.5V 3.3V
R FB Open 20k 10k 5.76k 4.02k 2.37k 1.62k
Input Capacitors
The LTM4604A 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:
D =
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 aluminum electrolytic capacitor, OS-CON or
polymer capacitor. 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 LTM4604A is designed for low output voltage ripple.
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 requirements. C OUT
can be a low ESR tantalum capacitor, a low ESR polymer
capacitor or an X5R/X7R ceramic capacitor. The typical
output capacitance range is 22μF to 100μF. Additional
output filtering may be required by the system designer
if further reduction of output ripple or dynamic transient
spikes 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.
The table optimizes the total equivalent ESR and total
bulk capacitance to maximize transient performance. The
LTpowerCAD GUI is available for further optimization.
Fault Conditions: Current Limit and Overcurrent
Foldback
The LTM4604A 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-
tion, the LTM4604A provides foldback current limiting as
the output voltage falls. The LTM4604A device has over-
temperature shutdown protection that inhibits switching
operation around 150°C.
10
For more information www.linear.com/LTM4604A
4604afa
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