SLMA002 datasheet(33/46 Pages) TI1

33 / 46 page

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RIPPLE1

RIPPLE1( total)

OUT1

max

RIPPLE1

0.661A

0.050 V

8.2 F

600 kHz

ESR1

0.024

0.661A

(42)

(

)

RIPPLE

RIPPLE(total)

OUT1

max

RIPPLE

0.547 A

0.050 V

8 12.4 F

600 kHz

ESR

0.033

0.547 A

(43)

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)

(

)

RMS _ CIN

OUT

1 D

3 A

0.5

1.5 A

(44)

BIAS

OUT

(45)

(

)

1.5 10

(1.5 10

6.68 10

)

OUT1

600000

5.82 10

13.2

5.0

19.7

50 10

19.7

50 10

8.2 H

TPS55383

,, TPS55386

www.ti.com ......................................................................................................................................................................................... SLUS818 – SEPTEMBER 2008

The maximum ESR to meet the ripple specification is given by:

A single 22-

µF ceramic capacitor with approximately 2.5 mΩ of ESR is selected to provide sufficient margin for

capacitance loss due to DC voltage bias.

Input Capacitor Selection

The TPS55386 datasheet recommends a 10

µF (minimum) ceramic bypass capacitor on each PVDD pin. While

out of phase operation reduces input RMS current, the input capacitors must be sized to support the greater of

the two input RMS currents, or 1.5A to allow operation when one channel is at maximum load and the other is

un-loaded. The ceramic capacitor must handle the RMS input ripple current of the converter.

The RMS current in the input capacitors is estimated by:

One 1210 size 10-

µF, 25-V, X5R ceramic capacitor with a 2-mΩ ESR and a 2-A RMS current rating are selected

to bypass each PVDD input. Higher voltage capacitors minimize capacitance loss under DC bias voltage,

ensuring the capacitors have sufficient capacitance at their working voltage.

Voltage Feedback

The primary feedback divider resistor (RFB) from VOUT to FB should be selected between 10 kΩ and 100 kΩ to

maintain a balance between power dissipation and noise sensitivity. For a 3.3-V and 5-V output, 20.5 k

Ω is

selected, so the lower resistor is given by:

For RFB = R2 = R9 = 20.5 kΩ and VFB = 0.80V, RBIAS1 = 3.90kΩ and RBIAS2 = 6.5kΩ (R4 = 3.83kΩ and

R7 = 6.49 k

Ω selected) for 5.0 V and 3.3 V respectively.

Compensation Components

The TPS55386 controller uses an internal transconductance error amplifier, which compares the feedback

voltage to the internal 0.80-V reference and sources a current proportional to the resulting error out of the COMP

pin. A series resistor and capacitor to ground generate an integrator with zero while a high frequency capacitor

provides a second pole to reduce the high frequency gain. The compensation loop components are selected by

the following equations with the 5.0-V output used in example calculations:

Calculate the modulator gain at DC:

(46)

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