LTM4623_15 Datasheet, PDF (9/26 Pages) Linear Technology – Ultrathin 20VIN, 3A Step-Down DC/DC Module Regulator

LTM4623
Applications Information
The typical LTM4623 application circuit is shown in
Figure 24. External component selection is primarily
determined by the input voltage, the output voltage and
the maximum load current. Refer to Table 7 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 LTM4623, 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 LTM4623. 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
=
0.6V
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
Pease note that for 3.3V and 5V output, a higher operating frequency
(2MHz) is required to optimize inductor current ripple. See Operating
Frequency section.
For parallel operation of N-channels LTM4623, tie all the
FB pins together and use the following equation to solve
for R_FB:
R_FB
=
0.6V
V_OUT – 0.6V
•
60.4k
N
Input Decoupling Capacitors
The LTM4623 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 source 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)
=
I_{OUT(MAX )}
η%
•
D • (1−D)
where η% is the estimated efficiency of the power module.
Output Decoupling Capacitors
With an optimized high frequency, high bandwidth design,
only a single low ESR output ceramic capacitor is required
for the LTM4623 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 7 shows a matrix of different output voltages and
output capacitors to minimize the voltage droop and
overshoot during a 1A 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.
Discontinuous Current Mode (DCM)
In applications where low output ripple and high efficiency
at intermediate current are desired, discontinuous current
mode (DCM) should be used by connecting the MODE pin
For more information www.linear.com/LTM4623
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