MIC4043

Micrel

MIC4043

6

May 2000

Applications Information

Replacement of ’431-Type Devices

Since the MIC4043 uses a high-side reference,

external

voltage dividers providing the feedback voltage will be

inverted when compared to those used with ’431-equiva-

lent devices.

The industry-standard ’431 is also typically used in series with

an opto-isolator LED. This configuration has a voltage drop of

at least 2.5V for the ’431 plus 1.4V for the LED (3.9V). More

recent lower-voltage shunt regulators require at least 1.25V

of headroom in addition to the 1.4V for the opto isolator, for

a total of 2.65V.

The MIC4043 effectively puts the regulator reference voltage

in parallel with the LED and drives the LED with a single,

series NPN transistor. The headroom required by this transis-

tor is its saturation voltage of 400mV over it’s operating

temperature range, reducing the overall headroom require-

ment to 1.4V + 0.4V = 1.8V.

Compensation

The noninverting side of the error amplifier is connected to the

high-side reference; the reference is connected to the IN pin.

The inverting side of the error amplifier is brought out to the

FB pin. For some applications, no compensation is needed,

but for most, some small value of capacitance is necessary

between the FB pin and SNK pin. The value of the feedback

capacitance is application specific, but for most applications

100pF to 3000pF is all that is needed. Changing the feedback

capacitor changes the loop response; that is, phase and gain

margin. An empirical way to check overall system loop

response, if a network analyzer is not available, is to step load

the output of the systems from 10% to 100% of nominal load.

The resultant small signal response at the output of the

systems will provide an idea of which direction to go based on

the overshoot and settling time of V

Voltage Detector

IN

R1

Logic

Output

R2

33k

GND FB

SNK

(FROM

POWER

SUPPLY)

MIC4043

DISABLED

ENABLED

V

1.245V

R2

R1

1









Figure 1. Voltage Detector

Figure 1 shows a simple voltage threshold detector with a

logic output.

High-Current Regulator

IN

R1

33k

Q1

≥500µA

R2

33k

GND FB

SNK

MIC4043

V

1.245V

R2

R1

1

R1 33k, R2

33k









∴=

=

Figure 2. High-Current Regulator

For the high-current regulator shown in Figure 2, headroom

is equal to the saturation voltage of Q1 plus the saturation

voltage of the MIC4043 (V