7

FN3182.8

April 5, 2006

Detailed Description

As shown in the Functional Diagram, the ICL7665S consists

of two comparators which compare input voltages on the

SET1 and SET2 terminals to an internal 1.3V bandgap

reference. The outputs from the two comparators drive

open-drain N-channel transistors for OUT1 and OUT2, and

open-drain P-channel transistors for HYST1 and HYST2

outputs. Each section, the Undervoltage Detector and the

Overvoltage Detector, is independent of the other, although

both use the internal 1.3V reference. The offset voltages of

The input impedance of the SET1 and SET2 pins are

extremely high, and for most practical applications can be

ignored. The four outputs are open-drain MOS transistors,

and when ON behave as low resistance switches to their

respective supply rails. This minimizes errors in setting up

the hysteresis, and maximizes the output flexibility. The

operating currents of the bandgap reference and the

comparators are around 100nA each.

Precautions

Junction isolated CMOS devices like the ICL7665S have an

inherent SCR or 4-layer PNPN structure distributed

throughout the die. Under certain circumstances, this can be

triggered into a potentially destructive high current mode.

This latchup can be triggered by forward-biasing an input or

output with respect to the power supply, or by applying

excessive supply voltages. In very low current analog

circuits, such as the ICL7665S, this SCR can also be

triggered by applying the input power supply extremely

rapidly (“instantaneously”), e.g., through a low impedance

battery and an ON/OFF switch with short lead lengths. The

rate-of-rise of the supply voltage can exceed 100V/

µs in

such a circuit. A low impedance capacitor (e.g., 0.05

µF disc

ceramic) between the V+ and GND pins of the ICL7665S

can be used to reduce the rate-of-rise of the supply voltage

in battery applications. In line operated systems, the rate-of-

rise of the supply is limited by other considerations, and is

normally not a problem.

If the SET voltages must be applied before the supply

voltage V+, the input current should be limited to less than

0.5mA by appropriate external resistors, usually required for

voltage setting anyway. A similar precaution should be taken

with the outputs if it is likely that they will be driven by other

circuits to levels outside the supplies at any time.

Simple Threshold Detector

Figure 9 shows the simplest connection of the ICL7665S for

threshold detection. From the graph 9B, it can be seen that

at low input voltage OUT1 is OFF, or high, while OUT2 is

ON, or low. As the input rises (e.g., at power-on) toward

Since the voltage to trip each comparator is nominally 1.3V,

and

1

2

3

4

8

7

6

5

OUT1

HYST1

SET1

GND

V+

OUT2

SET2

HYST2

INPUT

HYST2

OUT2

OUT1

V+

20

k

Ω

12

pF

12

pF

12

pF

12

pF

20

k

Ω

4.7k

Ω

HYST1

4.7

k

Ω

1.0V

1.6V

FIGURE 7. TEST CIRCUITS

1.6V

1.0V

V+

GND

GND

GND

GND

(5V)

V+

(5V)

V+

(5V)

V+

(5V)

INPUT

OUT1

HYST1

OUT2

HYST2

FIGURE 8. SWITCHING WAVEFORMS

+

()

--------------------------------

=

+

()

--------------------------------

=

+

()

----------------------------------

1.3

+

()

---------------------------------- for detector 1

=

=

+

()

----------------------------------

1.3

+

()

---------------------------------- for detector 2

=

=

ICL7665S