SLOS861B March 2015 – April 2015 DRV2700
The DRV2700 device is a single-chip piezo driver with an integrated 105-V boost switch, integrated power diode, and integrated fully-differential amplifier. This versatile device is capable of driving both high-voltage and low-voltage piezo loads. The input signal can be either differential or single-ended. The DRV2700 device supports four GPIO-controlled gains: 28.8 dB, 34.8 dB, 38.4 dB, and 40.7 dB.
The boost voltage is set using two external resistors. The boost current-limit is programmable through the R(REXT) resistor. The boost converter architecture does not allow the demand on the supply current to exceed the limit set by the R(REXT) resistor; therefore, allowing the user to optimize the DRV2700 circuit for a given inductor based on the desired performance requirements. Additionally, this boost converter is based on a hysteretic architecture to minimize switching losses and therefore increase efficiency.
A typical start-up time of 1.5 ms makes the DRV2700 device an ideal piezo driver for fast responses. Thermal overload protection prevents the device from damage when overdriven.
The DRV2700 device creates a boosted supply rail with an integrated DC-DC converter that can go up to 105 V. The switch-mode power supplies have a few different sources of losses. When boosting to very high voltages, the efficiency begins to degrade because of these losses. The DRV2700 device has a hysteretic boost design to minimize switching losses and therefore increase efficiency. A hysteretic controller is a self-oscillation circuit that regulates the output voltage by keeping the output voltage within a hysteresis window set by a reference voltage regulator and, in this case, the current-limit comparator. Hysteretic converters typically have a larger ripple as a trade off because of the minimized switching. This ripple may vary depending on the output capacitor and load. The power FET and power diode of the boost converter are both integrated within the device to provide the required switching while minimizing external components. Additionally, the boost voltage output (BST) can be easily fed into the high-voltage amplifier through the adjacent pin (PVDD) to help minimize routing inductance and resistance on the board.
When using the high-voltage amplifier in conjunction with the boost converter, the PVDD pin is located next to the BST pin to immediately feed the high voltage signal back into the device to power the amplifier. The DRV2700 device was designed as a differential amplifier. A major benefit of the fully differential amplifier is the improved common-mode rejection ratio (CMRR) over single-ended input amplifiers. The increased CMRR of the differential amplifier reduces sensitivity-to-ground offset that is related noise injection which is important in low-noise systems.
The high-voltage amplifier can be used in a single-ended DC input configuration to provide a DC output on the OUT+ and OUT– pins. The amplifier is very linear across the full voltage range and by using a DAC (digital-to-analog converter) input, the output can be controlled with very good granularity.
Precautions must be taken into thermal concerns of this amplifier because high frequencies, voltage, and capacitive load combinations can overheat the device. See the Piezo Load Selection section for a general guideline.
The DRV2700 device features a fast startup time, which is beneficial for the device come out of shutdown very quickly. When the EN pin transitions from low to high, the boost supply is turned on, the input capacitor is precharged to VDD / 2, and the amplifier is enabled in a 1.5 ms (typical) total start-up time.
When AC coupled with larger input capacitors, the input can require additional time to charge up to VDD / 2. Because the charging current on the input capacitors are not ensured to be exactly the same, a non-zero differential value can exist during startup. Although this differential output voltage (voltage pop) during startup is not specified, it should be fairly small and not exceed 2 V.
The DRV2700 device has programmable gains through the GAIN[1:0] bits. Table 2 lists the gain from IN+ or IN– to OUT+ or OUT–.
The gains are optimized to achieve approximately 50 VPP, 100 VPP, 150 VPP, or 200 VPP at the output without clipping from a 1.8-V peak source of a single-ended input signal.
The output voltage of the integrated boost converter is adjusted by a resistive feedback divider between the boost output voltage (BST) and the feedback pin (FB). The boost voltage should be programmed to a value greater than the maximum peak signal voltage that the user expects to create with the DRV2700 amplifier. Lower boost voltages achieve better system efficiency and therefore should be used when lower amplitude signals are applied. The minimum boost voltage that is required should be used to save on not only power but also heat dissipation. The maximum allowed boost voltage is 105 V.
The current-limit of the boost switch is adjusted through a resistor to ground placed on the REXT pin. In order to protect the device, the REXT pin value should remain between 7.5 kΩ and 32.5 kΩ as shown in Figure 20. To avoid damage to both the inductor and the DRV2700 device, the programmed current-limit must be less than the rated saturation limit of the inductor selected by the user. If the combination of the programmed limit and inductor saturation is not high enough, then the output current of the boost converter is not high enough to regulate the boost output voltage under heavy load conditions. This lower output current causes the boosted rail to sag which can possibly cause distortion of the output waveform.
The DRV2700 device has an integrated charge pump to provide gate drive for internal nodes. The output of this charge pump is placed on the VPUMP pin. An X5R or X7R storage capacitor with a value of 0.1 µF and a voltage rating of 10 V or greater must be placed at this pin for proper operation. This pin and voltage should not be used as an external reference or driver.
The DRV2700 device contains an internal temperature sensor that shuts down both the boost converter and the amplifier when the temperature threshold is exceeded. When the die temperature falls below the threshold, the device restarts operation automatically as long as the EN pin is high. Continuous operation of the DRV2700 device can cause the device to heat up if proper precautions and operating ranges are not followed. The thermal shutdown function protects the DRV2700 device from damage when overdriven, but usage models which drive the DRV2700 device into thermal shutdown should always be avoided.
Although a high-voltage amplifier can be used in a number of ways, the DRV2700 device was intended for two main configurations which are boost + amplifier mode and flyback mode.
In the boost + amplifier mode configuration, the boost converter is used in a boost configuration with a single inductor. The boost output (BST) is then fed into the high-voltage amplifier (PVDD) to drive the outputs. This configuration supports the boost converter up to 100 VP and the amplifier to drive 200 VPP or 0 to 100 VP. The Typical Applications section describes the various implementations of this mode.
In the flyback mode configuration, the boost converter is used in a flyback configuration which allows the boost converter to drive the output to even higher voltages. For example, with a 1:10 turn ratio of the transformer, the transformer can turn the 100 V on the SW node into 1 kV on the high-voltage output. Figure 37 shows a basic circuit diagram.