Linear Technology DC2518A Demo Manual page 16

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.
V
IN
50 A
SET
R
SET
240k
Figure 16. Regulator with SET Pin
The series resistors provide an equivalent 240k to GND.
The 50μA current develops a nominal +12V 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
resistor midpoint to make adjustments to V
V
IN
LT3081
50 A
+
–
SET
DC2518A F17
R
SET
215k
+1.25V
R
SET2
25k
Figure 17. DAC Connects to SET Resistors
16
LT3081
+
+12V
–
LOAD
DC2518A F16
and drives the SET
OUT
.
OUT
+12V
LOAD
LTC2975
HI RANGE
VDAC
0V TO 2.65V
The LTC2975's DAC is connected directly to the midpoint
of the two SET resistors. The R
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 higher or lower by ±1.2V.
OUT
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.
50 A
R
240k
SET
LT3090
V
–15V
IN
DC2518A F18
Figure 18. Negative LDO with SET Resistor
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.
+3.3V
R3
RDAC
130k
40k
PNP
20 A
50 A
SET
LT3090
–15V V
IN
Figure 19. Negative LDO with Servo
and R resistor
SET1 SET2
SET
+
10 F
V
–12V
OUT
LTC2975
VDAC
DC2518A F19
R
SET2
70k
R
SET1
200k
+
10 F
V
–12V
OUT
DC2518A F18
dc2518af