Datasheet
LT3988
16
3988f
TIME
Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3988 F07
Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
Figure 7. Two Different Modes of Output Voltage Tracking
applicaTions inForMaTion
Figure 8. Setup for Coincident and Ratiometric Tracking
Figure 9. Equivalent Input Circuit of Error Amplifier
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 TRACK/SS2
voltage is substantially higher than 0.75V at steady state
and effectively turns off D1. D2 and D3 will therefore con-
duct the same current and offer tight matching between
V
FB2
and the internal precision 0.75V reference. In the
ratiometric mode with R6 = R2, TRACK/SS2 equals 0.75V
at steady state. D1 will divert part of the bias current and
make V
FB2
slightly lower than 0.75V. Although this error
is minimized by the exponential I-V characteristic of the
diodes, it does impose a finite amount of output voltage
deviation. Further, when channel 1’s output experiences
dynamic excursions (under load transient, for example),
channel 2 will be affected as well. Setting R6 to a value
that pushes the TRK/SS2 voltage to 1V at steady state will
eliminate these problems while providing near ratiometric
tracking. The example shows channel 2 tracking channel 1,
however either channel may be set up to track the other.
Soft-Start
If a capacitor is tied from the TRACK/SS pin to ground,
then the internal pull-up current will generate a voltage
ramp on this pin. This results in a ramp at the output,
limiting the inductor current and therefore input current
during start-up. A good value for the soft-start capacitor
is C
OUT
/10,000, where C
OUT
is the value of the output
capacitor.
R5 R1
R6 R2
V
OUT2
R4
R3
TO
FB1
PIN
TO
TRK/SS2
PIN
TO
FB2
PIN
V
OUT1
COINCIDENT
R3
R4
R5 =
R6 =
RATIOMETRIC
R1
R1
V
OUT1
/1V – 1
SELECTING VALUES FOR R5 AND R6
3988 F08
1.36µA
–
+
I I
D1
TRACK/SS
0.75V
FB
D2
D3
3988 F09
g
m
R1
R2
=
V
OUT1
0.75
–1,
R3
R4
=
V
OUT2
0.75
–1