LM20124 datasheet(17/28 Pages) TI

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tSS =

0.8V x CSS

ISS

CC2 =

COUT x RESR

RC1

fZ(FIL) =

2 x S x COUT x RESR

LM20124, LM20124-Q1

www.ti.com

SNVS507F – OCTOBER 2007 – REVISED MARCH 2013

If the output filter zero, fZ(FIL) approaches the crossover frequency (FC), an additional capacitor (CC2) 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 fSW/2 degrades the phase margin. The output filter zero

is set by the output capacitor value and ESR as shown in the equation below.

(9)

If needed, the value for CC2 should be calculated using the equation shown below.

(10)

Where RESR is the output capacitor series resistance and RC1 is the calculated compensation resistance.

AVIN FILTERING COMPONENTS (CF and RF)

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 25. as CF and RF. The required value for RF is 1Ω. CF must be used. Recommended value of CF is 1.0

µF. The filter capacitor, CF 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 CF.

SUB-REGULATOR BYPASS CAPACITOR (CVCC)

The capacitor at the VCC pin provides noise filtering and stability for the internal sub-regulator. The

recommended value of CVCC 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 recommended for most

applications.

SETTING THE START UP TIME (CSS)

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 CSS will result in longer start up times. Table 3 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)

CSS (nF)

none

100

120

If different start up times are needed the equation shown below can be used to calculate the start up time.

(11)

As shown above, the start up time is influenced by the value of the Soft-Start capacitor CSS(F) and the 5 µA Soft-

Start pin current ISS(A). that may be found in the electrical characteristics 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 CSS 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 M

Ω resistor from SS/TRK to ground can be

used without greatly affecting the start-up time.

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