LTC2926IUFD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2926IUFD' PDF : 28 Pages View PDF
LTC2926
APPLICATIO S I FOR ATIO
Slave Control Without MOSFETs
The LTC2926 can control tracking and sequencing of a
slave supply without a MOSFET if the supply generator’s
output voltage is set by an accessible resistive voltage
divider and if its voltage reference is ground-based. Track-
ing currents mirrored to the FB pins are injected into the
feedback nodes of the supply generators to control the
output voltage. When master ramp signal has reached
it maximum voltage, the FB pin current is zero, and the
LTC2926 has no effect on the output voltage accuracy,
transient response or stability of the generator.
To control a supply generator (e.g., DC/DC converter) with
a feedback reference voltage VFB(GEN) of 0.75V or less,
connect the FB pin of the LTC2926 to the tap point of the
generator’s resistive divider, as shown in Figure 21a. Fol-
low steps 1 and 3 of the Three-Step Design Procedure to
set the ramp rates and tracking profile. Use the feedback
resistor values required by the supply generator for RFA
and RFB in step 3.
A generator with VFB(GEN)>0.75V may be controlled without
a MOSFET if the slave voltage is large enough (see Figure
22). First follow steps 1 and 3 of the Three-Step Design
Procedure to set the ramp rates and tracking profile.
Use the feedback resistor values required by the supply
generator in step 3. Next, split resistor RFA as in Figure
21b, so that
RFAA
≤ RFA
•
0.75 V
VFB(GEN)
and
RFAB = RFA – RFAA
and tie the LTC2926’s FB pin to the node in between. The
new tap point allows the LTC2926’s FB pin to see <0.75V
at the end of ramp-up. Finally, scale the track resistors to
match RFAA:
R′TA
= RFAA
RFAA + RFAB
•RTA
and
R′TB
=
RFAA
RFAA + RFAB
•RTB
Voltage regulators that force their reference voltage be-
tween their output and feedback nodes do not employ a
ground-based reference, and thus will not be controllable
by the LTC2926 without a series MOSFET.
SUPPLY GENERATOR
VIN
IN
OUT
VFB(GEN)
GND
FB
LTC2926
RAMPBUF SGATE NC
RTB
TRACK
FB
GND
RTA
SLAVE VIN
SUPPLY GENERATOR
IN
OUT
RFB
VFB(GEN)
GND
FB
RFA
LTC2926
RAMPBUF SGATE NC
RT′ B
TRACK
FB
GND
RT′ A
2926 F21
SLAVE
RFB
RFAB
RFAA
(a)
(b)
Figure 21. Slave Supply Control Without a MOSFET
(a) Generator Reference VFB(GEN) ≤ 0.75V and (b) Generator Reference VFB(GEN) > 0.75V
Slave Supply Control Without
a MOSFET
6
CONTROL VIA
5
FB PIN AND
SPLIT RFA
RESISTOR
4
3
CONTROL
VIA FB PIN
2
1
MOSFET
CONTROL ONLY
0
0 0.25 0.50 0.75 1.00 1.25 1.50
VFB(GEN) (V)
2926 F22
Figure 22. Regions of Possible
Slave Control Without a MOSFET
2926fa
23
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