LM2694MTSLASHNOPB datasheet(9/26 Pages) TI

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tON(min) =

VOUT x 305 ns

(VIN(min) - VOUT)

RON =

VOUT x (VIN ± 1.5V)

FS x 1.14 x 10

-10 x V

- 1.4 k:

tON =

VIN ± 1.5V

1.14 x 10

-10 x (R

ON + 1.4 k

+ 95 ns

LM2694

BST

VCC

ISEN

SGND

VOUT1

LM2694

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SNVS444A – MAY 2006 – REVISED APRIL 2013

Figure 10. Self Biased Configuration

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 inductor (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 RON resistor and the input voltage (VIN), and is calculated from:

(5)

See Figure 6. The inverse relationship with VIN results in a nearly constant frequency as VIN is varied. To set a

specific continuous conduction mode switching frequency (FS), the RON resistor is determined from the following:

(6)

In high frequency applications the minimum value for tON 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 VIN. The minimum allowed on-time to regulate the desired VOUT at the minimum

VIN is determined from the following:

(7)

The LM2694 can be remotely shut down by taking the RON/SD pin below 0.8V. See Figure 11. 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 VIN and the RON resistor.

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LM2694MT/NOPB Datasheet(PDF) 9 Page - Texas Instruments

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