“Charge-pump (inductorless) DC/DC converters are popular in space-constrained applications, which must require low to medium load currents. This converter uses a small package, low quiescent operating current, and requires minimal external components. However, noise production is a less desirable feature of many charge pumps.
Charge-pump (inductorless) DC/DC converters are popular in space-constrained applications, which must require low to medium load currents. This converter uses a small package, low quiescent operating current, and requires minimal external components. However, noise production is a less desirable feature of many charge pumps.
These unwanted noises can cause many problems. In wireless applications, the noise generated at the power input can interfere with RF transmission and reception, while the noise at the output can couple to those sensitive circuits and even produce audible noise. The new LTC3200 series boost charge pump adopts a new structure, which can minimize the noise at the input and output to avoid the above-mentioned unwanted interference.
Burst Mode®Burst Mode vs Constant Frequency
Most adjustable charge pump DC/DC converters use a Burst Mode architecture. Although this regulator configuration has the lowest quiescent current, it also produces the highest input and output noise. With Burst Mode devices, the charge pump switch supplies the maximum current to the output, or is turned off completely. To regulate the output, a hysteretic comparator and reference control the turn-on and turn-off of the charge pump. When there is low frequency ripple at the output, it needs to be adjusted (as shown in Figure 1). This one-on-one-off burst will result in a large input ripple current and, therefore, must be powered by the input power supply. Any impedance in the input supply will create voltage noise at the input that 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 boost charge pumps capable of delivering up to 100mA of output current. The LTC3200-5 provides a regulated 5V output voltage in a 6-pin SOT-23 package; the LTC3200 produces an adjustable output voltage in an 8-pin MSOP package.
Both devices use a constant frequency architecture to reduce low frequency output noise. Even when there is no load, the switching of the charge pump is continuous, and the linear control loop is responsible for regulating the total amount of charge delivered to the output during each clock cycle. Since the output regulation loop is linear, the peak-to-peak output ripple is approximately VRIPPLE = (ILOAD/COUT)/(2 • fOSC), due to the slow swing of the regulator
The device’s 2MHz oscillation frequency allows low output ripple even with small output capacitors. Figure 2 shows the LTC3200-5’s output ripple for a 100mA load current with different values of output capacitors.
Figure 2: Output ripple of the LTC3200-5.
reduce input noise
While constant frequency alone improves input noise, the LTC3200 family goes one step further by using a unique built-in control circuit to regulate the input current of the charge pump across both clock phases. , which minimizes the reference input ripple due to changing the input current. Figure 3 shows the input noise difference between the LTC3200 and a typical burst mode charge pump. As shown, both devices can draw 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 purposes. A typical burst-mode charge pump device uses 10μF ceramic capacitors at the input and output, and the LTC3200 uses 1μF ceramic capacitors with the same dielectric. As shown in Figure 3, using the LTC3200 can significantly reduce input noise even with a 1/10 reduction in bypass capacitance.
Figure 3: Input noise test circuit.
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 powering white LEDs to backlight small color LCD displays. The circuit shown in Figure 4 is a low-noise boost power supply that can drive up to six white LEDs. The FB pin of the LTC3200 is used to regulate the LED current flowing through each LED ballast. With the LTC3200, users can directly use the battery to provide boost power for the backlight circuit, eliminating the need for awkward low-frequency noise filtering.
Figure 4: Low noise white LED driver with LED current control.
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