Tool/software:
Hi team,
Regarding the recommended inductor on the datasheet of TPS551892-Q1, Which equation is the inductor value of 4.7uH calculated, inductor calculation for buck mode(equation 9) or inductor calculation for boost mode(equation 10)?
Also, what parameters did we use to derive the inductor value of 4.7uH?
VIN:
VIN(MAX):
VOUT:
VOUT(MAX):
fSW:
delta IL(P-P):
Best regards,
Shunsuke Yamamoto
Hi Shunsuke,
We recommend 4.7uH for two factors:
1. Inter loop stability requirement: As below shows, in 400kHz application, inductance should be larger than 1.2/0.4=3uH. Considering inductance derating, at least 4.7uH is needed.
2.Right half plane zero. With larger inductance, the boost right half plane zero will be lower, making it harder to optimize loop compensaiton.
So considering the above two factors, we recommend to use 4.7uH inductor. Actually the above parameters you consider is to calculate inductance by current ripple ratio. This is not that important in buck-boost design compared with the above two factors. Please kindly let me know if you have questions.
Regards,
Mulin
Hi Shunsuke,
Right half plane zero only exists in boost converter. When four-switch buck-boost converter works in boost most. there also exists right half plane zero.
About right-half-plane (RHP) zero: The right-half-plane (RHP) zero has the same 20 dB/decade rising gain magnitude as a conventional zero, but with 90° phase lag instead of lead. This characteristic is difficult if not impossible to compensate. The designer is usually forced to roll off the loop gain at a relatively low frequency. The crossover frequency may be a decade or more below what it otherwise could be, resulting in severe impairment of dynamic response.
For more details please refer to this file: 7230.slup084.pdf
Regards,
Mulin
