LM46002PWPR datasheet(30/52 Pages) TI

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SSC

FBT

FBB

2 f

/ /R

)

OUT

4.35

LM46002

SNVSA13B – APRIL 2014 – REVISED SEPTEMBER 2014

www.ti.com

9.2.2.6 Feed-Forward Capacitor

The LM46002 is internally compensated and the internal R-C values are 400 k

Ω 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 feedforward capacitor

CFF can be added in parallel with RFBT. CFF is chosen such 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)

The following equation for CFF was tested:

(22)

This equation 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 neeed 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 47 pF COG capacitor is selected.

9.2.2.7 Bootstrap Capacitors

Every LM46002 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.

9.2.2.8 VCC Capacitor

The VCC pin is the output of an internal LDO for LM46002. The input for this LDO comes from either VIN or

BIAS (please refer to Functional Block Diagram for LM46002). To insure stability of the part, place a minimum of

2.2 µF, 10 V capacitor from this pin to ground.

9.2.2.9 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

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

will affect 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.

9.2.2.10 Soft-Start Capacitors

The user can leave the SS/TRK pin floating and the LM46002 will implement a soft start time of 4.1 ms typically.

In order to use an external soft start capacitor, the capacitor should be sized such that the soft start time will be

longer than 4.1 ms. Use the following equation in order to calculate the soft start capacitor value:

(23)

Where,

CSS = Soft start capacitor value (µF)

ISSC = Soft start charging current (µA)

tSS = Desired soft start time (s)

For the desired soft start time of 10 ms and soft start charging current of 2.2 µA, the equation above yield a soft

start capacitor value of 0.022 µF.

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