30074123

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

LM22677 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 LM22677. 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

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 LM22677 this current can

be as high as 7.1A (typical). Assuming a worst case 1V drop

across the diode, the maximum diode power dissipation can

be as high as 7.1W.

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