UCC28C40: Internal EA circuit confirm

Evan Peng

Intellectual 1395 points

Part Number: UCC28C40

Tool/software:

Hi Expert,

This is Shenzhen FAE Evan. My customer is designing flyback by UCC28C40.

The question is that, how about the working principle of this Error Amplifier circuit.

1. If FB voltage is pulled below 2.5V, what is the max output voltage on COMP pin.

2. What is the forward voltage drop of the 2 diode in the red circle

Regards,

Evan

over 1 year ago

0 Steven Mappus1 over 1 year ago

TI__Mastermind 33115 points

Max COMP is ~7V and the forward drop of the two diode is not specified and the diodes are shown as functional description only and are not real diodes in the dcicuit but are only representing pn junction voltage drop between COMP and PWM comparator. Assume the two diodes to be ~1.5V which leaves 7V-1.5V=5.5V and 5.5V/3=1.83V and this is the max potential input to the PWM comparator but it is clamped to 1V.

Steve

0 Evan Peng over 1 year ago in reply to Steven Mappus1

TI__Intellectual 1395 points

Hi Steven,

So the real normal working voltage will reach about 7V, right? However, the abs. value is only 6.3V, will it result some risk?

Regards,

Evan

0 Steven Mappus1 over 1 year ago in reply to Evan Peng

TI__Mastermind 33115 points

You are correct, make sure COMP voltage less than 6.3V.

Steve

0 Evan Peng over 1 year ago in reply to Steven Mappus1

TI__Intellectual 1395 points

Hi Steven,

1. We find different ICs reach different COMP voltage in the same board in open loop mode, some is 5.3V and some is 5.8V. What result the diffenrence?

2. Do you have recommended method to ensure COMP voltage below 6.3V?

Regards,

Evan

0 Steven Mappus1 over 1 year ago in reply to Evan Peng

TI__Mastermind 33115 points

COMP is normally pulled up to VREF (5V) and this naturally limits the COMP voltage to Vcomp<5V. You are measuring COMP voltages of 5.3V and 5.8V. Can you provide a schematic showing how the COMP is connected and what is the pullup volatge?

Steve

Power management

Power management forum

UCC28C40: Internal EA circuit confirm