25X DC/DC？

如果想做一个输入2.4~3.6V,输出40V的charge pump,不想用inductor+diode的那种
1.单纯使用internal cap booster，25X (I know 40/2.4=17,but considering the efficiency)面积好像太大
不知道有什么更好的方法?效率有多高
2.使用2个charge pump, internal cap booster + external cap booster
这是我现在想到折衷的方法，但是为了提高效率，希望在低电压（2.4V）用更高的倍数（25X），在高电压时候用低倍数(15X), 不知道这样调会不会有问题呢?

Pls tell me what is your current driving ability first?

@Vout90uA

OK. Generally
(1) Theorectially, calculate the number of stage required N to generate 25X output voltage
Like Vout=(n+1)(VDD-VD)
(2) Using P+/N-Well Diode, NMOS or PMOS to replace the ideal Diode depending on your process chosen
If your process have HV device or HV technology you can choose any one of them. If not, should be very careful, youhave to deal with the breakdown voltage of N-Well/P-sub, which will limit your output voltage. You also need to pay attention to Capacitor break down voltage either.
(3) I assume you have HV device, then your charge pump architecure can be choose like pelliconi type, which is useful for high efficiency. In summary to avoid parasitc cp, and body-effect of NMOS-type diode is very important to maintain the high efficiency.
(4) Since your loading current is small, fully integrated such a charge pump is possible area around ~0.5mm^2 is reasonable assume using 0.6um process.
(5) By the way, what application your charge pump used?