LTM8046_15 Datasheet, PDF (11/20 Pages) Linear Technology – Isolated DC/DC Module Converter

LTM8046
Applications Information
BIAS Pin Considerations
The BIAS pin is the output of an internal linear regulator
that powers the LTM8046’s internal circuitry. It is set to
3V and must be decoupled with a low ESR capacitor of at
least 1μF. The LTM8046 will run properly without apply-
ing a voltage to this pin, but will operate more efficiently
and dissipate less power if a voltage greater than 3.1V is
applied. At low V_IN, the LTM8046 will be able to deliver
more output current if BIAS is 3.1V or greater. Up to 31V
may be applied to this pin, but a high BIAS voltage will
cause excessive power dissipation in the internal circuitry.
For applications with an input voltage less than 15V, the
BIAS pin is typically connected directly to the V_IN pin. For
input voltages greater than 15V, it is preferred to leave the
BIAS pin separate from the V_IN pin, either powered from
a separate voltage source or left running from the internal
regulator. This has the added advantage of keeping the
physical size of the BIAS capacitor small. Do not allow
BIAS to rise above V_IN.
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
significantly reduces the start-up current spike and out-
put voltage overshoot by applying a capacitor from SS to
GND. When the LTM8046 is enabled, whether from V_IN
reaching a sufficiently high voltage or RUN being pulled
high, the LTM8046 will source approximately 8µA out of
the SS pin. As this current gradually charges the capaci-
tor from SS to GND, the LTM8046 will correspondingly
increase the power delivered to the output, allowing for a
graceful turn-on ramp.
Isolation Working Voltage and Safety
The LTM8046 isolation is 100% hi-pot tested by tying
all of the primary pins together, all of the secondary pins
together and subjecting the two resultant circuits to a
differential of 3kVDC for one second. This establishes
the isolation voltage rating of the LTM8046 component.
The isolation rating of the LTM8046 is not the same as
the working or operational voltage that the application
will experience. This is subject to the application’s power
source, operating conditions, the industry where the end
product is used and other factors that dictate design
requirements such as the gap between copper planes,
traces and component pins on the printed circuit board,
as well as the type of connector that may be used. To
maximize the allowable working voltage, the LTM8046 has
three rows of solder balls removed to facilitate the printed
circuit board design. The ball to ball pitch is 1.27mm,
and the typical ball diameter is 0.78mm. Accounting for
the missing row and the ball diameter, the printed circuit
board may be designed for a metal-to-metal separation
of up to 4.3mm. This may have to be reduced somewhat
to allow for tolerances in solder mask or other printed
circuit board design rules.
To reiterate, the manufacturer’s isolation voltage rating
and the required operational voltage are often different
numbers. In the case of the LTM8046, the isolation voltage
rating is established by 100% hi-pot testing. The working
or operational voltage is a function of the end product
and its system level specifications. The actual required
operational voltage is often smaller than the manufacturer’s
isolation rating.
For those situations where information about the spacing
of LTM8046 internal circuitry is required, the minimum
metal to metal separation of the primary and secondary is
1.9mm. The LTM8046 is a UL recognized component under
UL 60950-1, file number E464570. The UL 60950-1 insula-
tion category of the LTM8046 transformer is Functional.
Considering UL 60950-1 Table 2N and the gap distances
stated above, 4.3mm external and 1.9mm internal, the
LTM8046 may be operated with up to 400V working
voltage in a pollution degree 2 environment. The actual
working voltage, insulation category, pollution degree and
other critical parameters for the specific end application
depend upon the actual environmental, application and
safety compliance requirements. It is therefore up to the
user to perform a safety and compliance review to ensure
that the LTM8046 is suitable for the intended application.
For more information www.linear.com/LTM8046
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