LINEAR LT2940 User Guide Manual

Unique Analog Multiplier Continuously Monitors Instantaneous Power and Simplifies Design of Power Control Loops Mitchell Lee and Thomas Di Giacomo

As energy consumption in electronics is increasingly scrutinized, the ability to accurately monitor and control power becomes an important part of any system design. To measure instantaneous power, one must simultaneously measure current and voltage and multiply the results. While traditional analog multiplier ICs can perform continuous multiplication, they typically lack the operating range and input sensitivity required for power monitoring and control. The high price of the multiplier itself and the necessary additional signal conditioning circuits make such a solution costly both in dollars and in board area. Digital solutions, on the other hand, 4-quadrant capability allows the LT2940 The LT2940 also includes a current moni- can provide sensitivity and dynamic to monitor bidirectional power flow tor output that allows the load current to range, but lack the ability to continu- such as in battery applications, or to be examined directly. The output signals ously monitor power. For instance, the measure power flow in reversible and for both the power and current are cur- LTC4151 combines a 7V to 80V operating regenerative motor drives. In AC appli- rent mode, a feature readily appreciated voltage range, a current sense amplifier, cations where 4-quadrant operation when using the LT2940 in a servo loop, a MUX, and an I2C interface with a 12-bit is necessary, there is plenty of band- or when simply filtering the output. An ADC to measure current and voltage. width to accurately track the results integrated comparator with complemen- Multiplication is performed in a host of a chopped sinusoid at common line tary open-collector outputs and selectable processor. This makes for an accurate frequencies of 50Hz, 60Hz and 400Hz. latching allows the LT2940 to be used for power monitoring solution, but the 7.5Hz direct control, so an entire control loop conversion rate of the ADC limits its util- can be implemented with a single IC. ity in closed loop applications, where it is unable to respond to rapid changes.

Figure 1. Block diagram of the LT2940

The LT2940 power monitor solves the

problems of creating power monitor GIMON = 1000

and control systems by combining all of

the necessary features in a single IC (see GND Figure 1). Here are a few of its features:

KPMON = 500

• Measures Power of a Supply or Load V2

• 4V–80V High Side Current PMON

Sense, 100V Max V–

• Full 4-Quadrant Operation 4-QUADRANT

MULTIPLIER CMPOUT

• 500k Hz Bandwidth

• Current Mode Power and

Current Monitor Outputs BGAP REF CLR

VCC CMPOUT

AND UVLC UVLC LE

A wide, 4V to 80V operating range makes the LT2940’s current sense suitable for LATCHLO

LATCH THREE-STATE

48V telecom power as well as intermedi- DECODE LATCHHI

ate bus voltages in the 5V to 24V range. LT2940

10 | July 2010 : LT Journal of Analog Innovation