HM722 datasheet(10/13 Pages) HMSEMI

10 / 13 page

Typical Application Circuits (continued)

and a lithium battery are housed in the probe assembly. A

conventional low-cost coaxial cable can be used to carry the

op-amp’s output signal to subsequent ICs for pH reading.

SHUNT-BASED CURRENT SENSING AMPLIFIER

The current sensing amplification shown in Figure 8 has a

slew rate of 2πfVPP for the output of sine wave signal, and

has a slew rate of 2fVPP for the output of triangular wave

signal. In most of motor control systems, the PWM

frequency is at 10kHz to 20kHz, and one cycle time is

100μs for a 10kHz of PWM frequency. In current shunt

monitoring for a motor phase, the phase current is

converted to a phase voltage signal for ADC sampling. This

sampling voltage signal must be settled before entering the

ADC.As theFigure 8 shown,thetotal settlingtimeof a

current shunt monitor circuit includes: the rising edge delay

time (tSR) due to the op-amp’s slew rate, and the

measurement settling time (tSET). If the minimum duty cycle

of the PWM is defined at 5%, and the tSR is required at 20%

of a total time window for a phase current monitoring, in

case of a 3.3V motor control system (3.3V MCU with 12-bit

ADC), the op-amp’s slew rate should be more than:

3.3V / (100μs× 5% × 20%) = 3.3 V/μs

At the same time, the op-amp’s bandwidth should be much

greater than the PWM frequency, like 10 time at least.

Figure 8. Current Shunt Monitor Circuit

M72x

To MCU

ADC pin

RSHUNT

To Motor Phase

VBUS

Low side

switch

High side

switch

Amplification

Offset

Filter

tSR

tSET

tSMP

tSR – Time delay due to op-amp slew rate

tSET – Measurement settling time

tSMP – Sampling time window

HM721, HM722, HM724

12MHz, High Slew Rate, RRIO CMOS Operational Amplifiers

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