Datasheet

LT3507A
18
3507af
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
outputs at start-up, they also disable the slaved channels
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 LT3507A 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, con-
nect 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 voltage
and an additional diode is used to match the shifted com-
mon mode voltage. The top two current sources are of
the same amplitude. In the coincident mode, the TRK/SS2
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
3507A F08
(8b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
1.25µA
–
+
I I
D1
TRK/SS
0.8V
FB
D2
D3
3507A F10
EA2