# Why can the offset voltage of the comparator let the transition time be longer or shorter?

When$V_{FB}>V_{REF}$,then the battery enter the CV mode,otherwise,battery enter CC mode,and because inherent offset voltage of the comparator, the transition from the CC stage to the CV stage may be too early or too late.

My question is why can the offset voltage of the comparator let the transition from the CC stage to the CV stage may be too early or too late?

• Well, if you want it to be at 1.5V, set the reference to that, but the comparator switches at a slightly different voltage? – PlasmaHH Mar 23 '18 at 14:06

If I added a small offset voltage in series with one of the comparator inputs like this: -

Wouldn't you now expect the comparator to switch and select CV mode when $V_{FB}$ is greater than $V_{REF}$ + 5 mV?

The 5 mV is, in effect, an input offset voltage.

This would mean that the time taken in CC mode would be a little longer because the battery terminal voltage has to rise to a slightly higher voltage in order to make the comparator switch to CV mode.

The actual transistion time in switching between CC and CV mode is unaffected - it will be rapid (sub microseconds) - it's just that the point in time that the transition occurs will be a little later in the scenario above. If the 5 mV was reversed, the transistion would occur a little earlier in the charging cycle.

• oh,so you mean the real delay is because one of voltage have to take some time(sub microseconds) to become bigger than other one? – electronic component Mar 23 '18 at 15:03
• How the voltages change with time is irrelevant except that the feedback voltage has to be 5 mV higher to cause the comparator to switch. How long it takes to become 5 mV higher is down to how much charge is being put into it. – Andy aka Mar 23 '18 at 15:09
• It looks like I'm right – electronic component Mar 23 '18 at 15:18
• I wouldn't have thought so - maybe tens of milliseconds or even seconds or a minute or so. – Andy aka Mar 23 '18 at 15:20