[Shishuo Chip Products] Want to learn more about the boost charge pump with a brand-new structure?

These unwanted noises can cause many problems. In wireless applications, noise generated by the power input can interfere with RF transmission and reception, while noise at the output can couple with sensitive circuits and even produce audible noise. The new LTC3200 series boost charge pump uses a new structure to minimize noise at the input and output to avoid the above unwanted interference.

Most adjustable charge pump DC/DC converters use a Burst Mode architecture. Although this regulator architecture has the lowest quiescent current, it also generates the highest input and output noise. In a burst mode device, the charge pump switch provides maximum current to the output or is completely turned off. To regulate the output, a hysteresis comparator and reference control the charge pump on and off. When low frequency ripple is generated at the output, it needs to be regulated (see Figure 1). This burst of on and off will result in a large input ripple current, so it must be powered from the input supply. Any impedance in the input supply will generate voltage noise at the input, which must be rejected by other circuits powered from the same supply.

Table 1: Typical burst mode output ripple.

The LTC3200 and LTC3200-5 charge pumps have been designed to minimize input and output noise. These devices are adjustable step-up charge pumps that can provide up to 100mA of output current. The LTC3200-5 provides a regulated 5V output voltage in a 6-pin SOT-23 package, while the LTC3200 generates an adjustable output voltage in an 8-pin MSOP package.

Both devices use a constant frequency architecture to reduce low frequency output noise. The charge pump switches continuously, even when there is no load, and a linear control loop regulates the amount of charge delivered to the output during each clock cycle. Because the output regulation loop is linear, the peak-to-peak output ripple is approximately V _RIPPLE = (I _LOAD /C _OUT )/(2 • f _OSC ), with no additional ripple due to regulator hysteresis.

The device’s 2MHz oscillation frequency allows low output ripple even with small output capacitors. Figure 2 shows the output ripple of the LTC3200-5 for 100mA load current with different values ​​of output capacitors.

While constant frequency alone can improve input noise, the LTC3200 family goes a step further by using a unique built-in control circuit to regulate the charge pump input current at both clock phases. This technique prevents the RC current from decaying during one or two half-clock cycles of the charge pump oscillator, which minimizes the reference input ripple by varying the input current. Figure 3 shows the difference in input noise between the LTC3200 and a typical burst-mode charge pump. As shown, both devices can achieve 100mA of output current and a regulated output voltage of 5V from an input voltage of 3.6V. The 0.1Ω input impedance is used for testing. Typical burst-mode charge pump devices use 10μF ceramic capacitors at both input and output, while the LTC3200 uses 1μF ceramic capacitors with the same dielectric. As shown in Figure 3, even if the bypass capacitor is reduced to 1/10, the use of the LTC3200 can significantly reduce input noise.

Charge pumps typically provide low power boost conversion for handheld electronic devices such as cell phones and PDAs. These devices, especially those containing RF communication circuits, are very sensitive to noise. A popular application for low noise charge pumps is to power white LEDs to provide backlighting for small color LCD displays. The circuit shown in Figure 4 is a low noise boost power supply that can drive up to 6 white LEDs. The FB pin of the LTC3200 is used to regulate the LED current flowing through each LED ballast tube. With the LTC3200, users can directly use the battery to provide boost power for the backlight circuit, eliminating the problem of awkward low frequency noise filtering.