Datasheet

V
OUT
V
SB
LMV431
COMP
OPTO
5V
LM3450/50A
5k
R
FB2
R
FB1
R
SC
C
SC
R
SBS
C
CMP
SOFT
START
FB
R
FB
LM3450
SNVS681D –NOVEMBER 2010–REVISED MAY 2013 www.ti.com
If the COMP pin voltage (V
CMP
) falls below 1.4V at any time, the device enters burst mode where the GATE is off
for 340µs then is turned on. If V
CMP
is still below 1.4V at the end of the on-time then another 340µs off-time
occurs. However, if V
CMP
has risen above 1.4V, the converter continues switching until it falls below the threshold
again. This feature is necessary to prevent the output of the converter from rising arbitrarily high because the
minimum on-time of the device prevents less energy transfer.
The LM3450/50A also implements both feedback short circuit protection and output over-voltage protection
(OVP) functions at the FB pin. If V
FB
exceeds 3V, then OVP is engaged and the part stops switching until V
FB
falls below 3V. In the same manner, if V
FB
falls below 168mV, then shutdown is engaged and switching stops
until V
FB
exceeds 188mV.
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 LM3450/50A internal error amplifier can be bypassed by placing 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)
compensator. The amplifier is frequently implemented with an LMV431. The output voltage resistor divider (R
FB1
,
R
FB2
) provides the sensed output voltage to the LMV431 inverting input. The PI compensation is achieved by
connecting R
SC
and C
SC
in between the LMV431 input and output, shown in Figure 23. In addition, C
CMP
is
placed from COMP to GND on the primary for higher frequency noise attenuation.
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 Figure 23. This secondary softstart circuit has no restart mechanism, therefore a
primary side softstart is recommended as described in the SOFTSTART section of this document.
Figure 23. Secondary Error Amplifier
PRECISION VOLTAGE REFERENCE
The LM3450/50A provides a 3V voltage reference (V
REF
) for biasing the V
ADJ
pin as well as any external circuitry.
V
REF
is regulated once V
CC
exceeds 3V. There is a 2mA current limit for the reference. A 10nF ceramic bypass
capacitor should be placed from V
REF
to GND.
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