LM22670INVEVAL National Semiconductor, LM22670INVEVAL Datasheet - Page 14

• Manufacturer Part Number: LM22670INVEVAL
• Description: BOARD EVALUATION FOR LM22670INV
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r

Specifications of LM22670INVEVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: -5V
• Current - Output: 1.5A
• Voltage - Input: 5 ~ 35V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM22670
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Other names: *LM22670INVEVAL

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Thermal Considerations
The two highest power dissipating components are the re-
circulating diode and the LM22670 regulator IC. The easiest
method to determine the power dissipation within the
LM22670 is to measure the total conversion losses (Pin –
FIGURE 6. Current Flow in a Buck Application
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14
Pout) then subtract the power losses in the Schottky diode
and output inductor. An approximation for the Schottky diode
loss is:
An approximation for the output inductor power is:
where R is the DC resistance of the inductor and the 1.1 factor
is an approximation for the AC losses. The regulator has an
exposed thermal pad to aid power dissipation. Adding several
vias under the device to the ground plane will greatly reduce
the regulator junction temperature. Selecting a diode with an
exposed pad will aid the power dissipation of the diode. The
most significant variables that affect the power dissipated by
the LM22670 are the output current, input voltage and oper-
ating frequency. The power dissipated while operating near
the maximum output current and maximum input voltage can
be appreciable. The junction-to-ambient thermal resistance of
the LM22670 will vary with the application. The most signifi-
cant variables are the area of copper in the PC board, the
number of vias under the IC exposed pad and the amount of
forced air cooling provided. The integrity of the solder con-
nection from the IC exposed pad to the PC board is critical.
Excessive voids will greatly diminish the thermal dissipation
capacity. The junction-to-ambient thermal resistance of the
LM22670 TO-263 THIN and PSOP-8 packages are specified
in the Electrical Characteristics table under the applicable
conditions. For more information regarding the TO-263 THIN
package,
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refer
P = (1 - D) x I
to
P = I
Application
OUT
2
x R x 1.1,
OUT
x V
Note
D
AN-1797
at