I have made a Li-ion charger BQ2057C 4.2V 1A.
I have made my design just like Figure4 on page 10.
The only components i have changed are RSNS =0.1R, so i can get 1A output and MOSFET is IRF7416
Other then that nothing is changed.
Ok my problems
Vin can be from 5V to a max of 18V.
But Vout should be no more then 4.2.
So when Vin is 5V, Vout it about 4.6V, and a Vin increase, Vout stablize to about 5V.
To me this seems its not right, at first i though it could be the IC, so i changed them. It made no differnce. But as the Vout is not linear to Vin, i knew the Mosfet is working.
And another , if i plugg in a battery even its full, the Vout, does become 4.2V. But if the battery is low and its charging, the voltage will be about 3.45V, but as increase Vin from 5V to higher, i see battery starts to take more current then 1A.
I now think the problem is with R2. But what effect does this have on the switching. If i am right in saying this value is the problem, how would i calculate to get a correct value?