Estimating Charge Pump Voltage Loss Under Load

A switched capacitor converter can produce the expected voltage with almost no load and fall short when the analog circuitry turns on. The nominal conversion ratio describes the topology, while effective output resistance and operating mode determine much of the loaded behavior. Review the rail at its real load before assuming a larger reservoir capacitor will solve the problem.

Identify where voltage is lost

Read the selected device’s model for switch resistance, flying capacitor behavior, capacitor ESR, and switching frequency. The Analog Devices LTC3260 data sheet describes effective open loop resistance as dependent on oscillator frequency, flying capacitance, switch resistance, nonoverlap timing, and external capacitor ESR.

Use this as a device specific example of why capacitance alone is not the complete model. Distinguish the flying capacitor that transfers charge from the output capacitor that supports the load between transfer events. Increasing one does not necessarily substitute for increasing the other.

Build a loaded voltage budget

Start with the ideal conversion voltage, then account for the documented load dependent loss and any subsequent regulator headroom. As an illustrative approximation, 20 mA through an effective resistance of 10 ohms produces 0.2 V of drop. Actual output resistance may vary with operating conditions, so avoid treating one typical value as a universal constant.

Check minimum input voltage, maximum load, temperature, and startup. If the generated negative rail supplies an amplifier, relate the available voltage to its required output swing and input range. A rail that appears close to nominal may still remove necessary analog headroom.

Review capacitor substitutions carefully

Compare effective capacitance at operating bias, ESR at relevant frequencies, and package limitations. Verify that the chosen values remain within the charge pump’s recommended range. Very large capacitors can affect startup time and charging current even if they reduce steady ripple.

Keep switching current loops consistent with the manufacturer’s layout guidance. A noisy measurement taken at a distant ground point can obscure whether the problem is average voltage loss or local ripple.

Measure steady and transient behavior

Record average output voltage, ripple, input current, and temperature across the intended load range. Apply representative load changes and observe whether the rail recovers within the system’s available time. Separate startup charging from steady state droop.

When evaluating a capacitor change, repeat the same input and load conditions rather than comparing unrelated captures. Save part numbers and operating mode with the result. A successful design shows sufficient loaded voltage and acceptable ripple; matching the unloaded conversion ratio is only an initial check.