LTM4637_15 Datasheet, PDF (10/30 Pages) Linear Technology – 20A DC/DC Module Step-Down Regulator

LTM4637
Applications Information
The typical LTM4637 application circuit is shown in
Figure 22. External component selection is primarily
determined by the maximum load current and output
voltage. Refer to Table 5 for specific external capacitor
requirements for particular applications.
V_IN to V_OUT Step-Down Ratios
There are restrictions in the V_IN to V_OUT step-down ratio
that can be achieved for a given input voltage. The duty
cycle is 94% typical at 500kHz operation. The V_IN to V_OUT
minimum dropout is a function of load current and operation
at very low input voltage and high duty cycle applications.
At very low duty cycles the minimum 100ns on-time must
be maintained. See the Frequency Adjustment section and
temperature derating curves.
Output Voltage Programming
The PWM controller has an internal 0.6V ±1% reference
voltage. As shown in the Block Diagram, a 60.4k internal
feedback resistor connects the V_{OUT_LCL} and V_FB pins
together. When the remote sense amplifier is used, then
DIFF_OUT is connected to the V_{OUT_LCL} pin. If the remote
sense amplifier is not used, then V_{OUT_LCL} connects to
V_OUT. The output voltage will default to 0.6V with no feed-
back resistor. Adding a resistor R_FB from V_FB to ground
programs the output voltage:
V_OUT
=
0.6V
•
60.4k +R
R_FB
FB
Table 1. V_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 (k) Open 90.9 60.4 40.2 30.1 19.1 13.3 8.25
For parallel operation of N LTM4637s, the following
equation can be used to solve for R_FB:
R_FB=
60.4k
V_OUT
/N
–1
0.6V
Tie the V_FB pins together for each parallel output. The
COMP pins must be tied together also.
Input Capacitors
The LTM4637 module should be connected to a low AC-
impedance DC source. Additional input capacitors are
needed for the RMS input ripple current rating. The I_CIN(RMS)
equation which follows can be used to calculate the input
capacitor requirement. Typically 22µF X7R ceramics are a
good choice with RMS ripple current ratings of ~ 2A each.
A 47µF to 100µF surface mount aluminum electrolytic bulk
capacitor can be used for more input bulk capacitance.
This bulk input capacitor is only needed if the input source
impedance is compromised by long inductive leads, traces
or not enough source capacitance. If low impedance power
planes are used, then this bulk capacitor is not needed.
For a buck converter, the switching duty cycle can be
estimated as:
D=
V_OUT
V_IN
Without considering the inductor ripple current, for each
output 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 mod-
ule. The bulk capacitor can be a switcher-rated aluminum
electrolytic capacitor or a Polymer capacitor.
Output Capacitors
The LTM4637 is designed for low output voltage ripple
noise. 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 require-
ments. C_OUT can be a low ESR tantalum capacitor, low
ESR Polymer capacitor or ceramic capacitors. The typical
output capacitance range is from 200µF to 800µF. Additional
output filtering may be required by the system designer
if further reduction of output ripple or dynamic transient
spikes is required. Table 5 shows a matrix of different output
voltages and output capacitors to minimize the voltage
droop and overshoot during a 10A/µs transient. The table
optimizes total equivalent ESR and total bulk capacitance
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