LTM4624
9
4624fc
For more information www.linear.com/LTM4624
applicaTions inForMaTion
The typical LTM4624 application circuit is shown in
Figure 20. External component selection is primarily
determined by the input voltage, the output voltage and
the maximum load current. Refer to Table 6 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 ratios that can be achieved for a given input voltage
due to the minimum off-time and minimum on-time limits
of the regulator. The minimum off-time limit imposes a
maximum duty cycle which can be calculated as:
D
MAX
= 1 – (t
OFF(MIN)
• f
SW
)
where t
OFF(MIN)
is the minimum off-time, typically 70ns
for LTM4624, and f
SW
(Hz) is the switching frequency.
Conversely the minimum on-time limit imposes a minimum
duty cycle of the converter which can be calculated as:
D
MIN
= t
ON(MIN)
• f
SW
where t
ON(MIN)
is the minimum on-time, typically 40ns
for LTM4624. In the rare cases where the minimum duty
cycle is surpassed, the output voltage will still remain
in regulation, but the switching frequency will decrease
from its programmed value. Note that additional thermal
derating may be applied. See the Thermal Considerations
and Output Current Derating section in this data sheet.
Output Voltage Programming
The PWM controller has an internal 0.6V reference voltage.
As shown in the Block Diagram, a 60.4k internal feedback
resistor connects the V
OUT
and FB pins together. Adding a
resistor, R
FB
, from FB pin to SGND programs the output
voltage:
R
FB
=
V
OUT
–0.6V
•60.4k
Table 1. R
FB
Resistor Table vs Various Output Voltages
V
OUT
(V) 0.6 1.0 1.2 1.5 1.8 2.5 3.3 5.0
R
FB
(kΩ) OPEN 90.9 60.4 40.2 30.1 19.1 13.3 8.25
Input Decoulping Capacitors
The LTM4624 module should be connected to a low AC
impedance DC source. For the regulator, a 10µF input
ceramic capacitor is required for RMS ripple current de
-
coupling. Bulk input capacitance is only needed when the
input source impedance is compromised by long inductive
leads, traces or not enough sour
ce capacitance. The bulk
capacitor can be an aluminum electrolytic capacitor or
polymer capacitor.
Without considering the inductor ripple current, the RMS
current of the input capacitor can be estimated as:
I
CIN(RMS)
=
OUT(MAX)
η%
• D• 1–D
( )
where η% is the estimated efficiency of the power module.
Output Decoulping Capacitors
With an optimized high frequency, high bandwidth design,
only a single low ESR output ceramic capacitor is required
for the LTM4624 to achieve low output ripple voltage and
very good transient response. Additional output filtering
may be required by the system designer if further reduction
of output ripple or dynamic transient spikes is required.
Table 6 shows a matrix of different output voltages and
output capacitors to minimize the voltage droop and
overshoot during a 1A and 2A load-step transient. The
Linear Technology LTpowerCAD™ design tool is available
to download online for output ripple, stability and transient
response analysis for further optimization.
