Datasheet

LTC3823
17
3823fd
mode, V
OUT1
must be set higher than V
OUT2
. To implement
the ratiometric tracking, the ratio of the divider should be
exactly the same as the master IC’s feedback divider. Note
that the pin Z1/SS
ENABLE
of the slave IC should be tied to
INTV
CC
so that the internal soft-start current is disabled
in both tracking modes or it will introduce a small error
on the tracking voltage depending on the absolute values
of the tracking resistive divider.
By selecting different resistors, the LTC3823 can achieve
different modes of tracking including the two in Figure 7.
So which mode should be programmed? While either
mode in Figure 7 satisfi es most practical applications,
there do exist some tradeoffs. The ratiometric mode saves
a pair of resistors, but the coincident mode offers better
output regulation. This can be better understood with the
help of Figure 9. At the input stage of the slave IC’s error
amplifi er, two common anode diodes are used to clamp
the equivalent reference voltage and an additional diode
is used to match the shifted common mode voltage. The
top two current sources are of the same amplitude. In the
coincident mode, the TRACK/SS voltage is substantially
higher than 0.6V at steady state and effectively turns off
D1. D2 and D3 will therefore conduct the same current
and offer tight matching between V
FB2
and the internal
precision 0.6V reference. In the ratiometric mode, however,
TRACK/SS equals 0.6V at steady state. D1 will divert part
of the bias current to make V
FB2
slightly lower than 0.6V.
Although this error is minimized by the exponential I-V
characteristic of the diode, it does impose a fi nite amount
of output voltage deviation. Furthermore, when the master
IC’s output experiences dynamic excursion (under load
transient, for example), the slave IC output will be affected
as well. For better output regulation, use the coincident
tracking mode instead of ratiometric.
APPLICATIONS INFORMATION
TIME
(7a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3823 F07
(7b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
Figure 7. Two Different Modes of Output Voltage Tracking
R3 R1
R4 R2
R3
V
OUT2
R4
(8a) Coincident Tracking Setup
TO
V
FB1
PIN
TO
TRACK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
R1
R2
R3
V
OUT2
R4
3823 F08
(8b) Ratiometric Tracking Setup
TO
V
FB1
PIN
TO
TRACK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
Figure 8. Setup for Coincident and Ratiometric Tracking
–
+
II
D1
TRACK/SS2
0.6V
V
FB2
D2
D3
3823 F09
EA2
Figure 9. Equivalent Input Current of Error Amplifi er