LM20323EVAL National Semiconductor, LM20323EVAL Datasheet - Page 15

• Manufacturer Part Number: LM20323EVAL
• Description: EVALUATION BOARD FOR THE LM20323
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM20323EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 3.3V
• Current - Output: 3A
• Voltage - Input: 4.5 ~ 25V
• Regulator Topology: Buck
• Frequency - Switching: 500kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM20323
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

the synchronous MOSFET body diode and eliminating re-
verse recovery losses.
The external Schottky conducts currents for a very small por-
tion of the switching cycle, therefore the average current is
low. An external Schottky rated for 1A will improve efficiency
by several percent in some applications. A Schottky rated at
a higher current will not significantly improve efficiency and
may be worse due to the increased reverse capacitance. The
forward voltage of the synchronous MOSFET body diode is
approximately 700 mV, therefore an external Schottky with a
forward voltage less than or equal to 700 mV should be se-
lected to ensure the majority of the dead time current is carried
by the Schottky.
THERMAL CONSIDERATIONS
The thermal characteristics of the LM20323 are specified us-
ing the parameter θ
to the ambient temperature. Although the value of θ
pendant on many variables, it still can be used to approximate
the operating junction temperature of the device.
To obtain an estimate of the device junction temperature, one
may use the following relationship:
and
Where:
T
P
θ
LM20323.
T
I
DCR is the inductor series resistance.
It is important to always keep the operating junction temper-
ature (T
temperature exceeds 170°C the device will cycle in and out
of thermal shutdown. If thermal shutdown occurs it is a sign
of inadequate heatsinking or excessive power dissipation in
the device.
Figure 8 and Figure 9 can be used as a guide to avoid ex-
ceeding the maximum junction temperature of 125°C provid-
ed an external 1A Schottky diode, such as Central
Semiconductor's CMMSH1-40-NST, is used to improve re-
verse recovery losses.
OUT
J
JA
A
IN
is the junction temperature in °C.
is the ambient temperature in °C.
is the input power in Watts (P
is the junction to ambient thermal resistance for the
is the output load current.
P
J
D
) below 125°C for reliable operation. If the junction
= P
IN
x (1 - Efficiency) - 1.1 x (I
JA
T
, which relates the junction temperature
J
= P
D
x θ
JA
IN
+ T
= V
A
IN
OUT
x I
IN
)
2
).
x DCR
JA
is de-
15
The dashed lines in the figures above show an approximation
of the minimum and maximum duty cycle limitations; while,
the solid lines define areas of operation for a given ambient
temperature. This data for the figure was derived assuming
the device is operating at 3A continuous output current on a
4 layer PCB with an copper area greater than 4 square inches
exhibiting a thermal characteristic less than 27 °C/W. Since
the internal losses are dominated by the FETs a slight reduc-
tion in current by 500mA allows for much larger regions of
operation, as shown in Figure 9.
Figure 10, shown below, provides a better approximation of
the θ
test consisted of 4 layers: 1oz. copper was used for the inter-
nal layers while the external layers were plated to 2oz. copper
weight. To provide an optimal thermal connection, a 5 x 4 ar-
ray of 12 mil thermal vias located under the thermal pad was
used to connect the 4 layers.
FIGURE 9. Safe Thermal Operating Areas (I
FIGURE 8. Safe Thermal Operating Areas (I
JA
for a given PCB copper area. The PCB used in this
OUT
30051588
30051590
www.national.com
OUT
= 2.5A)
= 3A)