LM22674MRX-ADJ/NOPB National Semiconductor, LM22674MRX-ADJ/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM22674MRX-ADJ/NOPB
• Description: IC REG SWITCH BUCK .5A ADJ 8PSOP
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Step-Down (Buck)r

Specifications of LM22674MRX-ADJ/NOPB

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 1.285V
• Current - Output: 500mA
• Frequency - Switching: 500kHz
• Voltage - Input: 4.5 ~ 42 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 8-PSOP
• For Use With: 551600236-001 - WEBENCH BUILD IT LM2267X 8-PSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Other names: LM22674MRX-ADJ

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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
-5.0 option
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
LM22674 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 LM22674. 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.
IN
FB
, plus some safety margin. A rule of thumb is to select
FIGURE 3. Resistive Feedback Divider
= 1.285V typical for the -ADJ option and 5V for the
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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 LM22674 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
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 LM22674, 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.
FIGURE 4. Current Flow in a Buck Application
possible.Figure 4
shows the current flow of a buck
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