LM2931 Series

7

MOTOROLA ANALOG IC DEVICE DATA

VO, OUTPUT VOLTAGE (V)

1.160

1.180

1.200

1.220

1.240

0

3.0

6.0

9.0

12

15

18

21

24

18.5

14

t, TIME (10

µs/DIV)

Vout = 5.0 V

RL = 500 Ω

CO = 100 µF

CO(ESR) = 0.3 Ω

TA = 25°C

100

0

t, TIME (10

µs/DIV)

VO, OUTPUT VOLTAGE (V)

2.0

2.1

2.2

0

3.0

6.0

9.0

12

15

18

21

24

Output “On”

Output “Off”

2.4

2.5

2.6

2.3

LM2931C Adjustable

IO = 10 mA

Vin = Vout + 1.0 V

TA = 25°C

Vin = 14 V

Vout = 5.0 V

Cin = 1000 µF

CO = 100 µF

CO(ESR) = 0.3 Ω

TA = 25°C

LM2931C Adjustable

IO = 10 mA

Vin = Vout + 1.0 V

TA = 25°C

Figure 13. Line Regulation

Figure 14. Load Regulation

Figure 15. Reference Voltage versus Output Voltage

Figure 16. Output Inhibit–Thresholds

versus Output Voltage

APPLICATIONS INFORMATION

The LM2931 series regulators are designed with many

protection features making them essentially blow–out proof.

These features include internal current limiting, thermal

shutdown, overvoltage and reverse polarity input protection,

and the capability to withstand temporary power–up with

mirror–image insertion. Typical application circuits for the

fixed and adjustable output device are shown in Figures 17

and 18.

The input bypass capacitor Cin is recommended if the

regulator is located an appreciable distance (

≥ 4″) from the

supply input filter. This will reduce the circuit’s sensitivity to

the input line impedance at high frequencies.

This regulator series is not internally compensated and

thus requires an external output capacitor for stability. The

capacitance value required is dependent upon the load

current, output voltage for the adjustable regulator, and the

type of capacitor selected. The least stable condition is

encountered at maximum load current and minimum output

voltage. Figure 22 shows that for operation in the “Stable”

region, under the conditions specified, the magnitude of the

output capacitor impedance |ZO| must not exceed 0.4 Ω. This

limit must be observed over the entire operating temperature

range of the regulator circuit.

With economical electrolytic capacitors, cold temperature

operation can pose a serious stability problem. As the

electrolyte freezes, around – 30

°C, the capacitance will

decrease and the equivalent series resistance (ESR) will

increase drastically, causing the circuit to oscillate. Quality

electrolytic capacitors with extended temperature ranges of

–40

° to +85°C and –55° to +105°C are readily available.

Solid tantalum capacitors may be a better choice if small size

is a requirement, however, the maximum

ZO limit over

temperature must be observed.

Note that in the stable region, the output noise voltage is

linearly proportional to

ZO. In effect, CO dictates the high

frequency roll–off point of the circuit. Operation in the area

titled “Marginally Stable” will cause the output of the regulator

to exhibit random bursts of oscillation that decay in an

under–damped fashion. Continuous oscillation occurs when

operating in the area titled “Unstable”. It is suggested that

oven testing of the entire circuit be performed with maximum

load, minimum input voltage, and minimum ambient

temperature.