Datasheet

t
SS_MIN
=
C
OUT
V
OUT
3A
t
SS
=
1.25V C
SS
I
SS
LM2696
www.ti.com SNVS375B –OCTOBER 2005–REVISED APRIL 2013
AV
IN
CAPACITOR
AV
IN
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 AV
IN
will drop to
approximately 12 µA, providing a low power sleep state.
In most cases of operation, AV
IN
is connected to PV
IN
; however, it is possible to have split rail operation where
AV
IN
is at a higher voltage than PV
IN
. AV
IN
should never be lower than PV
IN
. Splitting the rails allows the power
conversion to occur from a lower rail than the AV
IN
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
• I
SS
is the soft-start pin source current (1 µA typical) that may be found in the ELECTRICAL
CHARACTERISTICS table. (16)
While the C
SS
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 C
OUT
and V
OUT
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.
EXTV
CC
CAPACITOR
External V
CC
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 EXTV
CC
is for
internal use, it can be used as an external rail for extremely light loads (<50 µA). If EXTV
CC
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 EXTV
CC
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 V
IN
. 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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