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# UCC256403: FB pin

Part Number: UCC256403

Hi Sir,

My customer have the below questions.

1. What conditions it will turn off the AVDD and internal MOSFET of the FB pin?

UCC25640x LLC Resonant Controller Features Brief Overview and Bring up Guidelines

2. The IFB typical value of UCC256403 is 164uA, if the IFB current is over the value, may I know to have any side effects?
3. The CTR of the Opto-coupler(BPC-1009SC) is 200~400%, based on the below circuit, the current(If) flowing into the Opto-coupler, what are the values of minimum and maximum?

• Hi Henry,

1. What conditions it will turn off the AVDD and internal MOSFET of the FB pin?

1. The AVDD will be off during the fault state.

opto coupler current = IFB + I_RFB

From above equation, Internal PMOS will be off, when the opto coupler current is same as IFB source current.

2.

The IFB typical value of UCC256403 is 164uA, if the IFB current is over the value, may I know to have any side effects?

The opto coupler will be saturated when the current drawn equal to IFB. You cannot draw current more than 164uA. During this case, VFbreplica will be at its minimum value.

When continues to increase and reaches the value of , the FB pin voltage starts to drop because there is no current flow through the PMOS. FB pin pulled low will impact the system transient response, due to the extra delay introduced by charging the parasitic capacitor of the optocoupler to pull up the FB pin voltage.

3.

IF=Iopto/CTR

Ideally, IF can be anywhere between 0 to (164uA/CTR_minimum)

Regards

Manikanta P

• Hi Sir,

I_R712 = (18V-5.5V)/12K = 1.04mA, I_R719 = 1.5V/2K = 750uA

If = I_R712 - I_R719 = 290uA

CTR = 200~400%

Ic = CTR(300%) x If = 870uA

My customer hopes for system stability, could you help to review both calculation values of Ic and If, could they be used or not?

• Hi,

Please see the attached app note draft (final section of the app note) where I have solved an example for compensator. Please follow those guidelines and find the compensator values accordingly.

Tri-34b0609e-2c6e-40c5-8f6a-40c808450af2.pdf

Let me know if you still have questions.

Regards

Manikanta P