What is a charge pump?
A charge pump provides dc-dc voltage conversion using a switch network to charge and discharge two or more capacitors. The basic charge pump switch network toggles between charge and discharge states of the capacitors. As shown in the Figure 1, C1 (the “flying capacitor”) shuttles charge, and C2 (the “reservoir capacitor”) holds charge and filters the output voltage. Additional “flying capacitors” and switch arrays enable multiple gains.
What are typical charge pump applications?
Charge pumps began with unregulated single-output ICs and then to regulated ICs with multiple output voltages. Output power and efficiency have also improved so the charge pump can now deliver up to 250mA at 75% efficiency (average). Most of their applications are in battery-based systems, such as cellular phones, pagers, Bluetooth systems, portable electronic equipment, and handheld instrumentation. A major application is to power white LEDs for backlighting LCD (liquid crystal display) panels (Figure 2). Another application is to power milliwatt-range digital processors.
What operating modes are available with the charge pump?
A charge pump IC can operate as an inverter, splitter or booster. They can double voltages, triple voltages, halve voltages, invert voltages, fractionally multiply or scale voltages such as 3/2X, 4/3X, 2/3X, etc. and generate arbitrary voltages, depending on the controller and circuit topology. The inverter converts the input voltage to a negative output. As a splitter, the output is a fraction of the input voltage. As a booster it can produce an input-output gain of 1.5X or 2X. Many portable systems employ a single li-ion or two NiMH batteries, so operating in the 2X mode, the charge pump can supply the proper forward voltage for white LEDs, which are typically in the range of 3.3V to 4.0V. Figure 3 shows a typical 2X charge pump regulator IC.
What are the major benefits of the charge pump?
A major advantage of the charge pump is elimination of the magnetic fields and EMI that comes with the associated inductor or transformer Another advantage of the “splitter” charge pump is that it provides efficiency improvements over LDOs without the complexity of an inductive step-down regulator. There is one possible minor noise source, however, the high charging current that flows to a “flying capacitor” when it connects to an input source or another capacitor with a different voltage.
Is the charge pump output regulated?
The basic charge pump lacks regulation, so virtually all present-day charge pump ICs add either linear regulation or charge-pump modulation. Linear regulation (LDO) offers the lowest output noise, and therefore provides better performance at a lower efficiency. Figure 2 shows a charge pump IC with two LDOs. Charge-pump modulation (which controls the switch resistance) provides higher efficiency and offers more output current for a given die size (or cost), because the regulator IC doesn’t include the series pass transistor required in an LDO.
Can a charge pump’s output adapt to its input voltage?
Charge pumps can dynamically change their output according to the battery voltage input. For example, a charge pump can operate at either the 1.5X or 1X mode. When the battery input voltage is low, this charge pump produces an output 1.5X the input. The charge pump operates in the 1X mode when the battery voltage is high and the load does not require a voltage boost, so the charge pump merely passes the input voltage to the load. This reduces the input current and power dissipation when the input voltage is high.
What is the effect of increasing the capacitor switching frequency?
Increasing the switching frequency increases the IC’s quiescent current, but also allows smaller capacitance values for C1 and C2. A constant frequency architecture provides a low noise regulated output as well as lower input noise than conventional charge pump regulators. High frequency operation simplifies filtering to further reduce conducted noise.
What are the best capacitors types for a charge pump?
For highest performance, use capacitors with low equivalent series resistance (ESR). Low ESR capacitors should be used at the output of the IC to minimize output ripple, output resistance and to maximize efficiency. You can achieve this using ceramic capacitors, but certain types of tantalum capacitors may be adequate.
What is the effect of soft-start for the charge pump?
Soft-start prevents excessive current flow at VIN during start-up. Soft-start increases the amount of current available to the output charge storage capacitor linearly over a fixed time period. Soft-start is usually enabled whenever the device is brought out of shutdown, and disabled shortly after achieving regulation.
How can the charge pump IC minimize power consumption?
Using pulse frequency modulation (PFM) the IC pumps only when charge must be delivered to maintain output regulation. When the output is above the target regulation voltage, the IC idles and consumes minimal supply current because the charge stored in the output capacitor supplies the load current. As this capacitor discharges and the output voltage falls below the target regulation voltage, the charge pump activates, and delivers charge to the output. This charge supplies the load current and boosts the voltage on the output capacitor.
What are some of the currently available Charge Pump ICs?
High Efficiency, Negative-Charge-Pump White-LED
Maxim’s MAX8822 is a white-LED (WLED) driver that combines a highly efficient negative charge pump with two independent, low-dropout (LDO) regulators (Figure 2). It can drive up to four white light-emitting diodes (LEDs) with regulated constant current for display backlighting in cell phones, digital cameras, PDAs, and other handheld devices.
By utilizing a proprietary negative 0.5x inverting charge pump and independent low-dropout (LDO) adaptive current regulators, very high efficiency is achieved over the full one-cell Li+ battery voltage range, even with large LED VF mismatch. Its 1MHz fixed-frequency switching allows small external components. The regulation scheme is optimized to ensure low EMI and low input ripple.
Two 200mA, low-noise, high power-supply-rejection-ratio (PSRR) LDOs with programmable output voltages provide power for battery-based devices. The LDO regulators produce a low 45µVRMS noise and high 60dB PSRR output for noise-sensitive applications. It features a single-wire, serial-pulse control logic interface that programs LED current and the output voltages of the LDOs. The LED dimming range is pseudo-logarithmic from 24mA to 0.1mA in 31 steps. LDO output voltages are programmable in 16 different combinations to meet various load requirements. The MAX8822 includes soft-start, thermal shutdown, open- and short-circuit protection.
The MAX8822 is fully specified over the -40°C to +85°C extended temperature range.
500mA High Efficiency, Low Noise, Inductorless Step-Down DC-DC Converter
Linear Technologies’ LTC®3251/LTC3251-1.2/LTC3251-1.5 are two-phase charge pump step-down dc-dc converters that produce a regulated output from a 2.7V to 5.5V input. These ICs use switched capacitor fractional conversion to achieve twice the typical efficiency of a linear regulator. No inductors are required. VOUT is resistor programmable from 0.9V to 1.6Vor fixed at 1.2V or 1.5V, with up to 500mA of load current available.
Its two-phase spread spectrum architecture provides a very low noise regulated output as well as low noise at the input. The ICs have four operating modes: continuous spread spectrum, spread spectrum with Burst mode operation, Super Burst™ mode operation and shutdown.
Low operating current (35μA in Burst mode operation, 10μA in Super Burst mode operation) and low external parts count make the LTC3251/LTC3251-1.2/LTC3251-1.5 suited for space-constrained battery-powered applications. The ICs are short-circuit and overtemperature protected, and are available in a thermally enhanced 10-pin MSOP package.
To prevent excessive current flow at VIN during start-up, the LTC3251 family has built-in soft-start circuitry. Soft-start is achieved by increasing the amount of current available to the output charge storage capacitor linearly over a period of approximately 500μs. Soft-start is enabled whenever the device is brought out of shutdown, and is disabled shortly after regulation is achieved.
5V/140 mA Charge Pump
As shown in Figure 3, the Texas Instruments TPS60150 is a 2X switched capacitor voltage converter that produces a regulated, low noise, and low-ripple fixed 5V output from a 2.7V to 5.5V Input Voltage. The 5V output can supply a minimum of 140mA current. It has a 1.5 MHz switching frequency.
Only three external capacitors are needed to generate the output voltage. For a minimum output voltage ripple, the output capacitor (COUT) should be a surface-mount ceramic capacitor. Tantalum capacitors generally have a higher equivalent series resistance (ESR) and may contribute to higher output voltage ripple. Leaded capacitors also increase ripple due to the package’s higher inductance. To achieve the best operation with low input voltage and high load current, the input and flying capacitors (CIN and CF, respectively) should also be surface-mount ceramic types. Large transient currents flow in the VIN, VOUT, and GND traces. To minimize both input and output ripple, keep the capacitors as close as possible to the regulator using short, direct circuit traces.
The TPS60150 enters skip mode if the output voltage reaches 5V+0.15V and the load current is below 8mA (typ). In Skip Mode, the TPS60150 disables the oscillator and decreases the pre-bias current of the output stage to reduce the power consumption. Once the output voltage dips below threshold voltage, 5V+0.15V, the TPS60150 begins switching to increase output voltage until the output reaches 5V+0.15V. When the output voltage dips below 5V, the TPS60150 returns to normal PWM mode; thereby re-enabling the oscillator and increasing the pre-bias current of the output stage to supply output current.
The TPS60150 has internal short circuit protection to protect the IC if the output is shorted to ground. The short circuit protection circuit senses output voltage and clamps the maximum output current to 80mA (typ).
The regulator has thermal shutdown that protects it from damage caused by overload conditions. Thermal protection disables the output when the junction temperature reached approximately 160°C, allowing the device to cool. When the junction temperature cools to approximately 140°C, the output circuitry is automatically re-enabled. The regulator also provides current limit to protect itself and the load.
Other protection features include under voltage lock out protection (UVLO) and inrush current is limited by the soft start function during power on and power transient states.
Ultra-Low Voltage Charge Pump IC
Seiko Instruments’ S-882Z Series is a charge pump IC for step-up dc-dc converter startup, which differs from conventional charge pump ICs, in that it uses a fully depleted SOI (Silicon on Insulator) technology to enable ultra-low voltage operation.
It can step up an extremely low input voltage of 0.3 to 0.35 V, which enables the efficient use of very low energy levels. The stepped up electric power is stored in a startup capacitor, and is discharged as the startup power of the step-up dc-dc converter when the startup capacitor reaches the discharge start voltage level. A built-in shutdown function is also provided, so that when the output voltage of the connected step-up dc-dc converter rises above a given value, the operation is stopped, thereby achieving significant power saving and battery life extension in portable applications.
The S-882Z Series chips come in a small SOT-23-5 package, allowing high-density mounting.
- Current consumption during operation: 0.5 mA max. (at VIN = 0.3 V)
- Current during shutdown: 0.6 μA max. (at VIN = 0.3 V)
- Discharge start voltage: 1.8 to 2.4 V (selectable in 0.2 V steps)
- Shutdown voltage discharge start voltage: + 0.1 V (fixed)
- Oscillation frequency: 350 kHz typ. (at VIN = 0.3 V)
- External component Startup capacitor: (CCPOUT)