Datasheet

V
OUT
V
EN
Voltage
Time
V
PGOOD
Soft Start Time (t
ss
)
90% V
OUT
(V
UVP
)
0V
Enable
Delay
(t
RESETSS
)
t
SS
x I
SS
= C
SS
0.6V
LM21215A
SNOSB87B –MARCH 2011–REVISED MARCH 2013 www.ti.com
SOFT START
When EN has exceeded 1.35V, and both PVIN and AVIN have exceeded the UVLO threshold, the LM21215A-1
will begin charging the output linearly to the voltage level dictated by the feedback resistor network. The
LM21215A-1 employs a user adjustable soft start circuit to lengthen the charging time of the output set by a
capacitor from the soft start pin to ground. After enable exceeds 1.35V, an internal 2 µA current source begins to
charge the soft start capacitor. This allows the user to limit inrush currents due to a high output capacitance and
not cause an over current condition. Adding a soft-start capacitor can also reduce the stress on the input rail.
Larger capacitor values will result in longer startup times. Use the equation below to approximate the size of the
soft-start capacitor:
where
• I
SS
is nominally 2 µA
• t
SS
is the desired startup time (4)
If V
IN
is higher than the UVLO level and enable is toggled high the soft start sequence will begin. There is a small
delay between enable transitioning high and the beginning of the soft start sequence. This delay allows the
LM21215A-1 to initialize its internal circuitry. Once the output has charged to 90% of the nominal output voltage
the power good flag will transition high. This behavior is illustrated in Figure 31.
Figure 31. Soft Start Timing
As shown above, the size of the capacitor is influenced by the nominal feedback voltage level 0.6V, the soft-start
charging current I
SS
(2 µA), and the desired soft start time. If no soft-start capacitor is used then the LM21215A-1
defaults to a minimum startup time of 500 µs. The LM21215A-1 will not startup faster than 500 µs. When enable
is cycled or the device enters UVLO, the charge developed on the soft-start capacitor is discharged to reset the
startup process. This also happens when the device enters short circuit mode from an over-current event.
INDUCTOR SELECTION
The inductor (L) used in the application will influence the ripple current and the efficiency of the system. The first
selection criteria is to define a ripple current, ΔI
L
. In a buck converter, it is typically selected to run between 20%
to 30% of the maximum output current. Figure 32 shows the ripple current in a standard buck converter operating
in continuous conduction mode. Larger ripple current will result in a smaller inductance value, which will lead to
lower inductor series resistance, and improved efficiency. However, larger ripple current will also cause the
device to operate in discontinuous conduction mode at a higher average output current.
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