MIC4043 Datasheet, PDF (6/12 Pages) Micrel Semiconductor – Low-Voltage Secondary-Side Shunt Regulator

MIC4043
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_OUT.
Micrel
Voltage Detector
V_OUT
(FROM
POWER
SUPPLY)
V_TRIP
=
1.245V ^{}
R2
R1
+ 1^{}
MIC4043
IN SNK
GND FB
R1 R_PULL-UP
Logic
Output
R2
33k
DISABLED
ENABLED
Figure 1. Voltage Detector
Figure 1 shows a simple voltage threshold detector with a
logic output.
High-Current Regulator
V_IN
V_OUT = 2.5V
I_BIAS
≥500µA
R_BIAS
Q1
MIC4043
IN SNK
GND FB
V_OUT
=
1.245V

R2
R1
+ 1^{}
∴R1= 33k, R2 = 33k
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_SAT(min) = 200mV).
MIC4043
6
May 2000