Datasheet

function is set by the feedback resistor ratio, error amp gain,
and external compensation network.
To achieve a -20dB/decade slope, the error amplifier zero,
located at f
Z(EA)
, should positioned to cancel the output filter
pole (f
P(FIL)
). An additional error amp pole, located at f
P2(EA)
,
can be added to cancel the output filter zero at f
Z(FIL)
. Can-
cellation of the output filter zero is recommended if larger
value, non-ceramic output capacitors are used.
Compensation of the LM20124 is achieved by adding an RC
network as shown in Figure 5 below.
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FIGURE 5. Compensation Network for LM20124
A good starting value for C
C1
for most applictions is 4.7 nF.
Once the value of C
C1
is chosen the value of R
C1
should be
calculated using the equation below to cancel the output filter
pole (f
P(FIL)
) as shown in Figure 4.
A higher crossover frequency can be obtained, usually at the
expense of phase margin, by lowering the value of C
C1
and
recalculating the value of R
C1
. Likewise, increaseing C
C1
and
recalculating R
C1
will provide additional phase margin at a
lower crossover frequency. As with any attempt to compen-
sate the LM20124 the stability of the system should be verified
for desired transient droop and settling time.
If the output filter zero, f
Z(FIL)
approaches the crossover fre-
quency (F
C
), an additional capacitor (C
C2
) should be placed
at the COMP pin to ground. This capacitor adds a pole to
cancel the output filter zero assuring the crossover frequency
will occur before the double pole at f
SW
/2 degrades the phase
margin. The output filter zero is set by the output capacitor
value and ESR as shown in the equation below.
If needed, the value for C
C2
should be calculated using the
equation shown below.
Where R
ESR
is the output capacitor series resistance and
R
C1
is the calculated compensation resistance.
AVIN FILTERING COMPONENTS (C
F
and R
F
)
To prevent high frequency noise spikes from disturbing the
sensitive analog circuitry connected to the AVIN and AGND
pins, a high frequency RC filter is required between PVIN and
AVIN. These components are shown in Figure 2. as C
F
and
R
F
. The required values for R
F
and C
F
are 1Ω and 1 µF re-
spectively. The filter capacitor, C
F
should be placed as close
to the IC as possible with a direct connection from AVIN to
AGND. A good quality X5R or X7R ceramic capacitor should
be used for C
F
.
SUB-REGULATOR BYPASS CAPACITOR (C
VCC
)
The capacitor at the VCC pin provides noise filtering and sta-
bility for the internal sub-regulator. The recommended value
of C
VCC
should be no smaller than 1 µF and no greater than
10 µF. The capacitor should be a good quality ceramic X5R
or X7R capacitor. In general, a 1 µF ceramic capacitor is rec-
ommended for most applications.
SETTING THE START UP TIME (C
SS
)
The addition of a capacitor connected from the SS pin to
ground sets the time at which the output voltage will reach the
final regulated value. Larger values for C
SS
will result in longer
start up times. Table 3, shown below provides a list of soft
start capacitors and the corresponding typical start up times.
TABLE 3. Start Up Times for Different Soft-Start
Capacitors
Start Up Time (ms) C
SS
(nF)
1 none
5 33
10 68
15 100
20 120
If different start up times are needed the equation shown be-
low can be used to calculate the start up time.
As shown above, the start up time is influenced by the value
of the Soft-Start capacitor C
SS
(F) and the 5 µA Soft-Start pin
current I
SS
(A). that may be found in the electrical character-
istics table.
While the Soft-Start capacitor can be sized to meet many start
up requirements, there are limitations to its size. The Soft-
Start time can never be faster than 1ms due to the internal
default 1 ms start up time. When the device is enabled there
is an approximate time interval of 50 µs when the Soft-Start
capacitor will be discharged just prior to the Soft-Start ramp.
If the enable pin is rapidly pulsed or the Soft-Start capacitor
is large there may not be enough time for C
SS
to completely
discharge resulting in start up times less than predicted. To
aid in discharging the Soft-Start capacitor during long disable
periods an external 1 MΩ resistor from C
SS
to ground can be
used without greatly affecting the start-up time.
USING PRECISION ENABLE AND POWER GOOD
The precision enable (EN) and power good (PGOOD) pins of
the LM20124 can be used to address many sequencing re-
quirements. The turn-on of the LM20124 can be controlled
with the precision enable pin by using two external resistors
as shown in Figure 6.
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LM20124