LM22677EVAL/NOPB National Semiconductor, LM22677EVAL/NOPB Datasheet - Page 11

• Manufacturer Part Number: LM22677EVAL/NOPB
• Description: BOARD EVALUATION FOR LM22677
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r

Specifications of LM22677EVAL/NOPB

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 5A
• Voltage - Input: 4.5 ~ 42V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM22677
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Other names: *LM22677EVAL; LM22677EVAL

A maximum value of 10 kΩ is recommended for the sum of
R1 and R2 to keep high output voltage accuracy for the –ADJ
option. A maximum of 2 kΩ 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 kΩ.
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 kΩ 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-
FIGURE 5. Resistive Feedback Divider
30074123
11
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.
IN
, plus some safety margin. A rule of thumb is to select
www.national.com