Datasheet

f
SW
R
ADJ
=
54680
- 13.15
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
LM21212-2
www.ti.com SNVS715A –MARCH 2011–REVISED MARCH 2013
SOFT START
When EN has exceeded 1.35V, and both PVIN and AVIN have exceeded the UVLO threshold, the LM21212-2
will begin charging the output linearly to the voltage level dictated by the feedback resistor network. The
LM21212-2 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:
(4)
where I
SS
is nominally 2 µA and t
SS
is the desired startup time. 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 LM21212-2 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 28.
Figure 28. 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 LM21212-2
defaults to a minimum startup time of 500 µs. The LM21212-2 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.
RESISTOR-ADJUSTABLE FREQUENCY
The frequency adjust (FADJ) pin allows the LM21212-2 to be programmed to a predetermined switching
frequency between 300 kHz to 1.55 MHz by connecting a resistor between FADJ and AGND. To determine the
resistor (R
ADJ
) value for a desired frequency, the following equation can be used:
(5)
where R
ADJ
is resistance in kΩ, and f
SW
is frequency in kHz. The desired frequency must fall within the
operational frequency range, 300 kHz to 1550 kHz, and a corresponding resistor must be used for normal
operation.
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