LM4041 datasheet(13/16 Pages) MICREL

13 / 16 page

March 2005

M9999-031805

LM4040/4041

Micrel, Inc.

Applications Information

The stable operation of the LM4040 and LM4041 references

requires an external capacitor greater than 10nF connected

between the (+) and (–) pins. Bypass capacitors with values

between 100pF and 10nF have been found to cause the

devices to exhibit instabilities.

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 2. LM4041-ADJs use pin 3 as

the (–) output.

Conventional Shunt Regulator

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

an external series resistor (RS) is connected between the

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

RS determines the current that ﬂows through the load (IL)

and the reference (IQ). Since load current and supply volt-

age may vary, RS should be small enough to supply at least

the minimum acceptable IQ 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 maxi-

mum and IL 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.

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

operating current, (IL and IQ), and the reference’s reverse

breakdown voltage (VR):

Rs = (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 (VREF) and the ratio of the ex-

ternal feedback resistors as shown in Figure 2. The output

is found using the equation:

(1)

VO = VREF [ (R2/R1) + 1 ]

where VO is the desired output voltage. The actual value of

the internal VREF is a function of VO. The “corrected” VREF

is determined by:

(2)

VREF´ = VO (ΔVREF / ΔVO) + VY

where VO is the desired output voltage. ΔVREF /ΔVO is found

inthe“ElectricalCharacteristics”andistypically–1.3mV/Vand

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

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

Note that actual output voltage can deviate from that pre-

dicted using the typical ΔVREF / ΔVO in equation (2); for C-

grade parts, the worst-case ΔVREF / ΔVO is –2.5mV/V and

VY = 1.248V.

The following example shows the difference in output volt-

age resulting from the typical and worst case values of

ΔVREF / ΔVO.

Let VO = +9V. Using the typical values ofΔVREF /ΔVO , VREF

is 1.223V. Choosing a value of R1 = 10kΩ, R2 = 63.272kΩ.

Using the worst case ΔVREF / ΔVO 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

ΔVREF / ΔVO will work in most cases, requiring no further

adjustment.

Figure 4. Voltage Level Detector

Figure 3. Voltage Level Detector

Typical Application Circuits

120k

–

LM4041-ADJ

< –12V

LED ON

200

–5V

LM4041-

ADJ

120k

–

330

> –12V

LED ON

–5V

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