LM2745 datasheet(14/23 Pages) NSC

14 / 23 page

Application Information (Continued)

will be reduced in turn to a flat level equal to the current limit

threshold. The third benefit of the soft-start capacitor dis-

charge is a smooth, controlled ramp of output voltage when

the current limit condition is cleared.

SHUTDOWN

If the shutdown pin is pulled low, (below 0.8V) the LM2745/8

enters shutdown mode, and discharges the soft-start capaci-

tor through a MOSFET switch. The high and low-side MOS-

FETs are turned off. The LM2745/8 remains in this state as

long as V

SD sees a logic low (see the Electrical Character-

istics table). To assure proper IC start-up the shutdown pin

should not be left floating. For normal operation this pin

should be connected directly to V

CC or to another voltage

between 1.3V to V

CC (see the Electrical Characteristics

table).

DESIGN CONSIDERATIONS

The following is a design procedure for all the components

needed to create the Typical Application Circuit shown on

the front page. This design converts 3.3V (V

IN) to 1.2V

OUT) at a maximum load of 4A with an efficiency of 89%

and a switching frequency of 300 kHz. The same procedures

can be followed to create many other designs with varying

input voltages, output voltages, and load currents.

Input Capacitor

The input capacitors in a Buck converter are subjected to

high stress due to the input current trapezoidal waveform.

Input capacitors are selected for their ripple current capabil-

ity and their ability to withstand the heat generated since that

ripple current passes through their ESR. Input rms ripple

current is approximately:

Where duty cycleD=V

OUT/VIN.

The power dissipated by each input capacitor is:

where n is the number of paralleled capacitors, and ESR is

the equivalent series resistance of each capacitor. The equa-

tion above indicates that power loss in each capacitor de-

creases rapidly as the number of input capacitors increases.

The worst-case ripple for a Buck converter occurs during full

load and when the duty cycle (D) is 0.5. For this 3.3V to 1.2V

design the duty cycle is 0.364. For a 4A maximum load the

ripple current is 1.92A.

Output Inductor

The output inductor forms the first half of the power stage in

a Buck converter. It is responsible for smoothing the square

wave created by the switching action and for controlling the

output current ripple (

∆I

OUT). The inductance is chosen by

selecting between tradeoffs in efficiency and response time.

The smaller the output inductor, the more quickly the con-

verter can respond to transients in the load current. How-

ever, as shown in the efficiency calculations, a smaller in-

ductor requires a higher switching frequency to maintain the

same level of output current ripple. An increase in frequency

can mean increasing loss in the MOSFETs due to the charg-

ing and discharging of the gates. Generally the switching

frequency is chosen so that conduction loss outweighs

switching loss. The equation for output inductor selection is:

L = 1.6 µH

Here we have plugged in the values for output current ripple,

input voltage, output voltage, switching frequency, and as-

sumed a 40% peak-to-peak output current ripple. This yields

an inductance of 1.6 µH. The output inductor must be rated

to handle the peak current (also equal to the peak switch

current), which is (I

OUT + (0.5 x

∆I

OUT)) = 4.8A, for a 4A

design.

The Coilcraft DO3316P-222P is 2.2 µH, is rated to 7.4A

peak, and has a direct current resistance (DCR) of 12 m

Ω.

After selecting the Coilcraft DO3316P-222P for the output

inductor, actual inductor current ripple should be re-

calculated with the selected inductance value, as this infor-

mation is needed to select the output capacitor. Re-

arranging the equation used to select inductance yields the

following:

IN(MAX) is assumed to be 10% above the steady state input

voltage, or 3.6V at V

IN = 3.3V. The re-calculated current

ripple will then be 1.2A. This gives a peak inductor/switch

current will be 4.6A.

Output Capacitor

The output capacitor forms the second half of the power

stage of a Buck switching converter. It is used to control the

output voltage ripple (

∆V

OUT) and to supply load current

during fast load transients.

In this example the output current is 4A and the expected

type of capacitor is an aluminum electrolytic, as with the

input capacitors. Other possibilities include ceramic, tanta-

lum, and solid electrolyte capacitors, however the ceramic

type often do not have the large capacitance needed to

supply current for load transients, and tantalums tend to be

more expensive than aluminum electrolytic. Aluminum ca-

pacitors tend to have very high capacitance and fairly low

ESR, meaning that the ESR zero, which affects system

stability, will be much lower than the switching frequency.

The large capacitance means that at the switching fre-

quency, the ESR is dominant, hence the type and number of

output capacitors is selected on the basis of ESR. One

simple formula to find the maximum ESR based on the

desired output voltage ripple,

∆V

OUT and the designed out-

put current ripple,

∆I

OUT, is:

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