LTM4641
APPLICATIONS INFORMATION—POWER SUPPLY FEATURES
Figure 6 shows idealized output voltage waveforms for
(3) Choose output capacitors and input capacitors for the
design in the same manner as is done for nontracking
applications.
applications in which LTM4641’s output (V ) tracks
OUT
a master rail (V
respectively.
) coincidently and ratiometrically,
MASTER
To fulfill a coincident rail-tracking requirement, recognize
that when the output voltage of the master rail reaches
the tracking rail’s nominal FS voltage, the TRACK/SS pin
of the LTM4641 (tracking slave) needs to be 600mV. This
can be satisfied by forming a resistor-divider network
V
MASTER
OUT
composed of R and R , interfacing V
to
TAC
TBC
OUT_MASTER
V
TRACK/SSoftheLTM4641trackingslave,andterminating
to SGND of the LTM4641 tracking slave. In Figure 7 and
Figure 8, U1 generates a master rail while U2 generates a
coincident-tracking rail that follows U1’s output. Values
of R and R
are selected such that:
TAC
TBC
TIME
4641 F06a
V
OUT_SLAVE_C (FS OUTPUT) –1 •RTBC
(6a) Coincident Tracking
RTAC
=
(17)
0.6V
V
V
MASTER
IntheexamplecircuitofFigure7, themasterrailgenerated
by U1 ramps up its output to 1.8V. The coincident-tracking
rail is generated by U2 and has a nominal FS output
voltage of 1V. Values of R
and R
are determined
TAC
TBC
OUT
such that when U1’s output reaches 1V, the TRACK/SS
pin of U2 reaches ~600mV; choosing R
to be 10kΩ
TBC
yields R ꢀ=ꢀ(1V/0.6Vꢀ–ꢀ1)ꢀ•ꢀ10kΩ,ꢀorꢀ~6.65kΩ.ꢀItꢀisꢀ
TAC
common to choose resistor values of 10k or less for
this task, so that voltage offset errors introduced by
the 1µA current source on TRACK/SS working into the
TIME
4641 F06b
(6b) Ratioꢃetric Tracking
R
/R
network are sufficiently small.
TAC TBC
Figure 6. Two Different ꢁodes of Output Voltage Tracking
To fulfill a ratiometric rail-tracking requirement, recognize
that when the output voltage of the master rail reaches
its final FS value, the TRACK/SS pin of the LTM4641
(tracking slave) needs to reach 600mV. This can be satis-
fied by forming a resistor-divider network composed of
To configure LTM4641 for coincident or ratiometric
tracking, begin the design (initially) the same way as for
nontracking applications:
(1) Determine the R
, R
, and R
values ap-
SET2
SET1A
SET1B
R
and R , interfacing V
to TRACK/SS
TAR
TBR
OUT_MASTER
propriate for the final, “full-scale” (FS) output voltage.
of the LTM4641 tracking slave, and terminating to SGND
of the LTM4641 tracking slave. In Figure 7 and Figure 8,
U3 generates a ratiometric-tracking rail that follows U1’s
(2) Determine the R resistor needed to guarantee
fSET
ramp down of the output voltage to the desired value.
output. Values of R
and R
are selected such that:
TAR
TBR
For example, if it is necessary for V to ramp down
OUT
to 0.8V while tracking the master rail, then R
is
fSET
V
RTAR
=
OUT_MASTER (FS_OUTPUT) –1 •RTBR
recommended from Table 1 to be ~887kΩ. If ramp-
downtrackingisnotneeded,thenR canbechosen
(18)
0.6V
fSET
according to Table 1 (or Figure 3) and the FS output
voltage of the LTM4641 generated rail.
4641f
25
Linear [ Linear ]
