LTM4641
APPENDICES
thermore, using an R
(and R
) value of 8.2kΩ
SET1B
interactions—which might at worst result in exces-
sive output voltage ripple or non-monotonic output
voltage ramp-up, a sufficiently slow output voltage
SET1A
for 1.2V
and larger assures that the common mode
OUT
range of the remote-sense pins is within their valid range
of–0.3V,minimum,to3V,maximum—evenifvoltagedrop
between the module’s ground deviates from the POL’s
ground by as much as 0.6V.
ramp-up time can eliminate the danger of V
and
INH
on-time settling interactions influencing output volt-
age ripple—but properly, this requires investigation
and hardware evaluation on a case-by-case basis.
The differential remote-sense feedback signal is routed
from the load as a differential pair on PCB traces (or
twisted pair, if wires are used) to R
components. It is very important to place R
andallothercomponentsformingthefeedbackimpedance-
dividernetworkasclosetoLTM4641asispossible.Ground
shieldingofthedifferentialremote-sensesignalisstrongly
recommended, to prevent stray noise from contaminating
the feedback information.
Figure 47 shows an example where R
connects
fSET
between f and V —rather than the input source
SET
INH
/R
feedback
/R
SET1A SET1B
supply. Because MSP limits the V voltage during
INH
ON
SET1A SET1B
the input voltage surge, the correct I programming
current can only be made with a resistor interface to
V , in that example.
INH
Appendix D. Remote Sensing in Harsh Environments
Therationaleforusingthesymmetricalresistornetworkis
toprovideaconsistentfeedbackstructurethatenablesfully
differentialremote-senseofoutputvoltagesbetween0.6V
and 6V with the flexibility to filter differential and common
mode noise in harsh environments. See Figure 64. The
If good shielding of the feedback signals cannot be pro-
vided, it is proactive to leave space in one’s layout for a
+
small filtercapacitor, C , placed directly between V
DM
OSNS
–
and V
, as close to the pins of the module as pos-
sible—in anticipation of the possible need to attenuate
OSNS
useofnotgreaterthan8.2kΩnominalresistorsforR
SET1A
differential mode noise.
(and R
) assures that the remote-sense signal is not
SET1B
Finally, if the POL is very far from the LTM4641, such as:
attenuated at frequencies of interest by the pole formed
by the feedback resistors and parasitic capacitances. Fur-
the output power connection (V
and GND) is made
OUT
C
, C : If Appreciable Cable Length Connects the LTM4641’s Output
CMA CMB
to the Load (e.g., Through Several Feet of Wire), Leave Provision
for High Frequency Decoupling of Common Mode Ground Noise with
These Capacitors. These Are Not Needed in Purely PCB-Based Designs,
Where the LTM4641 Is Close to the Load
C
, C : Feedforward Capacitors
FFA FFB
Yeild Improved Transient Response
When Filtering V
Output Capacitors (C
with Only MLCC
OUT
)
OUT(MLCC)
V
OUT
V
OUT
+
LTM4641
C
FFA
C
C
OUT(MLCC)
OUT(BULK)
ICT
TEST
POINT
+
V
ORB
C
CMA
R
8.2k
8.2k V
SET1A
V
+
FB
OSNS
TO ERROR
AMPLIFIER
+
R
C
LOAD
SET2
C
DM
8.2k
R
SET1B
–
–
–
V
OSNS
8.2k
4641 F064
C
ICT
TEST
POINT
FFB
V
CMB
ORB
TRUE DIFFERENTIAL REMOTE
SENSE AMPLIFIER
SGND
GND
Route Feedback Signals as
a Differential Pair (or
Twisted Pair if Using Wires).
Sandwich Between Ground
Planes to Form a Protective Shield,
Guarding Against Stray Noise
Place All Feedback
Components Local
To The LTM4641
SGND CONNECTS TO GND INTERNAL TO MODULE. KEEP SGND
ROUTES/PLANES SEPARATE FROM GND ON MOTHERBOARD
If Effective Ground Shielding of the Feedback Signals Cannot
Be Implemented, Leave Provision for a Small Capacitor (C
To Attenuate Differential Mode Noise if Necessary
)
DM
Figure 64. Feedback Remote Sense Connections and Techniques for Harshest Operating Environments
4641f
58
Linear [ Linear ]
