Analysis of low noise positive and negative power supplies based on high voltage negative output charge pump

introduction

Electronic components such as operational amplifiers, drivers or sensors often require a bipolar supply to operate, but there is rarely one available at the point of load. The LTC3260 is a negative output charge pump (inductorless) DC/DC converter with two low noise LDO regulators that can generate positive and negative supplies from a single wide input (4.5V to 32V) supply. The device can switch between high efficiency Burst Mode operation and low noise constant frequency mode, making it popular in both portable and noise sensitive applications. The LTC3260 is available in a flat 3mm x 4mm DFN package or a thermally enhanced 16-pin MSOP package, which helps to achieve a compact solution with very few external components. Figure 1 shows a typical 12V to ±5V application using the LTC3260.

Negative Output Charge Pump

The LTC3260 can provide up to 100mA at its charge pump output (VOUT) from an inverting input voltage. VOUT also serves as the input supply for a negative LDO regulator (LDO-). The charge pump frequency is adjustable from 50kHz to 500kHz by a single external resistor. The MODE pin is used to select between high efficiency Burst Mode operation and constant frequency mode for low noise requirements.

Constant frequency mode

A single resistor at the RT pin sets the constant operating frequency of the charge pump. If the RT pin is grounded, the charge pump operates at 500kHz, where the open-loop output resistance (ROL) and output ripple are optimized to provide the maximum available output power with a peak-to-peak output ripple of only a few mV.

As shown in Figure 2, light load efficiency can be improved by reducing the operating frequency, but at the expense of increased output ripple. The lower operating frequency will produce a higher effective open-loop resistance (ROL), but the reduction in switching rate also reduces the input current, resulting in improved efficiency at light loads. In addition, when the load is relatively heavy, the increase in ROL will reduce the actual voltage difference between VOUT and LOD-, thereby reducing the power dissipation in the negative LDO. The cumulative result is an overall improvement in efficiency at high input voltage and/or light load conditions.

As shown in the following expression and Figure 3, reducing the frequency will increase the output ripple.

Burst Mode Operation

Figure 4 shows the light load efficiency of the charge pump in Burst Mode operation. Although the output ripple of Burst Mode operation increases compared to constant frequency mode, the increase in ripple is only a small percentage of VIN, as shown in Figure 5.

Burst Mode operation is achieved by charging VOUT to nearly –VIN. The LTC3260 then enters a low quiescent current sleep state (about 100μA with both LDO regulators enabled) until burst hysteresis is reached. The charge pump then wakes up and the cycle repeats. The average VOUT is approximately –0.94VIN. As the load increases, the charge pump will operate more frequently to keep the output in regulation. If the load increases enough, the charge pump will automatically switch to constant frequency mode to maintain regulation.

Dual LDO

Both LDOs of the LTC3260 (a positive LDO regulator powered from VIN and a negative LDO regulator powered from VOUT) can support 50mA loads. Each LDO has a 300mV dropout voltage and a 50mA output and an adjustment pin that allows the output voltage to be set with a simple resistor divider. The LDO regulators can be enabled individually. The EN- pin enables the negative output charge pump and LDO-. When both regulators are disabled, the device shuts down with a quiescent current of only 2μA. The LDO reference can be filtered by adding a capacitor to each bypass pin to further reduce noise on the output of the LDO regulator.

in conclusion

The LTC3260 generates low noise positive and negative supplies from a single positive supply. The LTC3260 can choose to perform Burst Mode operation, which improves light load efficiency in battery-powered equipment, or a low noise constant frequency mode designed to meet the requirements of noise-sensitive applications. The LTC3260 combines a negative output charge pump and two LDO regulators to provide an elegant solution for applications with 4.5V to 32V inputs.