Datasheet MIC4043 (Micrel) - 8

Micrel, Inc. MIC4043 Application 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 ’431equivalent devices. Voltage Detector
Figure 1 shows a simple voltage threshold detector with a
logic output. 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 removes the need to place the shunt
reference in parallel with the opto-isolator. The MIC4043
combines a 1.245V reference in conjunction with an error
amplifier that drives an NPN output transistor. The NPN
transistor is connected in series with the opto-isolator and
regulates the drive current in the opto-isolator. Unlike
conventional shunt regulators, the MIC4043 does not
have to connect the shunt reference in series with the
opto-isolator, so the voltage drop is just the saturation
voltage for one transistor, typically 160mV at full load. Figure 1. Voltage Detector 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 (VSAT(MIN) =
200mV). Compensation
The non-inverting 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 resistor
capacitor network is necessary between the FB pin and
GND 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 VOUT. October 10, 2014 Figure 2. High-Current Regulator 8 Revision 2.0