Datasheet
LM20333
www.ti.com
SNVS558D –MAY 2008–REVISED APRIL 2013
The external Schottky conducts currents for a very small portion 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 selected
to ensure the majority of the dead time current is carried by the Schottky.
THERMAL CONSIDERATIONS
The thermal characteristics of the LM20333 are specified using the parameter θ
JA
, which relates the junction
temperature to the ambient temperature. Although the value of θ
JA
is dependant 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:
T
J
= P
D
x θ
JA
+ T
A
(10)
and
P
D
= P
IN
x (1 - Efficiency) - 1.1 x (I
OUT
)
2
x DCR
where
• T
J
is the junction temperature in °C
• P
IN
is the input power in Watts (P
IN
= V
IN
x I
IN
)
• θ
JA
is the junction to ambient thermal resistance for the LM20333
• T
A
is the ambient temperature in °C
• I
OUT
is the output load current
• DCR is the inductor series resistance (11)
It is important to always keep the operating junction temperature (T
J
) below 125°C for reliable operation. If the
junction 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 32, Figure 33, Figure 34 and Figure 35 can be used as a guide to avoid exceeding the maximum junction
temperature of 125°C provided an external 1A Schottky diode, such as Central Semiconductor's CMMSH1-40-
NST, is used to improve reverse recovery losses.
Figure 32. Safe Thermal Operating Areas (I
OUT
= 3A, f
SW
= 350kHz)
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