Datasheet

LT3507
19
3507fb
For more information www.linear.com/LT3507
applications inForMation
Figure 8. Two Different Modes of Output Voltage Tracking
Figure 9. Setup for Coincident and Ratiometric Tracking
Figure 10. Equivalent Input Circuit of Error Amplifier
OUTPUT VOLTAGE TRACKING
The LT3507 allows the user to program how the output
ramps up by means of the TRK/SS pins. Through these
pins, any channel output can be set up to either coinci
-
dently or ratiometrically track any other channel output.
This example will show the channel 2 output tracking the
channel 1 output, as shown in Figure 8. The TRK/SS2
pin
acts as a clamp on channel 2’s reference voltage. V
OUT2
is referenced to the TRK/SS2 voltage when the TRK/SS2
< 0.8V and to the internal precision reference when TRK/
SS2 > 0.8V.
To implement the coincident tracking in Figure 8a, connect
an extra resistive divider to the output of channel 1 and
connect its midpoint to the TRK/SS2 pin (Figure 9). The
ratio of this divider should be selected the same as that
of channel 2’s feedback divider (R5=R3 and R6=R4). In
this tracking mode, V
OUT1
must be set higher than V
OUT2
.
To implement the ratiometric tracking in Figure 8b, change
the extra divider ratio to R5=R1 and R6=R2 + ΔR. The
extra resistance on R6 should be set so that the TRK/SS2
voltage is ≥1V when V
OUT1
is at its final value.
The need for this extra resistance is best understood with
the help of the equivalent input circuit shown in Figure 10.
At the input stage of the error amplifier, two common anode
diodes are used to clamp the equivalent reference volt
-
age 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 TRK/
SS2 voltage is substantially higher than 0.8V at steady
state and effectively turns off D1. D2 and D3 will there
-
fore conduct the same current and offer tight matching
between V
FB2
and the internal precision 0.8V reference. In
the ratiometric mode with R6=R2, TRK/SS2 equals 0.8V
at steady state. D1 will divert part of the bias current and
make V
FB2
slightly lower than 0.8V. Although this error
R1
R2
=
V
OUT1
0.8
– 1,
R3
R4
=
V
OUT2
0.8
– 1
⎛
⎝
⎜
⎞
⎠
⎟
R5 R1
R6 R2
V
OUT2
R4
R3
Tracking Setup
TO
V
FB1
PIN
TO
TRK/SS2
PIN
TO
V
FB2
PIN
V
OUT1
COINCIDENT
R3
R4
R5 =
R6 =
RATIOMETRIC
R1
R1
V
OUT1
/1V – 1
SELECTING VALUES FOR R5 AND R6
TIME
(8a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3507 F08
(8b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
1.25µA
–
+
I I
D1
TRK/SS
0.8V
FB
D2
D3
3507 F10
EA2
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