TS9002

TS9002 Rev. 1.0

Page 9

R1 =

R2 =

1.182 -

2

reference pin and should be maintained within the

maximum current the reference can source. It is safe

to maintain the current within 20µA. It is also

important to ensure that the current from reference is

much larger than the HYST pin input current. Given

R2 = 2.4M

Ω, the current sourced by the reference is

0.5

μA. This allows the hysteresis band and R1 to be

approximated as follows:

R1(k

Note

the

hysteresis

is

the

same

for

both

comparators.

Board Layout and Bypassing

While power-supply bypass capacitors are not

typically required, it is good engineering practice to

use 0.1

μF bypass capacitors close to the device’s

power

supply

pins

when

the

power

supply

impedance is high, the power supply leads are long,

or there is excessive noise on the power supply

traces. To reduce stray capacitance, it is also good

engineering practice to make signal trace lengths as

short as possible. Also recommended are a ground

plane and surface mount resistors and capacitors.

Window Detector

The schematic shown in Figure 3 is for a 4.5V

undervoltage

threshold

detector

and

a

5.5V

overvoltage threshold detector using the TS9002.

Resistor components R1, R2, and R3 can be

selected based on the threshold voltage desired

while resistors R4 and R5 can be selected based on

the hysteresis desired. Adding hysteresis to the

circuit will minimize chattering on the output when

the input voltage is close to the trip point. OUTA and

OUTB

generate

the

active

low

undervoltage

indication and active-low overvoltage indication,

respectively. If both OUTA and OUTB signals are

ANDed together, the resulting output of the AND

gate is an active-high, power-good signal. To design

the circuit, the following procedure needs to be

followed:

1.

As described below, determine the desired

hysteresis and select resistors R4 and R5

accordingly.

This

circuit

has

±5mV

of

hysteresis at the input where the input

input resistor divider.

2.

Choosing R1. As the leakage current at the

INB- pin is less than 1nA, the current

through R1 should be at least 100nA to

minimize offset voltage errors caused by the

input leakage current. Values within 100k

Ω

and

1M

Ω are recommended. In this

example, a 294k

Ω, 1% standard value

resistor is selected for R1.

3.

Calculating R2 + R3. As the input voltage

be 5.5V. Choose R2 + R3 as follows:

R1 + R3 = R1 x

- 1

= 294k

Ω x

5.5V

1.182V + 5mV

- 1

= 1.068M

Ω

4.

the undervoltage threshold should be 4.5V.

Choose R2 as follows:

R2 = (R1 + R2+ R3) x

- 294k

= (294k

Ω + 1.068MΩ) x

1.182V-5mV

4.5

- 294k

= 62.2k

Ω

Figure 3. Window Detector