Source: IC Skill Porter
Notes taken a long time ago, today there is time to take out and discuss with students learning the direction of charge pump, there are incorrect places welcome to point out.
Drive capability related
An ideal model for cross-coupled charge pumps
Suppose the switching cycle is
, the working capacitance is
(In the image above.)
and
), the output capacitance is
, the output voltage is
, the load current is
, the supply voltage is
, all devices are ideal originals, then there is the following equation:
The charge provided per cycle is:
The charge consumed per cycle is:
Charge conservation in one cycle:
The above is an expression of the output voltage, output current, operating frequency, and capacitance of an ideally cross-coupled charge pump (validated by ideal model simulation).
Think: From the direction of energy conservation, why is the result wrong?
The energy provided by each clock is:
The energy consumed per clock is:
Then you can get the result as:
Some discussion: Because there is an energy loss during charge sharing, it is easier to get the correct result from the charge conservation direction when analyzing this problem.
Output ripple correlation
First, the cross-coupled charge pump ripple frequency is 2 times the operating frequency, and because the structure is perfectly symmetrical, it only needs to be considered in
Work during time.
Assumption: The average voltage is:
, the voltage ripple is:
, the output current is:
, the charging clock rise time is:
, the clock period is:
, the output capacitance is
, then the output ripple can be calculated in two directions:
From the discharge point of view: Since the charging process time is very short (the rising edge of the clock), the discharge time can be considered
so:
From the charging point of view: Charging time:
, the voltage change brought by the load during this time is basically negligible, by
The available charge is:
, voltage ripple caused by charging charge:
Note: The above derivations are ideal models, if you want to consider the on loss of the switch, you need to correct the formula, the above results are verified by the simulation of the ideal model, the simulation results are shown in the following figure.
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