LTC2926IUFD View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2926IUFD' PDF : 28 Pages View PDF
LTC2926
APPLICATIO S I FOR ATIO
Final Sanity Checks
The collection of equations below is useful for identifying
unrealizable solutions.
As stated in step 3 of the design procedure, the slave
supply must finish ramping before the master signal has
reached its final voltage. This can be verified with the
following equation:
VMASTER
>
VTRACK
•
⎛
⎝⎜
1+
RTB
RTA
⎞
⎠⎟
Here, VTRACK = 0.8V. VMASTER is the final voltage of the
master signal, either the supply voltage ramped up through
the optional external MOSFET or VCC when no MOSFET
is present.
It is possible to choose resistor values that require the
LTC2926 to supply more current than the Electrical Char-
acteristics table guarantees. To avoid this condition, check
that each TRACK pin’s current, ITRACKn, does not exceed
1mA, and that the RAMPBUF pin current, IRAMPBUF, does
not exceed ±3mA.
To confirm that ITRACKn ≤ 1mA, verify that:
VTRACK ≤ 1mA
RTA ||RTB
Check that the RAMPBUF pin will not be forced to sink
more than 3mA when it is at 0V and will not be forced to
source more than 3mA when it is at VMASTER.
VTRACK + VTRACK ≤ 3mA and
RTA1||RTB1 RTA2 ||RTB2
VMASTER + VMASTER ≤ 3mA
RTA1 + RTB1 RTA2 + RTB2
Load Requirements
A weak resistive load can cause static and dynamic track-
ing errors. The behavior of the source-follower topology
of MOSFET-controlled tracking relies on the load’s abil-
ity to support the ramp rates and tracking currents of a
particular application. Consider the simplified slave load
schematic in Figure 23.
SUPPLY
MODULE
Qn
OUT
RFB
SGATEn
FBn
LTC2926
RFA
SLAVEn
LOAD
RL CL
2926 F23
Figure 23. Simplified Slave Supply Load
When the supplies are ramped down quickly, the load must
be capable of sinking enough current to support the ramp
rate. For example, if there is a large output capacitance and
a weak resistive load on a particular supply, that supply’s
falling rate will be limited by the RC time constant of the
load. In Figure 24, the falling 2.5V slave cannot keep up
with the falling master ramp.
When the supplies are near ground, the load must be
capable of sinking the tracking current without creating
a large offset voltage. For weak resistive loads and slave
voltage levels near ground, the tracking current (at its
maximum there) can be in excess of the load’s current
demand. Having no capability for sinking current, the
MOSFET shuts off. All of the mirrored tracking current that
flows through RFB also flows through the load resistance,
RL, which creates a voltage offset between the slave and
ground. In Figure 24, the 1.8V supply shows an offset
from ground.
2926fa
24
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