LM27762 Low-Noise Charge Pump
Created: Dec 13, 2016
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A charge pump is a type of an inductorless voltage regulator. Instead of using an inductor, this type of DC-to-DC converter uses ceramic or electrolytic capacitors as energy-storage elements. Charge pump is easy to use because it requires no expensive inductors or additional semiconductors. Although capacitors are more common and much cheaper than the coils used in other types of DC-to-DC converters, capacitors cannot change their voltage level abruptly. A changing capacitor voltage always follows the exponential function, which imposes limitations that inductive voltage converters can avoid. However, inductive voltage converters are more expensive. That is the advantage of charge pump over inductive voltage converters. A charge pump is capable of delivering high efficiency circuit while having a simple circuitry and lower cost.
This reference design features LM27762, a low-noise regulated switched-capacitor voltage inverter with both positive and negative LDOs. This IC is capable of delivering very low-noise positive and negative outputs that are adjustable between +/-1.5V and +/-5V. The input voltage range of this design is from 2.7V to 5.5V. The positive LDO provides a positive voltage configurable with external gain setting resistors R1, R2. The low-noise charge pump voltage converter inverts the input voltage to a negative output voltage. Charge pump is followed by the negative LDO, which regulates a negative output voltage configurable with external gain setting resistors R3, R4. Five low-cost capacitors are used in this circuit to provide up to 250mA of output current. The regulated output for the LM27762 is adjustable between ±1.8V and ±5.2V. Positive and negative outputs of LM27762 have dedicated enable inputs (EN+, EN-). These outputs support independent timing for the positive and negative rails for specific system power-sequence needs. Enable inputs can be also shorted together and connected to the input voltage. The LM27762 has an optional Power Good (PGOOD) feature. A 10kΩ pull-up resistor is connected to VIN since the PGOOD feature is used.
Charge pumps can be used as voltage regulator in wireless communication systems, cellular phones, interface power supplies, handheld instrumentation, Hi-Fi headphone and powering data converters. Charge pumps are often the best choice for powering applications requiring a combination of simplicity, low power and low cost.