LTC2923IDE datasheet(16/20 Pages) LINER

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LTC2923

2923fa

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:

where V

TRACK

10 8

⎛

⎝

⎜

⎞

⎠

⎟ <=

It is possible to choose resistor values that require the

LTC2923 to supply more current than the Electrical Char-

acteristics table guarantees. To avoid this condition, check

that ITRACKx does not exceed 1mA and IRAMPBUF does not

exceed ±2mA.

To confirm that ITRACKx < 1mA, the TRACKx pin’s maxi-

mum guaranteed current, verify that:

TRACK

< 1

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

sink more then 2mA when it is at 0V or be forced to source

more than 2mA when it is at VCC.

mA and

TRACK

Caution with Boost and Linear Regulators

Note that the LTC2923’s tracking cell is not able to control

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

to control one of these types of supplies, where the output

is not controllable through its feedback node, the series

FET can be used to control one supply’s output. For

example, boost regulators commonly 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 LTC2923’s track-

ing 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 drop-

out regulators (LDOs), may be problematic. Linear regu-

lators commonly contain circuitry that prevents driving

their outputs below their reference voltage. This may not

be obvious from the data sheets, so lab testing is recom-

mended whenever the LTC2923’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

resistive 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 18 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 immediately when

their input supplies are applied. If the LTC2923 tries to

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

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

defeating the tracking circuit (Figure 19). Often this delay

is intentionally configured by a soft-start capacitor. This

can be remedied either by reducing the soft-start capacitor

on the slave supply or by including a capacitor in the ON

pin’s resistive divider to delay the ramp up. See Figure 20.

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