I need your help desperately.
With E2E help, I built a prototype to test, see attached schematic. it consists of 2 portions: TPS61288 boost circuit and a Vin clamp/cut-off circuit (D1,Q1,Q2, D3).
Design goal: Battery voltage 8V~15V, boost output 12V 1.2A (max 1.4A).
The boost circuit alone works well. At Vin close to 12V, the ripple does very high(Vpp=1V). I add an 270uf aluminum cap, the ripple drops to 70mV. When Vin >12V, it directly goes to the Vout.
The Vin clamp/cut-off circuit alone works well too. When Vin <7.2V, Vin is cut off. When Vin in range 7.3V~12V, Q3 output 7.3V~12V. when Vin >12V, Q3 output 12V (when R6=11K).
However, when I connect Q3 output to boost input. It has an audible noise , and Vout is only 9V.
- I change L1 to 10uH, doesn’t help.
- I change zero/pole distribution by change C7 to 30pf, doesn’t help.
- I add a 100uf aluminum cap at Q3 output , doesn’t help.
I am really frustrated, seek help from you.