LM4040 datasheet(12/16 Pages) MICREL

12 / 16 page

LM4040/4041

Micrel

LM4040/4041

January 2000

Applications Information

The LM4040 and LM4041 have been designed for stable

operation without the need of an external capacitor con-

nected between the (+) and (–) pins. If a bypass capacitor is

used, the references remain stable.

Schottky Diode

LM4040-x.x and LM4041-1.2 in the SOT-23 package have a

parasitic Schottky diode between pin 2 (–) and pin 3 (die

attach interface connect). Pin 3 of the SOT-23 package must

float or be connected to pin 1. LM4041-ADJs use pin 3 as the

(–) output.

Conventional Shunt Regulator

In a conventional shunt regulator application (see Figure 1),

an external series resistor (R

S) is connected between the

supply voltage and the LM4040-x.x or LM4041-1.2 reference.

S determines the current that flows through the load (IL) and

the reference (I

Q). Since load current and supply voltage may

vary, R

S should be small enough to supply at least the

minimum acceptable I

Q to the reference even when the

supply voltage is at its minimum and the load current is at its

maximum value. When the supply voltage is at its maximum

and I

L is at its minimum, RS should be large enough so that

the current flowing through the LM4040-x.x is less than

15mA, and the current flowing through the LM4041-1.2 or

LM4041-ADJ is less than 12mA.

S is determined by the supply voltage (VS), the load and

operating current, (I

L and IQ), and the reference’s reverse

breakdown voltage (V

R).

s = (Vs – VR) / (IL + IQ)

Adjustable Regulator

The LM4041-ADJ’s output voltage can be adjusted to any

value in the range of 1.24V through 10V. It is a function of the

internal reference voltage (V

REF) and the ratio of the external

feedback resistors as shown in Figure 2. The output is found

using the equation

(1)

O = VREF´ [ (R2/R1) + 1 ]

where V

O is the desired output voltage. The actual value of

the internal V

REF is a function of VO. The “corrected” VREF is

determined by

(2)

REF´ = VO (∆VREF / ∆VO) + VY

where V

O is the desired output voltage. ∆VREF / ∆VO is found

in the Electrical Characteristics and is typically –1.3mV/V and

Y is equal to 1.233V. Replace the value of VREF´ in equation

(1) with the value found using equation (2).

Note that actual output voltage can deviate from that pre-

dicted using the typical

∆V

REF / ∆VO in equation (2); for C-

grade parts, the worst-case

∆V

REF / ∆VO is –2.5mV/V and

Y = 1.248V.

The following example shows the difference in output voltage

resulting from the typical and worst case values of

∆V

REF / ∆VO:

Let V

O = +9V. Using the typical values of ∆VREF /∆VO , VREF

is 1.223V. Choosing a value of R1 = 10k

Ω, R2 = 63.272kΩ.

Using the worst case

∆V

REF / ∆VO for the C-grade and D-

grade parts, the output voltage is actually 8.965V and 8.946V

respectively. This results in possible errors as large as 0.39%

for the C-grade parts and 0.59% for the D-grade parts. Once

again, resistor values found using the typical value of

∆V

REF / ∆VO will work in most cases, requiring no further

adjustment.

Figure 4. Voltage Level Detector

120k

–

LM4041-ADJ

< –12V

LED ON

200

–5V

LM4041-

ADJ

120k

–

330

> –12V

LED ON

–5V

Figure 3. Voltage Level Detector

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