MIC5190
Micrel
Feedback Resistors
Active Filter
VOUT
Another application for the MIC5190 is as an active filter on
the output of a switching regulator. This improves the power
supply in several ways.
IR3716S
MIC5190
R1
First, using the MIC5190 as a filter on the output can signifi-
cantly reduce high frequency noise. Switching power sup-
plies 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 MIC5190 as a post regulator.
FB
COUT
R2
GND
Figure 10. Adjustable Output
The feedback resistors adjust the output to the desired
voltage and can be calculated as follows:
R1
VOUT = VREF 1+
R2
V
is equal to 0.5V for the MIC5190. The minimum output
REF
voltage (R1=0) is 0.5V. For output voltages greater than 1V,
use the MIC5191.
VOUT = 1V
ILOAD = 10A
Theresistortoleranceaddserrortotheoutputvoltage. These
errorsareaccumulativeforbothR1 andR2. Forexample, our
resistors selected have a ±1% tolerance. This will contribute
to a ±2% additional error on the output voltage.
TIME (1µs/div)
Figure 11. Ripple Reduction
The feedback resistors must also be small enough to allow
enough current to the feedback node. Large feedback resis-
tors will contribute to output voltage error.
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.
V
= R1×IFB
ERROR
V
= 1kΩ ×12µA
= 12mV
ERROR
V
ERROR
For our example application, this will cause an increase in
output voltage of 12mV. For the percentage increase,
V
ERROR
V
ERROR% =
×100
×100
VOUT
12mV
1.5V
VERROR% =
VERROR% = 0.8%
TIME (100µs/div)
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.
Figure 12. 10A Load Transient
The transient response also contributes to the overall AC
output voltage deviation. Figure 12 shows a 1A to 10A load
transient.Thetoptraceistheoutputoftheswitchingregulator
(same circuit as Figure10). The output voltage undershoots
by 100mV. Just by their topology, linear regulators have the
ability to respond at much higher speeds than a switching
regulator. Linear regulators do not have the limitation or
restrictions of switching regulators which must reduce their
bandwidth to less than their switching frequency.
Applying the MIC5190
Linear Regulator
The primary purpose of the MIC5190 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.
December 2005
11
M9999-120105
MICREL [ MICREL SEMICONDUCTOR ]
