LM22671 datasheet(12/18 Pages) NSC

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nected to the FB pin. Other output voltages can use the -ADJ

option with a resistor divider.

The resistor values can be determined by the following equa-

tions:

-ADJ option:

-5.0 option:

Where V

FB = 1.285V typical for the -ADJ option and 5V for the

-5.0 option

30076823

FIGURE 5. Resistive Feedback Divider

A maximum value of 10 k

Ω is recommended for the sum of

R1 and R2 to keep high output voltage accuracy for the –ADJ

option. A maximum of 2 k

Ω is recommended for the -5.0 out-

put voltage option. For the 5V fixed output voltage option, the

total internal divider resistance is typically 9.93 k

Ω.

At loads less than 5 mA, the boot capacitor will not hold

enough charge to power the internal high side driver. The

output voltage may droop until the boot capacitor is

recharged. Selecting a total feedback resistance to be below

3 k

Ω will provide some minimal load and can keep the output

voltage from collapsing in such low load conditions.

Catch Diode

A Schottky type re-circulating diode is required for all

LM22671 applications. Ultra-fast diodes which are not Schot-

tky diodes are not recommended and may result in damage

to the IC due to reverse recovery current transients. The near

ideal reverse recovery characteristics and low forward volt-

age drop of Schottky diodes are particularly important diode

characteristics for high input voltage and low output voltage

applications common to the LM22671. The reverse recovery

characteristic determines how long the current surge lasts

each cycle when the N-channel MOSFET is turned on. The

reverse recovery characteristics of Schottky diodes mini-

mizes the peak instantaneous power in the switch occurring

during turn-on for each cycle. The resulting switching losses

are significantly reduced when using a Schottky diode. The

reverse breakdown rating should be selected for the maxi-

mum V

IN, plus some safety margin. A rule of thumb is to select

a diode with the reverse voltage rating of 1.3 times the max-

imum input voltage.

The forward voltage drop has a significant impact on the con-

version efficiency, especially for applications with a low output

voltage. ‘Rated’ current for diodes varies widely from various

manufacturers. The worst case is to assume a short circuit

load condition. In this case the diode will carry the output cur-

rent almost continuously. For the LM22671 this current can

be as high as 700 mA (typical). Assuming a worst case 1V

drop across the diode, the maximum diode power dissipation

can be as high as 700 mW.

Circuit Board Layout

Board layout is critical for switching power supplies. First, the

ground plane area must be sufficient for thermal dissipation

purposes. Second, appropriate guidelines must be followed

to reduce the effects of switching noise. Switch mode con-

verters are very fast switching devices. In such devices, the

rapid increase of input current combined with the parasitic

trace inductance generates unwanted L di/dt noise spikes.

The magnitude of this noise tends to increase as the output

current increases. This parasitic spike noise may turn into

electromagnetic interference (EMI) and can also cause prob-

lems in device performance. Therefore, care must be taken

in layout to minimize the effect of this switching noise.

The most important layout rule is to keep the AC current loops

as small as possible. Figure 6 shows the current flow of a buck

converter. The top schematic shows a dotted line which rep-

resents the current flow during the FET switch on-state. The

middle schematic shows the current flow during the FET

switch off-state.

The bottom schematic shows the currents referred to as AC

currents. These AC currents are the most critical since current

is changing in very short time periods. The dotted lines of the

bottom schematic are the traces to keep as short as possible.

This will also yield a small loop area reducing the loop induc-

tance. To avoid functional problems due to layout, review the

PCB layout example. Best results are achieved if the place-

ment of the LM22671, the bypass capacitor, the Schottky

diode and the inductor are placed as shown in the example.

It is also recommended to use 2oz copper boards or thicker

to help thermal dissipation and to reduce the parasitic induc-

tances of board traces.

30076824

FIGURE 6. Current Flow in a Buck Application

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