LMV710 datasheet(13/19 Pages) NSC

13 / 19 page

Application Note (Continued)

The capacitor C

1 is first discharged by applying a positive

pulse to the reset transistor. When a positive voltage V

IN is

applied to the input, the input voltage is higher than the

voltage across C

1. The output of the op amp goes high and

forward biases the diode D

1. The capacitor C1 is charged to

IN. When the input becomes less than the current capacitor

voltage, the output of the op amp A1 goes low and the diode

1 is reverse biased. This isolates the C1 and leaves it with

the charge equivalent to the peak of the input voltage. The

follower prevents unintentional discharging of C

1 by loading

from the following circuit.

5 and C1 are properly selected so that the capacitor is

charged rapidly to V

IN. During the holding period, the capaci-

tor slowly discharge through C

1, via leakage of the capacitor

and the reverse-biased diode, or op amp bias currents. In

any cases the discharging time constant is much larger than

the charge time constant. And the capacitor can hold its

voltage long enough to minimize the output ripple.

Resistors R

2 and R3 limit the current into the inverting input

of A1 and the non-inverting input of A2 when power is

disconnected from the circuit. The discharging current from

1 during power off may damage the input circuitry of the op

amps.

The peak detector can be reset by applying a positive pulse

to the reset transistor. The charge on the capacitor is

dumped into ground, and the detector is ready for another

cycle.

The maximum input voltage to this detector should be less

than (V

+ -V

D), where VD is the forward voltage drop of the

diode. Otherwise, the input voltage should be scaled down

before applying to the circuit.

HIGH SIDE CURRENT SENSING

The high side current sensing circuit (Figure 8) is commonly

used in a battery charger to monitor charging current to

prevent over-charging. A sense resistor R

sense is connected

to the battery directly. This system requires an op amp with

rail-to-rail input. The LMV710/711/715 are ideal for this ap-

plication because its common mode input range can go

beyond the positive rail.

10132523

FIGURE 7. Peak Detector

10132513

FIGURE 8. High Side Current Sensing

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