LSM-10A D5 Series
NON-ISOLA TED , 8-33W SMT DC/DC CONVER TERS
LOAD
20k
5-10
Turns
+INPUT
COMMON
+OUTPUT
TRIM
6
The highest temperatures in LSM D5 SMT's occur at their output inductor,
whose heat is generated primarily by I2R losses. The derating curves were
developed using thermocouples to monitor the inductor temperature and
varying the load to keep that temperature below +110°C under the assorted
conditions of air ow and air temperature. Once the temperature exceeds
+115°C (approx.), the thermal protection will disable the converter. Automatic
restart occurs after the temperature has dropped below +110°C.
As you may deduce from the derating curves and observe in the efciency
curves on the following pages, LSM D5 SMT's are more efcient at lower
current levels. Also I2R losses in the output inductor are signicantly less at
lower current levels. Consequently, LSN-D5 SMT's deliver very impressive
temperature performance if operating at less than full load.
Lastly, when LSM D5 SMT's are installed in system boards, they are obvi-
ously subject to numerous factors and tolerances not taken into account here.
If you are attempting to extract the most current out of these units under
demanding temperature conditions, we advise you to monitor the output-
inductor temperature to ensure it remains below +110°C at all times.
Output Reverse Conduction
Many DC/DC's using synchronous rectication suffer from Output Reverse
Conduction. If those devices have a voltage applied across their output before
a voltage is applied to their input (this typically occurs when another power
supply starts before them in a power-sequenced application), they will either
fail to start or self destruct. In both cases, the cause is the "freewheeling" or
"catch" FET biasing itself on and effectively becoming a short circuit.
LSM D5 SMT DC/DC converters do not suffer from Output Reverse Conduc-
tion. They employ proprietary gate drive circuitry that makes them immune
to applied output voltages.
Thermal Considerations and Thermal Protection
The typical output-current thermal-derating curves shown below enable
designers to determine how much current they can reliably derive from each
model of the LSM D5 SMT's under known ambient-temperature and air-ow
conditions. Similarly, the curves indicate how much air ow is required to
reliably deliver a specic output current at known temperatures.
Figure 7. Trim Connections Using Fixed Resistors
Figure 6. Trim Connections Using a Trimpot
Trim Equations
Note: Resistor values are in k
. Accuracy of adjustment is subject to
tolerances of resistors and factory-adjusted, initial output accuracy.
VO = desired output voltage. VO
NOM = nominal output voltage.
Note: LSM-0.8/10-D5 is not trimmable.
UP
VO
– 1
RT
(k
) =
1.296
DOWN
1
– VO
RT
(k
) =
– 1
1.62(VO 0.8)
UP
VO
– 1.2
RT
(k
) =
1.992
DOWN
1.2
– VO
RT
(k
) =
– 2.37
2.49(VO 0.8)
UP
RT
(k
) =
6
DOWN
RT
(k
) =
– 4.99
7.5(VO
– 0.8)
UP
VO
– VO
NOM
VO
– VO
NOM
RT
(k
) =
1.896
DOWN
VO
NOM – VO
VO
NOM – VO
RT
(k
) =
– 4.99
2.37(VO
– 0.8)
LSM-1/10-D5
Model
LSM-3.3/10-D5
LSM-1.2/10-D5
LSM-1.5/10-D5
LSM-1.8/10-D5
LSM-2/10-D5
LSM-2.5/10-D5
Trim Equations
Note:
Install either a xed
trim-up resistor
or a xed trim-down
resistor depending upon
desired output voltage.
Trim
Down
Trim
Up
LOAD
+INPUT
COMMON
+OUTPUT
TRIM
COMMON