LTC2926IGN datasheet(24/28 Pages) LINER

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LTC2926

2926fa

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:

MASTER

TRACK

>• +

⎛

⎝⎜

⎞

⎠⎟

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:

TRACK

≤1

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.

TRACK

+≤

and

MASTER

≤

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.

Figure 23. Simpliﬁed Slave Supply Load

LOAD

SLAVEn

2926 F23

OUT

SUPPLY

MODULE

LTC2926

SGATEn

FBn

RFB

RFA

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.

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