LM20145MHXSLASHNOPB datasheet(17/29 Pages) TI

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CC2 =

COUT x RESR

RC1

fZ(FIL) =

2 x S x COUT x RESR

RC1 =

CC1

COUT

IOUT

VOUT

10 x D

VIN

-1

1-D

fSW x L

COMP

CC1

RC1

CC2

LM20145

(optional)

LM20145

www.ti.com

SNVS530E – OCTOBER 2007 – REVISED MARCH 2013

Figure 29. Compensation Network for LM20145

A good starting value for CC1 for most applications is 4.7 nF. Once the value of CC1 is chosen the value of RC

should be calculated using the equation below to cancel the output filter pole (FP(FIL)) as shown in Figure 28.

(9)

A higher crossover frequency can be obtained, usually at the expense of phase margin, by lowering the value of

CC1 and recalculating the value of RC1. Likewise, increasing CC1 and recalculating RC1 will provide additional

phase margin at a lower crossover frequency. As with any attempt to compensate the LM20145 the stability of

the system should be verified for desired transient droop and settling time.

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.

(10)

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

where

•

RESR is the output capacitor series resistance

•

RC1 is the calculated compensation resistance

(11)

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 26. 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, shown below

provides a list of soft start capacitors and the corresponding typical start up times.

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