LTC2925

17

2925f

1V/DIV

MASTER

SLAVE2

SLAVE1

1ms/DIV

2925 F19

APPLICATIO S I FOR ATIO

Final Sanity Checks

The collection of equations below is useful for identifying

unrealizable solutions.

As stated in step 2, the slave supply must finish ramping

before the master signal has reached its final voltage. This

can be verified by the following equation:

V

R

R

V

TRACK

TB

TA

MASTER

1

+









<

master signal, either the supply voltage ramped up through

It is possible to choose resistor values that require the

LTC2925 to supply more current than the Electrical Char-

acteristics table guarantees. To avoid this condition, check

exceed

±3mA.

mum guaranteed current, verify that:

V

RR

mA

TRACK

TA

TB

< 1

Finally, check that the RAMPBUF pin will not be forced to

sink more than 3mA when it is at 0V or be forced to source

V

RR

V

RR

V

RR

mA and

V

RR

V

RR

V

RR

mA

TRACK

TA

TB

TRACK

TA

TB

TRACK

TA

TB

MASTER

TA

TB

MASTER

TA

TB

MASTER

TA

TB

1

1

22

33

1

1

22

33

3

3

++

<

+

+

+

+

+

<

Caution with Boost Regulators and Linear Regulators

Note that the LTC2925’s tracking cell is not able to control

the outputs of all types of power supplies. If it is necessary

to control a supply, where the output is not controllable

through its feedback node, the series FET can be used to

control its output. For example, boost regulators com-

monly contain an inductor and diode between the input

supply and the output supply providing a DC current path

when the output voltage falls below the input voltage.

Therefore, the LTC2925’s tracking cell will not effectively

drive the supply’s output below the input.

Special caution should be taken when considering the use

of linear regulators. Three-terminal linear regulators have

a reference voltage that is referred to the output supply

rather than to ground. In this case, driving current into the

regulator’s feedback node will cause its output to rise rather

than fall. Even linear regulators that have their reference

voltage referred to ground, including low-dropout regula-

tors (LDOs), may be problematic. Linear regulators com-

monly contain circuitry that prevents driving their outputs

below their reference voltage. This may not be obvious

from the datasheets, so lab testing is recommended when-

ever the LTC2925’s tracking cell is used to control linear

regulators.

Load Requirements

When the supplies are ramped down quickly, either the

load or the supply itself must be capable of sinking enough

current to support the ramp rate. For example, if there is

a large output capacitance on the supply and a weak resis-

tive load, supplies that do not sink current will have their

falling ramp rate limited by the RC time constant of the load

and the output capacitance. Figure 19 shows the case when

the 2.5V supply does not track the 1.8V and 3.3V supplies

near ground.

Start-Up Delays

Often power supplies do not start-up immediatley when

their input supplies are applied. If the LTC2925 tries to

ramp-up these power supplies as soon as the input supply

is present, the start-up of the outputs may be delayed

Figure 19. Weak Resistive Load