TSM921 datasheet(12/19 Pages) TOUCHSTONE

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TSM921-TSM924

Page 12

TSM921_24DS r1p0

RTFDS

THEORY OF OPERATION

The TSM921

–TSM924 family of single/dual/quad,

low-voltage,

micropower

analog

comparators

provide excellent flexibility and performance while

sourcing continuously up to 40mA of current. The

TSM921, TSM923, and the TSM924 provide an

on-board

1.182V

±1%

reference

voltage.

minimize glitches that can occur with parasitic

feedback or due to less than optimal board layout,

the design of the TSM921-TSM924 output stage is

optimized to eliminate crowbar glitches as the output

switches. To minimize current consumption while

providing flexibility, the TSM921 and the TSM923

have an on-board HYST pin in order to add

additional hysteresis.

Power-Supply and Input Signal Ranges

The TSM921-TSM924 can operate from a single

supply voltage range of +2.5V to +11V, provide a

wide common mode input voltage range of V- to

V+-1.3V, and accept input signals ranging from V- to

V+ - 1V. The inputs can accept an input as much as

300mV above and below the power supply rails

without damage to the part. While the TSM921 and

the TSM924 are able to operate from a single supply

voltage range, a GND pin is available that allows for

a dual supply operation with a range of ±1.25V to

±5.5V. If a single supply operation is desired, the

GND pin needs to be tied to V-. In a dual supply

mode, the TSM921 and the TSM924 are TTL/CMOS

compatible with a ±5V voltage and the TSM922 and

the TSM923 are TTL compatible with a single +5V

supply.

Low-Voltage Operation: V+ = 1.5V (TSM924 Only)

Due to a decrease in propagation delay and a

reduction in output drive, the TSM921-TSM923

cannot be used with a supply voltage much lower

than 2.5V. However, the TSM924 can operate down

to a supply voltage of 2V; however, as the supply

voltage reduces, the TSM924 supply current drops

and the performance is degraded. When the supply

voltage drops to 2.2V, the reference voltage will no

longer function; however, the comparators will

function

down

1.5V

supply

voltage.

Furthermore, the input voltage range is extended to

just below 1.5V the positive supply rail. For

applications with a sub-2.5V power supply, it is

recommended to evaluate the circuit over the entire

power supply range and temperature.

Comparator Output

The TSM921 and the TSM924 have a GND pin that

allows the output to swing from V+ to GND while the

V- pin can be set to a voltage below GND as long as

the voltage difference between V+ and V- is within

11V. Having a different voltage on V- will not affect

the output swing. For TTL applications, V+ can be

set to +5V±10% and V- can be set anywhere

between 0V and -5V±10%. On the other hand, the

TSM922 and the TSM923 do not have a GND pin;

hence, for TTL applications, V+ needs to be set to a

+5V power supply and V- to 0V. Furthermore, the

output design of the TSM921-TSM924 can source

and sink more than 40mA and 5mA, respectively,

while simultaneously maintaining a quiescent current

in the microampere range. If the power dissipation of

the package is maintained within the max limit, the

output can source pulses of 100mA of current with

V+ set to +5V. In an effort to minimize external

component count needed to address power supply

feedback, the TSM921-TSM924 output does not

produce crowbar switching current as the output

switches.

With

10mV

input

overdrive,

the

propagation delay of the TSM921-TSM924 is 12

μs.

Voltage Reference

The TSM921, TSM923, and TSM924 have an on-

board 1.182V reference voltage with an accuracy of

±1%.

The REF pin is able to source and sink 15μA

and 8μA of current, respectively. The REF pin is

referenced to V- and it should not be bypassed.

Noise Considerations

Noise can play a role in the overall performance of

the TSM921-TSM924. Despite having a large gain, if

the input voltage is near or equal to the input offset

voltage, the output will randomly switch HIGH and

LOW. As a result, the TSM921-TSM924 produces a

peak-to-peak noise of approximately 0.3mVPP while

the reference voltage produces a peak-to-peak

noise of approximately 1mVPP.

Furthermore, it is

important

design

layout

that

minimizes

capacitive coupling from a given output to the

reference pin as crosstalk can add noise and, as a

result, degrade performance.

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