VOUT

VSB

LMV431

COMP

OPTO

5V

LM3450/50A

5k

RFB2

RFB1

RSC

CSC

RSBS

CCMP

SOFT

START

FB

RFB

LM3450

SNVS681D – NOVEMBER 2010 – REVISED MAY 2013

www.ti.com

If the COMP pin voltage (VCMP) falls below 1.4V at any time, the device enters burst mode where the GATE is off

for 340µs then is turned on. If VCMP is still below 1.4V at the end of the on-time then another 340µs off-time

occurs. However, if VCMP 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 VFB exceeds 3V, then OVP is engaged and the part stops switching until VFB

falls below 3V. In the same manner, if VFB falls below 168mV, then shutdown is engaged and switching stops

until VFB 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 5.11k

Ω

resistor from FB to GND. This engages a 5k

Ω 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 (RFB1,

RFB2) provides the sensed output voltage to the LMV431 inverting input. The PI compensation is achieved by

connecting RSC and CSC in between the LMV431 input and output, shown in Figure 23. In addition, CCMP 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 (VREF) for biasing the VADJ pin as well as any external circuitry.

VREF is regulated once VCC exceeds 3V. There is a 2mA current limit for the reference. A 10nF ceramic bypass

capacitor should be placed from VREF to GND.

16

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