Datasheet

LTC3865/LTC3865-1
20
3865fb
APPLICATIONS INFORMATION
shown in Figure 6a. In this tracking mode, V
OUT1
must
be set higher than V
OUT2
. To implement the ratiometric
tracking, the ratio of the V
OUT2
divider should be exactly
the same as the master channel’s internal feedback divider.
By selecting different resistors, the LTC3865/LTC3865-1
can achieve different modes of tracking including the two
in Figure 5.
So which mode should be programmed? The coincident
mode offers better output regulation. This can be better
understood with the help of Figure 7. At the input stage
of the slave channel’s error amplifi er, two common an-
ode 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 magnitude. In coincident mode, the TK/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 at steady state. In the
ratiometric mode, however, TK/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 characteristics of the diode, it does
impose a fi nite amount of output voltage deviation.
TIME
(5a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
V
OUT1
V
OUT2
TIME
3865 F05
(5b) Ratiometric Tracking
OUTPUT VOLTAGE
Figure 5. Two Different Modes of Output Voltage Tracking
nR3 R1
nR4 R2
R3
V
OUT2
R4
(6a) Coincident Tracking Setup
TO
EA1
TO
TK/SS2
PIN
TO
EA2
V
OUT1
38551 F06
(6b) Ratiometric Tracking Setup
nR1 R1
nR2 R2
R3
V
OUT2
R4
TO
EA1
TO
TK/SS2
PIN
TO
EA2
V
OUT1
Figure 6. Setup for Coincident and Ratiometric Tracking
–
+
D3
3865 F07
D2D1
TK/SS2
0.6V
V
FB2
II
EA2
Figure 7. Equivalent Input Circuit of Error Amplifi er