Application Analysis of LDO Regulator LTC3260

Photocoupler

Electronic components such as op amps, drivers, or sensors typically require a bipolar power supply, but there is rarely one available bipolar power supply 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 switches between high-efficiency BurstMode operation and low-noise constant-frequency mode, making it popular for both portable and noise-sensitive applications. Available in a low profile 3mm x 4mm DFN package or a thermally enhanced 16-lead MSOP package, the LTC3260 helps achieve a compact solution with few external components. Figure 1 shows a typical 12V to ±5V application using the LTC3260.

Negative output charge pump

The LTC3260 can supply up to 100mA at its charge pump output (VOUT) with an inverting input voltage. In addition, VOUT acts as an input supply to a negative LDO regulator (LDO-). The charge pump frequency can be adjusted from a single external resistor from 50kHz to 500kHz. The MODE pin is used to select between high efficiency burst mode operation and constant frequency mode designed to meet low noise requirements.

Constant frequency mode

A single resistor on the RT pin is responsible for setting 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 maximum available output power with peak-to-peak output ripple of only a few mV.

As shown in Figure 2, the efficiency at light loads can be improved by lowering the operating frequency, but at the cost of increased output ripple. A lower operating frequency will result in a higher effective open-loop resistance (ROL), but a decrease in the switching rate of the switch 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-, thus reducing power dissipation in the negative LDO. The cumulative result is an increase in overall efficiency at high input voltages and/or light load conditions.

As shown in the following expression and Figure 3, lowering 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 the burst mode operation is increased compared to the constant frequency mode, the ripple increase is only a small percentage of VIN, as shown in Figure 5.

Burst mode operation is achieved by charging VOUT close to –VIN. The LTC3260 then enters a low quiescent current sleep state (current consumption is approximately 100μA with both LDO regulators enabled) until a sudden hysteresis is reached. Then, the charge pump is woken up and the above cycle is repeated. The average VOUT is approximately –0.94 VIN. As the load increases, the charge pump will be running more frequently to keep the output in regulation. If the load is increased enough, the charge pump will automatically switch to constant frequency mode to maintain regulation.

Dual LDO

The LTC3260's two LDOs (positive LDO regulators from VIN and negative LDO regulators from VOUT) are capable of supporting 50mA loads. Each LDO has a 300mV dropout and a 50mA output and an adjustment pin that allows a simple resistor divider to set the output voltage. The LDO regulator can be individually enabled. The EN- pin is responsible for enabling the negative output charge pump and the LDO. When both regulators are deactivated, the device will shut down and the quiescent current is only 2μA. It can be added to each bypass pin. A capacitor filters the LDO reference to further reduce noise on the output of the LDO regulator.

in conclusion

The LTC3260 can generate low noise positive and negative supplies with a single positive supply. The LTC3260 can be selected to perform burst mode operation or low noise constant frequency mode, the former can increase efficiency at light loads in battery-powered devices, while the latter is designed to meet the requirements of noise-sensitive applications. Combining a negative output charge pump with two LDO regulators, the LTC3260 provides a compact solution for applications with 4.5V to 32V inputs.