LM2694 datasheet(11/19 Pages) NSC

11 / 19 page

Start-Up Regulator, V

(Continued)

Regulation Comparator

The feedback voltage at FB is compared to the voltage at the

Softstart pin (2.5V). In normal operation (the output voltage

is regulated), an on-time period is initiated when the voltage

at FB falls below 2.5V. The buck switch stays on for the

programmed on-time, causing the FB voltage to rise above

2.5V. After the on-time period, the buck switch stays off until

the FB voltage falls below 2.5V. Input bias current at the FB

pin is less than 100 nA over temperature.

Over-Voltage Comparator

The voltage at FB is compared to an internal 2.9V reference.

If the voltage at FB rises above 2.9V the on-time pulse is

immediately terminated. This condition can occur if the input

voltage or the output load changes suddenly, or if the induc-

tor (L1) saturates. The buck switch remains off until the

voltage at FB falls below 2.5V.

ON-Time Timer, and Shutdown

The on-time for the LM2694 is determined by the R

ON resis-

tor and the input voltage (V

IN), and is calculated from:

(4)

See Figure 4. The inverse relationship with V

IN results in a

nearly constant frequency as V

IN is varied. To set a specific

continuous conduction mode switching frequency (F

S), the

ON resistor is determined from the following:

(5)

In high frequency applications the minimum value for t

ON is

limited by the maximum duty cycle required for regulation

and the minimum off-time of 265 ns, ±15%. The minimum

off-time limits the maximum duty cycle achievable with a low

voltage at V

IN. The minimum allowed on-time to regulate the

desired V

OUT at the minimum VIN is determined from the

following:

(6)

The LM2694 can be remotely shut down by taking the

RON/SD pin below 0.8V. See Figure 10. In this mode the SS

pin is internally grounded, the on-timer is disabled, and bias

currents are reduced. Releasing the RON/SD pin allows

normal operation to resume. The voltage at the RON/SD pin

is normally between 1.5V and 3.0V, depending on V

IN and

the R

ON resistor.

20187014

FIGURE 9. Self Biased Configuration

20187019

FIGURE 10. Shutdown Implementation

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