LTC2927CDDB datasheet(13/16 Pages) LINER

13 / 16 page

LTC2927

2927fb

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:

TRACK

MASTER

⎛

⎝⎜

⎞

⎠⎟

Here, VTRACK = 0.8V. VMASTER is the ﬁnal voltage of the

master signal (VCC if RAMP pin).

It is possible to choose resistor values that require the

LTC2927 to supply more current than the Electrical Char-

acteristics table guarantees. To avoid this condition, check

that ITRACK does not exceed 1mA and IRAMPBUF does not

exceed ±2mA.

To conﬁrm that ITRACK < 1mA, the TRACK pin’s maximum

guaranteed current, verify that:

TRACK

< 1

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

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

more than 2mA when it is at VMASTER.

mA and

TRACK

MASTER

ON Pin Resistive Divider

Check that the ON pin voltage is above the 1.25V maximum

threshold at the lowest possible supply voltage value.

ONB

ONA

CC MIN

()

–

125

Also check that the supply voltage is above the minimum

LTC2927 operating supply voltage of 2.9V before the ON

pin is above the 1.21V minimum threshold voltage.

Figure 17. Weak Resistive Load

2927 F17

10ms/DIV

1V/DIV

MASTER

SLAVE

ONB

ONA

121

–

For example, if the typical application shown on page 1

has a 3.3V ±10% VIN, the lowest possible operating supply

voltage will be 2.97.

ONB

ONA

297

125

1 1 376

–.

If RONA is 100k then RONB must be greater than 137.6k.

Therefore, 138k is chosen. These values must be checked

to ensure the supply reaches the LTC2927 minimum op-

erating supply voltage of 2.9V before the ON pin is above

the minimum threshold.

138

121

1 1 389

–.

Load Requirements

When the supply is 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 17 shows the

case when the slave supply does not track the master

near ground.

APPLICATIO S I FOR ATIO

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