Forum Post: RE: Buck-Boost Regulator around LM5118 - ramp generation

Hi Sonu

If ignore the additional term (50uA) from the equation (2), it results. I_RAMP=5u*(VIN-VOUT) [A]. And then we can say dV_RAMP/dT = 5u*(VIN-VOUT)/C_RAMP since I=C*dV/dT

Assuming a resistor sensing which senses the rising slope of inductor current in BUCK mode and the gain of sense amplifier is A(=10), we can say dV/dT of the current sense amplifier output is (Vin-Vout)/L*R_S*A  since V=L*dI/dT.

Therefore,

5u*(VIN-VOUT)/C_RAMP = (Vin-Vout)/L* R_S*A

5u / C_RAMP = R_S*A /L

C_RAMP= 5u*L/R_S/A  =gm * L /R_S/A

For proper current emulation, the sample and hold pedestal value and the ramp amplitude must have the same relative relationship to the actual inductor current

==> In other word, the sum of the pedestal value (DC)  and the ramp (AC) should be similar with the actual inductor current sensing during Ton.

Regards,

Eric