LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
WithR
,R
,andR
determined,double-check
closeinproximitytothemoduleaspossible(seeFigure43).
If external MOSFET MSP is not used (Figure 45), two 10μF
or four 4.7μF ceramic capacitors should be electrically
SET1A SET1B
the output voltage setting with:
SET2
RSET1A 2•RSET1A
8.2kΩ
connected directly between the V
and GND pins. If
VOUT = 0.6V • 1+
+
INH
(14)
RSET2
MSP is used (Figure 46, Figure 47 and Figure 49), then
MSP must be placed as close to the LTM4641’s V pins
INH
SomerecommendedvaluesforR
for popular output voltages are shown in Table 2.
,R
,andR
SET2
SET1A SET1B
as possible, and two 10μF or four 4.7μF ceramic capaci-
tors should be electrically connected directly between the
drain of MSP and GND (see Figure 44). A 47μF to 100μF
surface mount bulk capacitor can be used to supplement
input power bypassing, and can share the burden of any
local ceramic capacitors in filtering the power stage’s
ripple current. If low impedance power planes are used
Table 2. Recoꢃꢃended RSET1A, RSET1B and RSET2 Values
for Soꢃe Popular Output Voltages, cf. Figure 5 Feedback
Connections.
V
OUT
R
, R
R
SET2
SET1A SET1B
0.6V
0.7V
0.8V
0.9V
1.0V
1.2V
1.5V
1.8V
2.0V
2.5V
3.3V
5.0V
6.0V
0Ω
∞ (Not Used)
∞ (Not Used)
∞ (Not Used)
∞ (Not Used)
∞ (Not Used)
∞ (Not Used)
33.2kΩ
1.37kΩ
2.74kΩ
4.12kΩ
5.49kΩ
8.2kΩ
8.2kΩ
8.2kΩ
8.2kΩ
8.2kΩ
8.2kΩ
8.2kΩ
8.2kΩ
to bring V to the vicinity of the module, input source
IN
impedance will be low enough that bulk capacitors will
not be needed. A localized bulk input capacitor is needed
when an underdamped LC-resonant tank is formed by
routing long input leads or traces (low ESR inductance)
bypassed only with MLCCs (ultralow ESR capacitance).
16.5kΩ
Neglecting the inductor peak-to-peak current ripple, the
RMS current of the input capacitor can be estimated as:
12.4kΩ
7.5kΩ
I
ICIN(RMS)
=
OUT(MAX) • D• 1–D
(15)
4.7kΩ
(
)
2.61kΩ
η
2.05kΩ
whereη isthepowerconversionefficiencyoftheLTM4641
See Appendix D for a detailed discussion on the following
topics:
module and D is the duty cycle on-time of M . The bulk
TOP
capacitor can be a switcher-rated electrolytic aluminum
capacitor or a polymer capacitor.
•ꢀ Whatꢀisꢀtheꢀrationaleꢀforꢀusingꢀaꢀsymmetricalꢀresistorꢀ
network?
For a buck converter, the switching duty cycle of M
can be estimated as:
TOP
•ꢀ WhatꢀshouldꢀIꢀdoꢀifꢀIꢀcannotꢀshieldꢀtheꢀdifferentialꢀsenseꢀ
feedbacklineswithGND?(Ianticipatedifferentialmode
noise in the feedback signal?)
VOUT
D=
(16)
V
IN
•ꢀ WhatꢀshouldꢀIꢀdoꢀifꢀtheꢀmoduleꢀandꢀtheꢀload(s)ꢀareꢀ
separated by a significant distance (~50cm or more),
or if the load current flows through a cable assembly
or power connector? (I anticipate common mode noise
in the feedback signal?)
Output Capacitors and Loop Stability/Loop
Coꢃpensation
The current mode constant on-time architecture enables
very high step-down input-to-output ratios with compel-
ling transient response. It also enables cycle-by-cycle fast
current limit and foldback current limit in an overcurrent
condition.TheLTM4641isinternallycompensatedtoyield
stability over all operating conditions.
Input Capacitors
The LTM4641 module should be connected to a low AC
impedance, nominally DC output voltage source. MLCC
input bypass capacitors must be provided externally, as
4641f
23
Linear [ Linear ]
