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LTM8057IY#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LTM8057
10
8057f
For more information www.linear.com/LTM8057
APPLICATIONS INFORMATION
For most applications, the design process is straight
forward, summarized as follows:
1. Look at Table 1a and find the row that has the desired
input range and output voltage.
2. Apply the recommended C
IN
, C
OUT
and R
ADJ
if required.
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 T
able 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 necessar
y. 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 service. 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 LTM8057. A
ceramic input capacitor combined with trace or cable
inductance
forms a high-Q (underdamped) tank circuit. If
the LTM8057 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.
LTM8057 Table 1a. Recommended Component Values and Configuration for Specific V
OUT
Voltages (T
A
= 25°C)
V
IN
V
OUT
V
BIAS
C
IN
C
OUT
R
ADJ
3.1V to 31V 2.5V 3.1V to 15V or Open 2.2µF, 50V, 1206 100µF, 6.3V, 1210 12.4k
3.1V to 31V 3.3V 3.1V to 15V or Open 2.2µF, 50V, 1206 100µF, 6.3V, 1210 10k
3.1V to 29V 5V 3.1V to 15V or Open 2.2µF, 50V, 1206 22µF, 16V, 1210 6.98k
3.1V to 26V 8V 3.1V to 15V or Open 2.2µF, 50V, 1206 22µF, 10V, 1206 4.53k
3.1V to 24V 12V 3.1V to 15V or Open 2.2µF, 25V, 0805 10µF, 16V, 1210 3.16k/8.2pF*
9V to 15V 2.5V V
IN
2.2µF, 50V, 1206 100µF, 6.3V, 1210 12.4k
9V to 15V 3.3V V
IN
2.2µF, 50V, 1206 47µF, 6.3V, 1210 10k
9V to 15V 5V V
IN
2.2µF, 50V, 1206 22µF, 16V, 1210 6.98k
9V to 15V 8V V
IN
2.2µF, 50V, 1206 22µF, 10V, 1206 4.53k
9V to 15V 12V V
IN
2.2µF, 25V, 0805 10µF, 16V, 1210 3.16k
18V to 31V 2.5V 3.1V to 15V or Open 2.2µF, 50V, 1206 100µF, 6.3V, 1210 12.4k
18V to 31V 3.3V 3.1V to 15V or Open 2.2µF, 50V, 1206 47µF, 6.3V, 1210 10k
18V to 29V 5V 3.1V to 15V or Open 2.2µF, 50V, 1206 22µF, 16V, 1210 6.98k
18V to 26V 8V 3.1V to 15V or Open 2.2µF, 50V, 1206 22µF, 10V, 1206 4.53k
18V to 24V 12V 3.1V to 15V or Open 2.2µF, 50V, 1206 10µF, 16V, 1210 3.16k/8.2pF*
Note:
Do not allow BIAS to exceed V
IN
, a bulk input capacitor is required. If BIAS is open, the minimum V
IN
is 4.3V.
*Connect 3.16k in parallel with 8.2pF from ADJ to GND
LTM8057
11
8057f
For more information www.linear.com/LTM8057
APPLICATIONS INFORMATION
BIAS Pin Considerations
The BIAS pin is the output of an internal linear regulator
that powers the LTM8057’s internal circuitry. It is set to
3V and must be decoupled with a low ESR capacitor of at
least 4.7μF. The LTM8057 will run properly without applying
a voltage to this pin, but will operate more efficiently and
dissipate less power if a voltage between 3.1V and V
IN
is
applied. At low V
IN
, the LTM8057 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 output
voltage overshoot by applying a capacitor from SS to GND.
When the LTM8057 is enabled, whether from V
IN
reaching
a sufficiently high voltage or RUN being pulled high, the
LTM8057 will source approximately 10µA out of the SS
pin. As this current gradually charges the capacitor from
SS to GND, the LTM8057 will correspondingly increase
the power delivered to the output, allowing for a graceful
turn-on ramp.
Isolation, Working Voltage and Safety Compliance
The LTM8057 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 3kV DC for one second. This establishes the isolation
voltage rating of the LTM8057 component.
The isolation rating of the LTM8057 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 re
-
quirements such as the gap between copper planes, traces
and component pins on the printed cir
cuit board, as well
as the type of connector that may be used. T
o maximize
the allowable working voltage, the LTM8057 has two
columns 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 columns and the ball diameter, the printed circuit
board may be designed for a metal-to-metal separation of
up to 3.03mm. This may have to be reduced somewhat to
allow for tolerances in solder mask or other printed circuit
board design rules. For those situations where informa
-
tion about the spacing of LTM8057 internal circuitry is
required, the minimum metal to metal separation of the
primary and secondary is 0.75mm.
To reiterate, the manufacturer’s isolation voltage rating
and the required working or operational voltage are often
different numbers. In the case of the LTM8057, the isola
-
tion 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 ac
-
tual required operational voltage is often smaller than the
manufacturer’s isolation rating.
The LTM8057 is a UL recognized component under
UL 60950, file number 464570. The UL 60950 insula-
tion category of the LTM8057 transformer is Functional.
Considering UL 60950 T
able 2N and the gap distances
stated above, 3.03mm external and 0.75mm internal,
the LTM8057 may be operated with up to 250V 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 LTM8057 is suitable for the intended application.
LTM8057
12
8057f
For more information www.linear.com/LTM8057
APPLICATIONS INFORMATION
ADJ and Line Regulation
For V
OUT
greater than 8V, parasitics in the transformer
interacting with the controller cause a localized increase in
minimum load. A small capacitor may need to be applied
from ADJ to GND to ensure proper line regulation. Care
must be taken when choosing this capacitor value. Too
small or no capacitor will result in poor line regulation;
in general, a larger capacitor is needed for higher V
OUT
.
Too large of a capacitance will require excessive minimum
load to maintain regulation.
The plots in Figure 1 show LTM8057 line regulation at
three different capacitor values applied from ADJ to GND.
The plots in Figure 2 show the minimum load requirement
for the same three capacitors.
Carefully choose the appropriate capacitor value for the
intended application.
V
OUT
to V
OUT
–
Reverse Voltage
The LTM8057 cannot tolerate a reverse voltage from V
OUT
to
V
OUT
–
during operation. If V
OUT
–
raises above V
OUT
dur-
ing operation, the LTM8057 may be damaged. To protect
against this condition, a low for
ward drop power Schottky
diode has been integrated into the LTM8057, 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
LTM8057 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 drive V
OUT
negative.
Avoid these conditions.
Safety Rated Capacitors
Some applications require safety rated capacitors, which
are high voltage capacitors that are specifically designed
and rated for AC operation and high voltage surges. These
capacitors are often certified to safety standards such as UL
60950, IEC 60950 and others. In the case of the LTM8057,
a common application of a safety rated capacitor would
be to connect it from GND to V
OUT
–
. To provide maximum
flexibility, the LTM8057 does not include any components
between GND and V
OUT
–
. Any safety capacitors must be
added externally.
The specific capacitor and circuit configuration for any
application depends upon the safety requirements of
the system into which the LTM8057 is being designed.
Table 2 provides a list of possible capacitors and their
manufacturers. The application of a capacitor from GND
to V
OUT
–
may also reduce the high frequency output noise
on the output.
V
IN
(V)
0
–5
DEVIATION (%)
–4
–2
–1
0
5
2
6
12
8057 F01
–3
3
4
1
18
24
NO CAP
8.2pF CAP
12pF CAP
Figure 1. V
OUT
Line Regulation vs V
IN
INPUT VOLTAGE (V)
0
MINIMUM REQUIRED LOAD (mV)
10
15
24
8057 F02
5
0
6
12
18
25
BIAS = 5V FOR V
IN
≥ 5V
BIAS = V
IN
FOR V
IN
< 5V
20
NO CAP
8.2pF CAP
12pF CAP
Figure 2. Minimum Required Load vs Input Voltage
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LTM8057IY#PBF

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Buy LTM8057IY#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 2kVAC Isolated, 1.5W Module DC/DC Converter
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