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LTM8046IY

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LTM8046
10
8046fb
For more information www.linear.com/LTM8046
applicaTions inForMaTion
For most applications, the design process is straight-
forward, summarized as follows:
1. Look at Table 1 and find the row that has the desired
input range and output voltage.
2. Apply the recommended C
IN
, C
OUT
and R
FB
.
3. Connect BIAS as indicated, or tie to an external source
up to 15V or V
IN
, whichever is less.
While these component combinations have been tested for
proper operation, it is incumbent upon the user to verify
proper operation over the intended system’s line, load and
environmental conditions. Bear in mind that the maximum
output current may be limited by junction temperature,
the relationship between the input and output voltage
magnitude and polarity and other factors. Please refer
to the graphs in the Typical Performance Characteristics
section for guidance.
Capacitor Selection Considerations
The C
IN
and C
OUT
capacitor values in Table 1 are the
minimum recommended values for the associated oper-
ating conditions
. Applying capacitor values below
those
indicated in Table 1 is not recommended, and may result
in undesirable operation. Using larger values is generally
acceptable, and can yield improved dynamic response, if
it is necessary. Again, it is incumbent upon the user to
verify proper operation over the intended system
’s line,
load and environmental conditions.
Ceramic capacitors are small, robust
and have very low
ESR. However, not all ceramic capacitors are suitable.
X5R and X7R types are stable over temperature and ap
-
plied voltage
and give dependable ser
vice. Other types,
including Y5V and Z5U have very large temperature and
voltage coefficients of capacitance. In an application cir
-
cuit they may have only a small fraction of their nominal
capacitance resulting in much higher output voltage ripple
than expected.
A final precaution regarding ceramic capacitors concerns
the maximum input voltage rating of the LTM8046. A
ceramic input capacitor combined with trace or cable
inductance forms a high-Q (underdamped) tank circuit. If
the LTM8046 circuit is plugged into a live supply, the input
voltage can ring to much higher than its nominal value,
possibly exceeding the device’s rating. This situation is
easily avoided; see the Hot-Plugging Safely section.
Table 1. Recommended Components and Configuration (T
A
= 25°C)
V
IN
V
OUT
C
IN
C
OUT
R
FB
3.2V to 32V 1.8V 1µF, 50V, 0805 X5R
2 × 100µF, 6.3V, 1206 X5R
18.7k
3.2V to 31V 2.5V 1µF, 50V, 0805 X5R
2 × 100µF, 6.3V, 1206 X5R
14.7k
3.2V to 29V 3.3V 1µF, 50V, 0805 X5R 100µF, 6.3V, 1206 X5R 11.8k
3.2V to 26V 5V 1µF, 25V, 0603 X5R 100µF, 6.3V, 1206 X5R 8.45k
3.2V to 20V 8V 1µF, 25V, 0603 X5R 47µF, 10V, 1206 X5R 5.49k
3.2V to 12V 12V 1µF, 25V, 0603 X5R
2 × 10µF, 16V, 1210 X5R
3.83k
3.2V to 25V 2.5V 1µF, 25V, 0603 X5R
2 × 100µF, 6.3V, 1206 X5R
14.7k
3.2V to 25V 3.3V 1µF, 25V, 0603 X5R 100µF, 6.3V, 1206 X5R 11.8k
CBIAS = 1µF 10V 0402 X5R
BIAS = 3.3V for V
IN
≥ 3.3V, V
IN
for V
IN
< 3.3V. If BIAS = V
IN
, the minimum input voltage is 4.3V.
LTM8046
11
8046fb
For more information www.linear.com/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 comp
onent under
UL 60950-1, file number E464570. The UL 60950-1 insula-
tion categor
y 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.
LTM8046
12
8046fb
For more information www.linear.com/LTM8046
applicaTions inForMaTion
Figure 1 for a suggested layout. Ensure that the grounding
and heat sinking are acceptable.
A few rules to keep in mind are:
1. Place the R
ADJ
resistor as close as possible to its re-
spective pin.
2.
Place the C
IN
capacitor as close as possible to the V
IN
and GND connections of the LTM8046.
3. Place the C
OUT
capacitor as close as possible to V
OUT
and V
OUT
–
.
4. Place the C
IN
and C
OUT
capacitors such that their
ground current flow directly adjacent or underneath
the LTM8046.
5. Connect all of the GND connections to as large a copper
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8046.
6. Use vias to connect the GND copper area to the board’s
internal ground planes. Liberally distribute these GND
vias to provide both a good ground connection and
thermal path to the internal planes of the printed circuit
board. Pay attention to the location and density of the
thermal vias in Figure 1. The LTM8046 can benefit from
the heat sinking afforded by vias that connect to internal
Figure 1. Layout Showing Suggested External Components,
Planes and Thermal Vias
8046 F01
GND
GND
V
IN
THERMAL/INTERCONNECT VIAS
V
OUT
–
V
OUT
RUN
FB
BIAS
SS
V
OUT
to V
OUT
–
Reverse Voltage
The LTM8046 cannot tolerate a reverse voltage from V
OUT
to V
OUT
–
during operation. If V
OUT
–
raises above V
OUT
dur-
ing operation, the LTM8046 may be
damaged. To protect
against this condition, a low forward drop power Schottky
diode has been integrated into the LTM8046, anti-parallel
to V
OUT
/V
OUT
–
. This can protect the output against many
reverse voltage faults. Reverse voltage faults can be both
steady state and transient. An example of a steady state
voltage reversal is accidentally misconnecting a powered
LTM8046 to a negative voltage source. An example of
transient voltage reversals is a momentary connection to
a negative voltage. It is also possible to achieve a V
OUT
reversal if the load is short-circuited through a long cable.
The inductance of the long cable forms an LC tank circuit
with the V
OUT
capacitance, which drives V
OUT
negative.
Avoid these conditions.
Minimum Load
The LTM8046 requires a minimum load in order to main
-
tain regulation. If less than the
minimum load is applied,
the output voltage may rise beyond the intended value
uncontrollably, possibly damaging the LTM8046 or the
application system. Avoid this situation
. The Typical
Performance Characteristics section provides graphs of
the minimum required load for several input and output
conditions at room temperature.
The LTM8046 is designed to skip switching cycles, if
necessary, to maintain regulation. While cycle skipping,
the output ripple may be higher than when the LTM8046
is not skipping cycles. The user must validate the perfor
-
mance of
the LTM8046 application over the appropriate
temperature, line, load and other operating conditions.
PCB Layout
Most of the headaches associated
with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8046. The LTM8046 is neverthe
-
less a switching power supply, and care must be taken to
minimize electrical noise to ensure proper operation. Even
with the high level of integration, you may fail to achieve
specified operation with a haphazard or poor layout. See
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTM8046IY

Mfr. #:
Buy LTM8046IY
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 3.1VIN to 32VIN, 2kVAC Isolated DC/DC Module Converter
Lifecycle:
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