MIC5209
Micrel, Inc.
Fixed Regulator Circuits
MIC5209BM
VIN
VOUT
2
1
3
4
IN
OUT
ADJ
GND
2
3
BYP
R1
R2
EN
1
4
2.2µF
EN
5–8
GND
5–8
470pF
Figure 1. Low-Noise Fixed Voltage Regulator
Figure 4. Ultra-Low-Noise Adjustable Application.
Figure1showsabasicMIC5209-x.xBM(SO-8)fixed-voltage
regulator circuit. See Figure 5 for a similar configuration us-
ing the more thermally-efficient MIC5209-x.xBS (SOT-223).
A 1µF minimum output capacitor is required for basic fixed-
voltage applications.
Figure 4 includes the optional 470pF bypass capacitor from
ADJ to GND to reduce output noise.
Slot-1 Power Supply
Intel’s Pentium II processors have a requirement for a 2.5V
±5% power supply for a clock synthesizer and its associated
loads. The current requirement for the 2.5V supply is depen-
dant upon the clock synthesizer used, the number of clock
outputs, and the type of level shifter (from core logic levels to
2.5V levels). Intel estimates a worst-case load of 320mA.
2
3
1
4
The MIC5209 was designed to provide the 2.5V power
requirement for Slot-1 applications. Its guaranteed perfor-
mance of 2.5V ±3% at 500mA allows adequate margin for
all systems, and its dropout voltage of 500mV means that it
operates from a worst-case 3.3V supply where the voltage
can be as low as 3.0V.
5–8
470pF
Figure 2. Ultra-Low-Noise Fixed Voltage Regulator
Figure 2 includes the optional 470pF noise bypass capacitor
between BYP and GND to reduce output noise. Note that the
minimum value of C
capacitor is used.
must be increased when the bypass
OUT
1
3
Adjustable Regulator Circuits
2,TAB
2
1
3
GND
4
R1
R2
Figure 5. Slot-1 Power Supply
5–8
A Slot-1 power supply (Figure 5) is easy to implement. Only
two capacitors are necessary, and their values are not criti-
cal. C bypasses the internal circuitry and should be at least
IN
0.1µF. C
provides output filtering, improves transient
Figure 3. Low-Noise Adjustable Voltage Regulator
OUT
response, and compensates the internal regulator control
The MIC5209BM/U can be adjusted to a specific output volt-
age by using two external resistors (Figure 3). The resistors
set the output voltage based on the equation:
loop. Its value should be at least 22µF. C and C
may
IN
OUT
be increased as much as desired.
Slot-1 Power Supply Power Dissipation
R2
R1
Powered from a 3.3V supply, the Slot-1 power supply of
Figure 5 has a nominal efficiency of 75%. At the maximum
anticipated Slot 1 load (320mA), the nominal power dissipa-
tion is only 256mW.
V
= 1.242V 1 +
OUT
This equation is correct due to the configuration of the
bandgap reference. The bandgap voltage is relative to the
output, as seen in the block diagram. Traditional regula-
tors normally have the reference voltage relative to ground;
therefore, their equations are different from the equation for
the MIC5209BM/U.
The SOT-223 package has sufficient thermal characteristics
for wide design margins when mounted on a single layer
copper-clad printed circuit board. The power dissipation of
the MIC5209 is calculated using the voltage drop across the
device ×
output current plus supply voltage ×
ground current.
AlthoughADJisahigh-impedanceinput,forbestperformance,
R2 should not exceed 470kΩ.
M9999-060906
10
June 2006
MICREL [ MICREL SEMICONDUCTOR ]
