## Thursday, May 5, 2016

### DRV2700 Piezo Driver

DRV2700 is a single chip Piezo driver which is 4 mm x 4 mm x 0.9 mm in size. It can be used in 100 V boost or 1 kV flyback configuration. Supply voltage is 3 V - 5.5 V. In this article, evaluation of DRV2700EVM module is discussed.

The following figure illustrate boost + amplifier configuration of DRV2700.

Where BST is boost output voltage. BST is used as supply voltage (PVDD) for the amplifier. EN is chip enable. FB is boost feedback used together with RFB1 and RFB2 to produce required boost voltage. VFB = 1.3 V and,the relation between RFB1, RFB2, and VBST in boost + amplifier configuration is as follow.
$$\begin{equation} V_{BST}=V_{FB}\left( 1+ \frac{R_{FB1}}{R_{FB2}} \right) \end{equation}$$ For example, if we want VBST of 105 V, we can use RFB1 = 806 kΩ and RFB2 = 10.1 kΩ. When RFB1 = 806 kΩ and RFB2 = 16.45 kΩ, VBST will be 65 V. Jumper settings and VBST table for Evaluation module DRV2700EVM is shown below.

GAIN0 and GAIN1 pins define the gain of amplifier which is ( $$\frac{OUT^+-OUT^- }{IN^+-IN^-}$$ ).

Table. Amplifier gain
GAIN[1:0] GdB GAMP
00 28.8 27.54
01 34.8 54.95
10 38.4 83.18
11 40.7 108.4

The performance of DRV2700 largely depends on the choice of inductor also. The recommended values for the inductor is between 3.3 μH and 22 μH. In general, the smaller the inductance, the higher the saturation current limit. When a large inductance value is used, DRV2700 will automatically switch to lower switching frequency which gives smaller switching losses. On the other hand, it will has higher parasitic inductor losses. Typically, smaller inductance values are used to obtain higher current limit. In this evaluation module, 4.7 μH, 2.7 A inductor is used.

REXT is used to limit boost current by connecting a resistor to ground. The value of resistor should be between 6 kΩ and 35 kΩ. The relationship between Current limit ( $$I_{LIM}$$ ) and resistor ( $$R_{EXT}$$ ) is shown below, where K=10500, VREF=1.35 V, and RINT=60 Ω.

$$\begin{equation} R_{EXT}=K.\frac{V_{REF}}{I_{LIM}}-R_{INT} \end{equation}$$
For example, when REXT=6.04 kΩ, ILIM wil be 2.32 A. To obtain ILIM = 1 A, the value of REXT should be 14 kΩ.

The minimum value of boost capacitor should be 50 nf. X7R type 100 nF capacitor is recommended. It is found that the lower the boost voltage, the better the efficiency. The efficiency at 80 V is around 50 % as shown in the following charts.

A system example which uses DC coupled input is shown below.

A piezo element can be modeled as a capacitor. Then, the peak value for its load current - Ipeak can be calculated as follow. $$\begin{equation} I_{p}=2.\pi.f.C. V_{p} \end{equation}$$ If you want the value of peak supply current for DRV2700, IDD, the voltage ratio, and efficiency - μ can be used to calculate as follow. $$\begin{equation} I_{DD}=2.\pi.f.C. V_{p}.\frac{V_{BST}}{V_{DD}.\mu} \end{equation}$$ For example, Stack Multilayer Piezo Actuator, PICMA P-885.91 (Physik Instrumente, Karlsruhe, Germany) has capacitance 3.1 μF and travel range 32 μm. For Vp= 50 V and frequency 10 Hz, Ipeak is 9.7 mA. Using VBST= 105 V, VDD= 3.3 V, and efficiency μ=45%, peak IDD is obtained as 689 mA.

As an another example, Noliac plate stack actuator NAC2003-H50-A01 has capacitance 24620 nF and travel range 72 μm. For Vp= 30 V and frequency 10 Hz, Ipeak is 46.4 mA. Using VBST= 65 V, VDD= 3.3 V, and efficiency μ=55%, peak IDD will be 1.66 A.

DRV2700 is compared against DRV8662 which is used in PDu100D. For Piezo actuator with large capacitive load, DRV2700 is said to be more suitable.

To reduce noise at output voltage, output resistor Ro is normally used to connect piezo element in series. Bandwidth is same as that of first order RC low pass filter. $$\begin{equation} f_{c}=\frac{1}{2.\pi.R_o.C} \end{equation}$$ The result of comparison between these two piezo drivers is shown below.

Table. Driving NAC2003-H50-A01, 25 μF with 60V peak to peak sinusoidal @ VBST= 60 V, VDD= 5 V, Ro= 1 kΩ
Driver DC 1 Hz 10 Hz
DRV2700 13.5 mA 10 ~ 110 mA 170 ~ 250 mA
pDu100B 33 mA 30 ~ 130 mA 200 ~ 300 mA

Table. Driving PICMA P-885.91, 3 μF with 60V peak to peak sinusoidal @ VBST= 100 V, VDD= 5 V, Ro= 1 kΩ
Driver DC 1 Hz 10 Hz
DRV2700 34 mA 32 ~ 57 mA 120 ~ 167 mA
pDu100B 73 mA 80 ~ 125 mA 160 ~ 220 mA

I have designed a 3 channel piezo driver which is shown in the following figure. It uses three DRV2700. It is 4 layered PCB with only 6 mm x 69 mm in size.