Datasheet

LT3507
18
3507fa
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 Amplifi er
if the master channel falls out of regulation (due to a short
circuit or a collapsing input voltage).
Finally, be aware that the circuit in Figure 7e does not
work, because the power good comparators are disabled
in shutdown.
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 fi nal 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 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,
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
1μA
–
+
I I
D1
TRK/SS
0.8V
FB
D2
D3
3507 F10
EA2