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LM2940

1.25A High Current Low Dropout Voltage Regulator Adjustable

Unspecified 

APPLICATION HINTS

The Bay Linear LM2941 incorporates protection against over-current

faults, reversed load insertion, over temperature operation, and

positive and negative transient voltage. However, the use of an output

capacitor is required in order to insure the stability and the

performances.

Thermal Consideration

Although the LM2941 offers limiting circuitry for overload

conditions, it is necessary not to exceed the maximum junction

temperature, and therefore to be careful about thermal resistance. The

heat flow will follow the lowest resistance path, which is the Junction-

to-case thermal resistance. In order to insure the best thermal flow of

the component, a proper mounting is required. Note that the case of

the device is electrically connected to the output. The

case has to be electrically isolated, a thermally conductive spacer can

be used. However do not forget to consider its contribution to thermal

resistance.

Assuming:

VIN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 90°C, θCASE= 1°C/W (no

external heat sink, no wind)

Power dissipation under these conditions

PD = (VIN – VOUT) * IOUT = 7.5W

Junction Temperature

TJ = TA + PD * (θCASE+ θJC)

For the Control Section

TJ = 90°C + 7.5W*(1°C/W + 0.6°C/W) = 102°C

114°C < TJUNCTION MAX for the control section.

For the Power Section

TJ = 90°C + 7.5W*(1°C/W + 1.6°C/W) = 104.5°C

109.5°C < TJUNCTION MAX for the power transistor.

In both case reliable operation is insured by adequate junction

temperature.

Capacitor Requirements

The output capacitor is needed for stability and to minimize the output

noise. The required value of the capacitor varies with the load.

However, a minimum value of 10µF Aluminum will guarantee

stability over load. A tantalum capacitor is recommended for a fast

load transient response.

If the power source has high AC impedance, a 0.1µF capacitor

between input & ground is recommended. This capacitor should have

good characteristics up to 250 kHz.

LM2940/2941

Minimum Load Current

To ensure a proper behavior of the regulator at light load, a

minimum load of 5mA for LM2941 is required.

Adjustable Regulator Design

B29152/53 are adjustable regulators and maybe programmed for

any value between 1.25V and 26V using two resistors. The relation

between the resistors is given by:

R1=R2 (VOUT/1.240 –1)

Resistors have a large value up to 1mΩ in order to reduce the

current consumption. This might be interesting in the case of

widely varying load currents.

Enable Input

LM2941 features enable input allowing turning ON & OFF the

device. EN has been designed to be compatible with TTL/CMOS

logic. When the regulator is ON, the current flowing through this

pin is approximately 20µA.

VIN

BAY

V OU

T

B2940

Fig.1 Basic Fixed Output Regulator

1 BAY 4

VOUT

VIN

2

B2941

3

5

R1

VOUT = VREF X [1 + (R1/

R2

R2)]

For best results, the total series resistance should be small

enough to pass the minimum regulator load current

Fig. 2 Adjustable Output Voltage Regulator

Bay Linear, Inc 2478 Armstrong Street, Livermore, CA 94550 Tel: (925) 989-7144, Fax: (925) 940-9556

www.baylinear.com

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