LM2696MXAXSLASHNOPB datasheet(13/28 Pages) TI

13 / 28 page

tSS_MIN =

COUTVOUT

tSS =

1.25V CSS

ISS

LM2696

www.ti.com

SNVS375B – OCTOBER 2005 – REVISED APRIL 2013

AVIN CAPACITOR

AVIN is the analog bias rail of the device. It should be bypassed externally with a small (1 µF) ceramic capacitor

to prevent unwanted noise from entering the device. In a shutdown state the current needed by AVIN will drop to

approximately 12 µA, providing a low power sleep state.

In most cases of operation, AVIN is connected to PVIN; however, it is possible to have split rail operation where

AVIN is at a higher voltage than PVIN. AVIN should never be lower than PVIN. Splitting the rails allows the power

conversion to occur from a lower rail than the AVIN operating range.

SOFT-START CAPACITOR

The SS capacitor is used to slowly ramp the reference from 0V to its final value of 1.25V (during shutdown this

pin will be discharged to 0V). This controlled startup ability eliminates large in-rush currents in an attempt to

charge up the output capacitor. By changing the value of this capacitor, the duration of the startup may be

changed accordingly. The startup time may be calculated using the following relationship:

Where

•

ISS is the soft-start pin source current (1 µA typical) that may be found in the ELECTRICAL

CHARACTERISTICS table.

(16)

While the CSS capacitor can be sized to meet the startup requirements, there are limitations to its size. If the

capacitor is too small, the soft-start will have little effect as the reference voltage is rising faster than the output

capacitor can be charged causing the part to go into current limit. Therefore a minimum soft-start time should be

taken into account. This can be determined by:

(17)

While COUT and VOUT control the slew rate of the output voltage, the total amount of time the LM2696 takes to

startup is dependent on two other terms. See the “ Startup” section for more information.

EXTVCC CAPACITOR

External VCC is a 3.65V rail generated by an internal sub-regulator that powers the parts internal circuitry. This

rail should be bypassed with a 1 µF ceramic capacitor (X5R or equivalent dielectric). Although EXTVCC is for

internal use, it can be used as an external rail for extremely light loads (<50 µA). If EXTVCC is accidentally

shorted to GND the part is protected by a 5 mA current limit. This rail also has an under-voltage lockout that will

prevent the part from switching if the EXTVCC voltage drops.

SHUTDOWN

The state of the shutdown pin enables the device or places it in a sleep state. This pin has an internal pull-up

and may be left floating or connected to a high logic level. Connecting this pin to GND will shutdown the part.

Shutting down the part will prevent the part from switching and reduce the quiescent current drawn by the part.

This pin must be bypassed with a 1 nF ceramic capacitor (X5R or Y5V) to ensure proper logic thresholds.

CBOOT CAPACITOR

The purpose of an external bootstrap capacitor is to turn the FET on by using the SW node as a pedestal. This

allows the voltage on the CBOOT pin to be greater than VIN. Whenever the catch diode is conducting and the

SW node is at GND, an internal diode will conduct that charges the CBOOT capacitor to approximately 4V.

When the SW node rises, the CBOOT pin will rise to approximately 4V above the SW node. For optimal

performance, a 0.1 µF ceramic capacitor (X5R or equivalent dielectric) should be used.

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