LM46000PWPR datasheet(32/56 Pages) TI2

32 / 56 page

SSC

FBT

FBB

2 f

/ /R

)

OUT

1.5

LM46000

SNVSA45B – JUNE 2014 – REVISED MARCH 2018

www.ti.com

Product Folder Links: LM46000

Submit Documentation Feedback

8.2.2.7 Feed-Forward Capacitor

The LM46000 is internally compensated and the internal R-C values are 400 kΩ and 50 pF, respectively.

Depending on the VOUT and frequency FS, if the output capacitor COUT is dominated by low ESR (ceramic types)

capacitors, it could result in low phase margin. To improve the phase boost an external feed-forward capacitor

CFF can be added in parallel with RFBT. Choose CFF so that phase margin is boosted at the crossover frequency

without CFF. A simple estimation for the crossover frequency without CFF (fx) is shown in Equation 21, assuming

COUT has very small ESR.

(21)

Equation 22 was tested for CFF:

(22)

Equation 22 indicates that the crossover frequency is geometrically centered on the zero and pole frequencies

caused by the CFF capacitor.

For designs with higher ESR, CFF is not required when COUT has very high ESR and CFF calculated from

Equation 22 should be reduced with medium ESR.Table 2 can be used as a quick starting point.

For the application in this design example, a 33-pF COG capacitor is selected.

8.2.2.8 Bootstrap Capacitors

Every LM46000 design requires a bootstrap capacitor, CBOOT. The recommended bootstrap capacitor is 0.47 μF

and rated at 6.3 V or higher. The bootstrap capacitor is located between the SW pin and the CBOOT pin. The

bootstrap capacitor must be a high-quality ceramic type with X7R or X5R grade dielectric for temperature

stability.

8.2.2.9 VCC Capacitor

The VCC pin is the output of an internal LDO for LM46000. The input for this LDO comes from either VIN or

BIAS (see Functional Block Diagram for LM46000). To insure stability of the part, place a minimum of 2.2-µF, 10-

V capacitor from this pin to ground.

8.2.2.10 BIAS Capacitors

For an output voltage of 3.3 V and greater, the BIAS pin can be connected to the output in order to increase light

load efficiency. This pin is an input for the VCC LDO. When BIAS is not connected, the input for the VCC LDO is

internally connected into VIN. Because this is an LDO, the voltage differences between the input and output

affects the efficiency of the LDO. If necessary, a capacitor with a value of 1 μF can be added close to the BIAS

pin as an input capacitor for the LDO.

8.2.2.11 Soft-Start Capacitors

The user can leave the SS/TRK pin floating, and the LM46000 implement a soft-start time of 4.1 ms typically. In

order to use an external soft-start capacitor, size the capacitor so that the soft-start time is longer than 4.1 ms.

Use Equation 23 to calculate the soft-start capacitor value:

where

•

CSS = Soft-start capacitor value (µF)

•

ISS = Soft-start charging current (µA)

•

tSS = Desired soft-start time (s)

(23)

For the desired soft start time of 10 ms and soft-start charging current of 2.2 µA, Equation 23 yields a soft-start

capacitor value of 0.022 µF.

LM46000PWPR Datasheet(PDF) 32 Page - Texas Instruments

LM46000PWPR Datasheet(HTML) 32 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

Datasheet Download

Link URL