TSM921 datasheet(14/20 Pages) SILABS

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TSM921-TSM924

Page 14

TSM921/24 Rev. 1.0

high impedance on the feedback. The following

procedure explains the steps to design the circuit for

a desired hysteresis:

Choose R3. As the leakage current at the

IN+ pin is less than 1nA, the current through

R3 should be at least 100nA to minimize

offset voltage errors caused by the input

leakage current. For R3 = 11.8M

Ω, the

current through R3 is VREF/R3 at the trip

point. In this case, a 10M

Ω resistor is a good

standard value for R3.

Next, the desired hysteresis band (VHB) is

set. In this example, VHB is set to 50mV.

Calculate R1.

R1 = R3 x

VHB

V+

= 10M

Ω x

50mV

100kΩ

In this example, a 100k

Ω, 1% standard

value resistor is selected for R1.

Choose the trip point for VIN rising (VTHR),

which is the threshold voltage at which the

comparator switches its output from low to

high as VIN rises above the trip point. In this

example, choose VTHR = 3V.

Calculate R2.

R2 =

VTHR

VREF x R1

1.182V x 100k

Ω -

100k

Ω -

10M

Ω

65.44kΩ

In this example, a 64.9k

Ω, 1% standard

value resistor is selected for R2.

The last step is to verify the trip voltages and

hysteresis

band

using

the

standard

resistance values:

VTHR = VREFx R1 x

VTHF = VTHR –

R1 x V+

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.

TYPICAL APPLICATION CIRCUITS

Auto-Off Power Source

A timed auto power-off circuit can be designed as

shown in Figure 4 where the output of the TSM921

is the switched power-supply output. With an internal

reference, hysteresis, high current output, and a

2.5

μA supply current, the TSM921 provides a

wealth of features that make it perfect for this

application.

While

consuming

only

3.5

μA

quiescent current with a 10mA load, the TSM921 is

able to generate a voltage of VBATT – 0.12V. As

shown in Figure 4, three resistors are used to

generate a hysteresis band of 100mV and sets the

IN+ trip point to 50mV when IN+ is going low. The

maximum power-on period of the OUT pin before

power-down occurs can be determined by the RC

time constant as follows:

Figure 3. External Hysteresis

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