Datasheet

Total Startup Time
T
SS
200 Ps
730 Ps
V
OUT
ExtV
CC
V
IN
LM2696
SNVS375B –OCTOBER 2005–REVISED APRIL 2013 www.ti.com
PGOOD RESISTOR
The PGOOD resistor is used to pull the PGOOD pin high whenever a steady state operating range is achieved.
This resistor needs to be sized to prevent excessive current from flowing into the PGOOD pin whenever the open
drain FET is turned on. The recommendation is to use a 10 kΩ–100 kΩ resistor. This range of values is a
compromise between rise time and power dissipation.
CATCH DIODE
The catch or freewheeling diode acts as the bottom switch in a non-synchronous buck switcher. Because of this,
the diode has to handle the full output current whenever the FET is not conducting. Therefore, it must be sized
appropriately to handle the current. The average current through the diode can be calculated by the equation:
I
D_AVG
= I
OUT
•(1–D) (18)
Care should also be taken to ensure that the reverse voltage rating of the diode is adequate. Whenever the FET
is conducting the voltage across the diode will be approximately equal to V
IN
. It is recommended to have a
reverse rating that is equal to 120% of V
IN
to ensure adequate guard banding against any ringing that could
occur on the switch node.
Selection of the catch diode is critical to overall switcher performance. To obtain the optimal performance, a
Schottky diode should be used due to their low forward voltage drop and fast recovery.
BYPASS CAPACITOR
A bypass capacitor must be used on the AV
IN
line to help decouple any noise that could interfere with the analog
circuitry. Typically, a small (1 µF) ceramic capacitor is placed as close as possible to the AVIN pin.
EXTERNAL OPERATION STARTUP
The total startup time, from the initial V
IN
rise to the time V
OUT
reaches its nominal value is determined by three
separate steps. Upon the rise of V
IN
, the first step to occur is that the EXTV
CC
voltage has to reach its nominal
output voltage of 3.65V before the internal circuitry is active. This time is dictated by the output capacitance (1
µF) and the current limit of the regulator (5 mA typical), which will always be on the order of 730 µs. Upon
reaching its steady state value, an internal delay of 200 µs will occur to ensure stable operation. Upon
completion the LM2696 will begin switching and the output will rise. The rise time of the output will be governed
by the soft-start capacitor. To highlight these three steps a timing diagram please refer to Figure 20.
Figure 20. Startup Timing Diagram
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