LM3000 datasheet(21/41 Pages) TI1

21 / 41 page

ICIN(RMS) | IO x

D x (1 ± D)

PCIN

| IO

x D x (1 ± D) x RCIN

2 x S x CO x VP

fC t

2 x VP x VL

L x 'IO

CO t

VP x VL

L x 'IO

CO t

1 +

1 -

RC x 'IO

VP x VL

L x 'IO

CO t

RC d

'IO

LM3000

www.ti.com

SNVS612A – JULY 2009 – REVISED JULY 2009

Output Capacitor Design Procedure

For the design example VIN = 12V, VOUT = 3.3V, D = VOUT / VIN = 0.275, L = 2.7 μH, ΔIL = 1.8A, ΔIO = 8A and VP

= 0.15V.

To meet the transient voltage specification, the maximum RC is:

(28)

For the design example, the maximum RC is 18.75 mΩ. Choose RC = 15 mΩ as the design limit.

From the equation for VP, the minimum value of CO is:

(29)

For D < 0.5, VL = VOUT

For D > 0.5, VL = VIN - VOUT

With RC = VP / ΔIO this reduces to:

(30)

With RC = 0 this reduces to:

(31)

Since D < 0.5, VL = VOUT. With RC = 15 mΩ, the minimum value for CO is 218 μF.

The minimum control loop bandwidth fC is given by:

(32)

For the design example, the minimum value for fC is 39 kHz. A 220 μF, 15 mΩ polymer capacitor in parallel with

a 22

μF, 3 mΩ ceramic will meet the target output voltage ripple and transient specification.

For the 1.2V, 15A output, two 220

μF, 15 mΩ polymer capacitors in parallel with a 22 μF, 3 mΩ ceramic are

chosen to meet the target design specifications.

INPUT CAPACITORS

The input capacitors for a buck regulator are used to smooth the large current pulses drawn by the inductor and

load when the high-side MOSFET is on. Due to this large ac stress, input capacitors are usually selected on the

basis of their ac rms current rating rather than bulk capacitance. Low ESR is beneficial because it reduces the

power dissipation in the capacitors. Although any of the capacitor types mentioned in the Output Capacitor

section can be used, ceramic capacitors are common because of their low series resistance. In general the input

to a buck converter does not require as much bulk capacitance as the output.

The input capacitors should be selected for rms current rating and minimum ripple voltage. The equation for the

rms current and power loss of the input capacitor in a single phase can be estimated as:

(33)

Where IO (A) is the output load current and RCIN (Ω) is the series resistance of the input capacitor. Since the

maximum values occur at D = 0.5, a good estimate of the input capacitor rms current rating in a single phase is

one-half of the maximum output current.

Neglecting the series inductance of the input capacitance, the input voltage ripple for a single phase can be

estimated as:

Submit Documentation Feedback

Product Folder Links: LM3000

LM3000 Datasheet(PDF) 21 Page - Texas Instruments

LM3000 Datasheet(HTML) 21 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

Datasheet Download

Link URL