Datasheet

LTM8048
12
8048ff
For more information www.linear.com/LTM8048
APPLICATIONS INFORMATION
Isolation and Working Voltage
The LTM8048 isolation is 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
±725VDC for one second. This establishes the isolation
voltage rating, but it does not determine the working volt
-
age rating, which is subject to the application board layout
and possibly other factors. The metal to metal separation
of the primary and secondary throughout the LTM8048
substrate is 0.44mm.
V
OUT2
Post Regulator
V
OUT2
is produced by a high performance low dropout
300mA regulator. At full load, its dropout is less than
430mV over temperature. Its output is set by applying a
resistor from the R
ADJ2
pin to GND; the value of R
ADJ2
can be calculated by the equation:
R
ADJ2
=
608.78
V
OUT2
– 1.22
kΩ
V
OUT1
to V
OUT
–
Reverse Voltage
The LTM8048 cannot tolerate a reverse voltage from V
OUT1
to V
OUT
–
during operation. If V
OUT
–
raises above V
OUT1
during operation, the LTM8048 may be damaged. To protect
against this condition, a low forward drop power Schottky
diode has been integrated into the LTM8048, anti-parallel
to V
OUT1
/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
LTM8048 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
OUT1
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
OUT1
capacitance, which drive V
OUT1
negative.
Avoid these conditions.
V
OUT2
Post Regulator Bypass Capacitance and Low
Noise Performance
The V
OUT2
linear regulator may be used with the addition
of a 0.01μF bypass capacitor from V
OUT
to the BYP pin
to lower output voltage noise. A good quality low leakage
capacitor, such as a X5R or X75 ceramic, is recommended.
This capacitor will bypass the reference of the regulator,
lowering the output voltage noise to as low as 20µV
RMS
.
Using a bypass capacitor has the added benefit of improv-
ing transient response.
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 L
TM8048,
a common application of a safety rated capacitor would
be to connect it from GND to V
OUT
–
. To provide maximum
flexibility, the LTM8048 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 LTM8048 is being designed.
Table 2 provides a list of possible capacitors and their
manufacturers.
Table 2. Safety Rated Capacitors
MANUFACTURER PART NUMBER DESCRIPTION
Murata Electronics GA343DR7GD472KW01L 4700pF, 250VAC, X7R,
4.5mm × 3.2mm
Capacitor
Johanson Dielectrics 302R29W471KV3E-****-SC 470pF, 250VAC,
X7R, 4.5mm × 2mm
Capacitor
Syfer Technology 1808JA250102JCTSP 100pF, 250VAC, C0G,
1808 Capacitor
The application of a capacitor from GND to V
OUT
–
may
also reduce the high frequency output noise on the output.