Texas Instruments 36V, 2A PowerWise® Adjustable Frequency Synchronous Buck Regulator Evaluation Module LM20242EVAL/NOPB LM20242EVAL/NOPB Datenbogen
Produktcode
LM20242EVAL/NOPB
SNVS534E – OCTOBER 2007 – REVISED MARCH 2013
Similar to the soft-start function, the fastest start up possible is 1ms regardless of the rise time of the tracking
voltage. When using the track feature the final voltage seen by the SS/TRACK pin should exceed 1V to provide
sufficient overdrive and transient immunity.
voltage. When using the track feature the final voltage seen by the SS/TRACK pin should exceed 1V to provide
sufficient overdrive and transient immunity.
BENEFIT OF AN EXTERNAL SCHOTTKY
During dead time, the body diode of the synchronous MOSFET acts as a free-wheeling diode and conducts the
inductor current. The MOSFET is optimized for high breakdown voltage, but this makes an inefficient body diode
reverse recovery charge. The power loss is proportional to load current and switching frequency. The loss
increases at higher input voltages and switching frequencies. One simple solution is to use a small 1A external
Schottky diode between SW and GND as shown in
inductor current. The MOSFET is optimized for high breakdown voltage, but this makes an inefficient body diode
reverse recovery charge. The power loss is proportional to load current and switching frequency. The loss
increases at higher input voltages and switching frequencies. One simple solution is to use a small 1A external
Schottky diode between SW and GND as shown in
, diodes D1 and D2. The external Schottky diode
effectively conducts all inductor current during the dead time, minimizing the current passing through the
synchronous MOSFET body diode and eliminating reverse recovery losses.
synchronous MOSFET body diode and eliminating reverse recovery losses.
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.
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 LM20242 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
(14)
and
P
D
= P
IN
x (1 - Efficiency) - 1.1 x (I
OUT
)
2
x DCR
(15)
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 LM20242.
T
A
is the ambient temperature in °C.
I
OUT
is the output load current.
DCR is the inductor series resistance.
It is important to always keep the operating junction temperature (T
J
) below 125°C for reliable operation. If the
junction temperature exceeds 160°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.
occurs it is a sign of inadequate heatsinking or excessive power dissipation in the device.
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