MIC5191_06 Datasheet, PDF (12/15 Pages) Micrel Semiconductor – Ultra High-Speed, High-Current Active Filter / LDO Controller

Micrel, Inc.
Feedback Resistors
IR3716S
V_OUT
MIC5191
FB
R1
R2
COUT
GND
Figure 10. Adjustable Output
The feedback resistors adjust the output to the desired
voltage and can be calculated as follows:
V_OUT
=
V_REF
⎜⎛1
⎝
+
R1
R2
⎟⎞
⎠
V_REF is equal to 1.0V for the MIC5191. The minimum
output voltage (R1=0) is 0.5V. For output voltages less
than 1V, use the MIC5190.The resistor tolerance adds
error to the output voltage. These errors are
accumulative for both R1 and R2. For example, our
resistors selected have a ±1% tolerance. This will
contribute to a ±2% additional error on the output
voltage. The feedback resistors must also be small
enough to allow enough current to the feedback node.
Large feedback resistors will contribute to output voltage
error.
V_ERROR = R1 x 1_FB
V_ERROR = 1kΩ x 12µA
V_ERROR = 12mV
For our example application, this will cause an increase
in output voltage of 12mV. For the percentage increase,
V_ERROR %
=
V_ERROR
V_OUT
× 100
V_ERROR %
=
12mV
1.5V
× 100
V_ERROR% = 0.8%
By reducing R1 to 100Ω, the error contribution by the
feed-back resistors and feedback current is reduced to
less than 0.1%. This is the reason R1 should not be
greater than 100Ω.
MIC5191
Applying the MIC5191
Linear Regulator
The primary purpose of the MIC5191 is as a linear
regulator, which enables an input supply voltage to drop
down through the resistance of the pass element to a
regulated output voltage.
Active Filter
Another application for the MIC5191 is as an active filter
on the output of a switching regulator. This improves the
power supply in several ways.
First, using the MIC5191 as a filter on the output can
significantly reduce high frequency noise. Switching
power supplies tends to create noise at the switching
frequency in the form of a triangular voltage ripple. High
frequency noise is also created by the high-speed
switching transitions. A lot of time, effort, and money are
thrown into the design of switching regulators to
minimize these effects as much as possible. Figure 9
shows the MIC5191 as a post regulator.
Figure 11. Ripple Reduction
Figure 11 shows the amount of ripple reduction for a 500
KHz switching regulator. The fundamental switching
frequency is reduced from greater than 100mV to less
than 10mV.
December 2006
Figure 12. 10A Load Transient
12
M9999-122206