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dc2518af
DEMO MANUAL DC2518A
ADVANCED DEMO BOARD OPERATIONS
Servo Circuits
A number of different servo circuits are utilized on the
DC2518. The servo circuits control a regulator’s SET pin,
a feedback (FB) pin, and an FBX pin.
The +12V rail on CH0 is based on the LT3081 linear regulator.
This device derives its output from a 50μA current source
(SET pin) that connects to a single resistor tied to GND.
The series resistors provide an equivalent 240k to GND.
The 50μA current develops a n12V on the SET
pin which is buffered to the OUT pin. The output voltage
may have some error due to SET current and external
resistor tolerances. The LTC2975 can servo the output by
accurately measuring the divided V
OUT
and drives the SET
resistor midpoint to make adjustments to V
OUT
.
Figure 16. Regulator with SET Pin
Figure 17. DAC Connects to SET Resistors
The LTC2975’s DAC is connected directly to the midpoint
of the two SET resistors. The R
SET1
and R
SET2
resistor
values are chosen such that the midpoint is approximately
1.25V, a voltage that is half of the DAC’s maximum output.
After the DAC finds a suitable voltage, it is connected to
the divider and controls this node, making it capable of
changing V
OUT
higher or lower by ±1.2V.
How to Servo a Negative LDO
A servo circuit that needs more thought is the LT3090
servo. The SET pin sinks 50μA and a 240k SET resistor
develops 12V below GND.
To add a servo circuit that translates a positive DAC volt-
age to a current pushing into a node that is below GND,
a PNP transistor is used.
+
LT3090
V
IN
SET
V
OUT
–15V
–12V
10μF
R
SET
240k
50μA
DC2518A F18
Figure 18. Negative LDO with SET Resistor
+
DC2518A F19
LTC2975
VDAC
LT3090
V
IN
SET
V
OUT
–15V
–12V
10μF
R
SET1
200k
R
SET2
70k
50μA
DC2518A F18
20μA
PNP
RDAC
40k
R3
130k
+3.3V
Figure 19. Negative LDO with Servo
R
SET
240k
DC2518A F16
+
–
LT3081
50μA
+12V
SET
V
IN
LOAD
R
SET
215k
DC2518A F17
+
–
R
SET2
25k
LT3081
+1.25V
LTC2975
HI RANGE
0V TO 2.65V
50μA
+12V
SET
V
IN
VDAC
LOAD
