Linear Technology DC2518A Demo Manual page 17

ADVANCED DEMO BOARD OPERATIONS
There are two SET resistors and the midpoint is fed with
a 20μA pre-bias current that sets the floating voltage. The
SET resistor R always sees 50μA and R
SET1
This arrangement allows the DAC, via the PNP , to adjust
the pre-bias current, thereby moving the resistor midpoint,
and therefore the output voltage. The LTC2975 DAC can
soft-connect to the emitter which sits ~0.6V above GND.
With the DAC disconnected, the PNP emitter current is
defined by R3. The collector pushes 20μA into the SET
resistors. Once the DAC is soft-connected to Q5's emitter,
the DAC resistor limits the current added or subtracted
to the R3 current.
Enable/Disable
Not all regulators have a RUN, SHDN, or other "enable" pin
that allows easy turn-on and shut-off. Additionally, there
might be no way to ensure V
regulator is disabled. Buck regulators are straightforward
in this regard. However, the regulator you choose for your
design may require some thought.
The LT3081 linear regulator does not have any such pin,
so an alternate method was devised to force V
An external NFET is connected to the SET pin that when
activated pulls the SET pin to GND, thereby bringing V
to GND. When the channel is disabled, the VOUT_EN pin
is low, thereby turning the PFET on which pulls the NFET
gate high, activating the NFET which pulls the SET pin to
GND. When the channel is enabled, the PFET gate pulls
sees 30μA.
SET2
goes to ground when the
OUT
OUT
DEMO MANUAL DC2518A
high and the NFET is released, allowing the LT3081's
50μA current source to ramp the SET node via the R
resistor. The TON_RISE timer begins to count. Once the
timer expires, the DAC starts its soft-connect operation.
+3.3V
R1
10k
LTC2975
V
OUTEN
Figure 20. Disable Output with Input PFET
Another potential issue you may encounter is the proper
disabling of a boost regulator. Look at the CH2: +48V
supply. The LT8330 is configured as a boost topology
which has a DC path (inductor and diode) from the input
to output. There is an enable pin on the device. However
when disabled, the output does not go to ground but rather
only goes down to the input voltage. To solve this problem,
a PFET was added in series with the input to disconnect
V , allowing the output to go to GND.
IN
off.
PFET
+12V
OUT
R2
10k
NFET
LTC2975
V
OUTEN
Figure 21. Disable Output with Input PFET
LT3081
V V
+15V
IN OUT
+
SET
PFET
R
SET
NFET
240k
R2
10k
D
L
S
IN
+3.3V
V V
IN OUT
R1
10k
LT8330
R
TOP
EN
309k
FBX
R
BOT
10.7k
SET
+12V
10 F
DC2518A F20
+48V
+
4.7 F
DC2518A F21
dc2518af
17