LM3450AEV230V30/NOPB National Semiconductor, LM3450AEV230V30/NOPB Datasheet - Page 15

• Manufacturer Part Number: LM3450AEV230V30/NOPB
• Description: LM3450A EVAL BOARD 230V30
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM3450AEV230V30/NOPB

• Current - Output / Channel: 700mA
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 50V
• Features: -
• Voltage - Input: 180 ~ 265VAC
• Utilized Ic / Part: LM3450A
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

ZERO CURRENT DETECTION
ZCD is implemented with a 100kΩ resistor from the ZCD pin
to a coupled winding on the transformer or inductor as shown
in
start-up. When Q
(V
winding. The circuit arms when V
when the energy is fully transferred, V
zero. When V
to be fully demagnetized, the circuit disarms, and Q
turned back on as shown in
low until Q
repeats.
SWITCHING FREQUENCY
With a constant on-time and variable off-time, there is a vari-
able switching frequency:
Figure 4
at the peak of the rectified AC waveform, while the maximum
switching frequency occurs at the valley.
ERROR AMPLIFIER
The LM3450 internal error amplifier is used for non-isolated
designs (boost) where the output voltage can be directly
sensed, via a resistor divider, at the FB pin. The FB pin is the
inverting input of the trans-conductance amplifier which is
regulated to 2.5V. The COMP pin is the output of the amplifier
and external compensation is placed from COMP to GND in
the form of a single capacitor (C
series resistor and capacitor, or both. The output of the am-
plifier sources or sinks current as necessary to force the
inputs of the amplifier to be equal. The compensation method
depends upon the transient performance desired and re-
quires a loop gain analysis. This analysis can be somewhat
ZCD
Figure
FIGURE 6. ZCD Waveforms for Flyback Design
) increases as energy is transferred through the auxiliary
shows that the minimum switching frequency occurs
5. This winding is also used to bootstrap V
SW
ZCD
is turned off via peak detection and the cycle
falls below 1.3V, the transformer is assumed
SW
turns off, the voltage at the ZCD pin
Figure
CMP
6. The ZCD pin will remain
ZCD
) as shown in
ZCD
exceeds 1.5V. Then,
decreases towards
Figure
CC
SW
after
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5, a
is
15
complex and cumbersome. To aid the designer, Table 1 in the
Appendix gives compensation recommendations for a variety
of applications. If a detailed analysis is desired, see Applica-
tion Note AN-2098.
If the COMP pin voltage (V
the device enters burst mode where the GATE is off for 340µs
then is turned on. If V
on-time then another 340µs off-time occurs. However, if
V
until it falls below the threshold again. This feature is neces-
sary to prevent the output of the converter from rising arbi-
trarily high because the minimum on-time of the device
prevents less energy transfer.
The LM3450 also implements both feedback short circuit pro-
tection and output over-voltage protection (OVP) functions at
the FB pin. If V
part stops switching until V
manner, if V
and switching stops until V
The flyback topology is frequently used to provide isolation
from input to output. Since, the current transfer ratio (CTR) of
standard optical isolation varies over temperature, proper
regulation using primary error amplifiers is difficult. An error
amplifier is usually placed in the secondary to regulate the
output voltage accurately. To accommodate isolated designs,
the LM3450 internal error amplifier can be bypassed by plac-
ing a 5.11kΩ resistor from FB to GND. This engages a 5kΩ
pull-up resistor from COMP to an internal 5V rail.
SECONDARY ERROR AMPLIFIER
For isolated designs, the error amplifier on the secondary
should take the form of a proportional integral (PI) compen-
sator. The amplifier is frequently implemented with a LM431
(low cost with internal reference). The output voltage resistor
divider (R
LM431 inverting input. The PI compensation is achieved by
connecting R
output, shown in
In addition to the basic error amplifier, a soft-start circuit can
be implemented using a capacitor, two diodes and a Zener
diode as shown in
GND for high frequency noise attenuation.
CMP
has risen above 1.4V, the converter continues switching
FB1
FIGURE 7. Secondary Error Amplifier
FB
, R
SC
falls below 168mV, then shutdown is engaged
FB
FB2
and C
Figure
exceeds 3V, then OVP is engaged and the
) provides the scaled output voltage to the
Figure
CMP
SC
7.
is still below 1.4V at the end of the
in between the LM431 input and
FB
7. C
CMP
FB
exceeds 188mV.
) falls below 1.4V at any time,
CMP
falls below 3V. In the same
is placed from COMP to
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