LM27

V+

VTEMP

GND

HYST

OS/OS/US/

US

CLOAD

R

0.12f

Heavy Capacitive

Load, Cable/Wiring

LM27

V+

VTEMP

GND

HYST

OS/OS/US/

US

CLOAD

R

Heavy Capacitive

Load, Cable/Wiring

0.12F

LM27

www.ti.com

SNIS124D – AUGUST 2002 – REVISED JULY 2013

5.

(a) Gradually raise VTEMP until OS goes HIGH. Note VTEMP.

(b) Calculate THYST using Equation 3.

VTEMP LOADING

The VTEMP output has very weak drive capability (1 µA source, 40 µA sink). So care should be taken when

attaching circuitry to this pin. Capacitive loading may cause the VTEMP output to oscillate. Simply adding a resistor

in series as shown in Figure 6 will prevent oscillations from occurring. To determine the value of the resistor

follow the guidelines given in Table 3. The same value resistor will work for either placement of the resistor. If an

additional capacitive load is placed directly on the LM27 output, rather than across CLOAD, it should be at least a

factor of 10 smaller than CLOAD.

Table 3. Resistive compensation for capacitive loading of VTEMP

CLOAD

R (

Ω)

≤100pF

0

1nF

8200

10nF

3000

100nF

1000

≥1µF

430

Resistor placement for capacitive loading compensation of VTEMP

Figure 5. R in series with capacitor

Figure 6. R in series with signal path

NOISE CONSIDERATIONS

The LM27 has excellent power supply noise rejection. Listed below is a variety of signals used to test the LM27

power supply rejection. False triggering of the output was not observed when these signals where coupled into

the V+ pin of the LM27.

•

square wave 400kHz, 1Vp-p

•

square wave 2kHz, 200mVp-p

•

sine wave 100Hz to 1MHz, 200mVp-p

Testing was done while maintaining the temperature of the LM27 one degree centigrade way from the trip point

with the output not activated.

Copyright © 2002–2013, Texas Instruments Incorporated

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