LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
The output voltage at the POL is differentially sensed via
a symmetrical impedance-divider network. In Figure 1
and Figure 5, it is seen that the control loop regulates the
Use of 0.1% tolerance resistors (or better) for R
,
SET1A
R
, and R
are recommended—with temperature
SET1B
SET2
coefficientsofresistancesuitableforone’soperatingrange
of PCB temperature—to assure that output voltage error
introduced by resistor value variation is acceptable for the
application. SMT resistors with T.C.R.s of 25ppm/°C and
better are readily available in the marketplace.
+
–
outputvoltagesuchthatthedifferentialV
-to-V
OSNS
OSNS
feedback signal voltage is the lesser of the TRACK/SS
pin voltage or the regulator’s nominal bandgap voltage
of 600mV. The arrangement and values of the resistors
in the symmetrical impedance-divider network set the
output voltage.
For output voltage settings less than or equal to 1.2V
,
OUT
R
SET2
is not needed, and R
and R
are given by:
SET1A
SET1B
+
–
The remote sense pins (V
, V
) have redundant
OSNS
OSNS
V
0.6V
+
OUT
connectionsinternaltothemoduletoreadbackpins(V
,
RSET1A =RSET1B
=
–1 •8.2kΩ
ORB
(9)
–
V
). The readback pins provide a means to verify the
ORB
integrity of the feedback signal connection during moth-
erboard ICT (in circuit test). The importance of verifying
the integrity of the connection of the feedback signal to
the output voltage prior to powering up the input voltage
cannot be understated. If one or both feedback pins are
left electrically floating due to manufacturing assembly
defect, for example, or if the remote-sense pins are short
circuited to each other, the control loop and overvoltage-
detector circuitry have no awareness of the actual output
voltage condition. A compromised feedback connection
presents a very real danger of (1) the control loop com-
For output voltages above 1.2V , R
(and R
)
OUT SET1A
SET1B
should be set equal to 8.2kΩ (or less, if 8.2kΩ is not a
convenient value for the user), and
is then given by:
RSET2
2•RSET1A
RSET2
=
(10)
VOUT RSET1A
–
−1
0.6 8.2kΩ
It is always permissible to select a value for R
(and
SET1A
R
) less than that given by Equation 9—and then
SET1B
calculate a valid value for R
from Equation 10—as
SET2
long as R
higher resulting power dissipation.
and R
are designed to withstand the
SET1A
SET1B
manding on M at the highest possible duty cycle—due
TOP
to the lack of negative feedback—and (2) the LTM4641’s
protection circuitry being unaware of any issue. In a pro-
ductionenvironment,moderndayICTcaneasilycatchany
such stuffing or assembly errors; in a lab or prototyping
environment, an ohmmeter can do the job.
+
When V
OSNS
is in regulation, the voltages at V
and
OSNS
OUT
are given by:
–
V
0.6V
∆VGND
+
VVOSNS
=
+
(11)
8.2kΩ||RSET1A ||RSET2
R
(
)
SET1A
For many applications that use a mixture of MLCC and
bulk(lowESRtantalumorpolymer)outputcapacitors, the
symmetrical impedance-divider network that feeds back
the POL’s voltage to the module need only be constructed
• R
(
||16.4kΩ
)
SET1A
and
with resistors R
and R
SET2
, for output voltages
SET1A
and lower. R
SET1B
–
+
of 1.2V
must be present for output
V
= V
– 0.6V
(12)
OUT
VOSNS
VOSNS
voltages in excess of 1.2V . R
and R
should
SET1B
OUT SET1A
respectively. ∆V
is the voltage drop between ground at
GND
always have the same nominal value. Applications with
MLCC-only output capacitors (see Output Capacitors
and Loop Stability in following pages) will demonstrate
improved transient response when feedforward capaci-
the POL and LTM4641’s SGND pins in volts. This voltage
dropꢀisꢀusuallyꢀentirelyꢀaꢀresultꢀofꢀIꢀ•ꢀRꢀdropꢀinꢀtheꢀoutputꢀ
distribution path—largest when maximum load current
is being drawn:
tors C and C , nominally equal in value, are installed
FFA
FFB
∆V
= V
– V
SGND(LTM4641)
(13)
electricallyinparallelwithR
andR ,respectively.
SET1B
GND
GND(POL)
SET1A
4641f
22
Linear [ Linear ]
