Micrel
MIC5205
0.33µF for currents below 1mA.
Application Information
No-Load Stability
Enable/Shutdown
The MIC5205 will remain stable and in regulation with no
load (other than the internal voltage divider) unlike many
other voltage regulators. This is especially important in
CMOS RAM keep-alive applications.
Forcing EN (enable/shutdown) high (> 2V) enables the
regulator. EN is compatible with CMOS logic gates.
If the enable/shutdown feature is not required, connect
EN (pin 3) to IN (supply input, pin 1). See Figure 1.
Thermal Considerations
Input Capacitor
The MIC5205 is designed to provide 150mA of
continuous current in a very small package. Maximum
power dissipation can be calculated based on the output
current and the voltage drop across the part. To
determine the maximum power dissipation of the
package, use the junction-to-ambient thermal resistance
of the device and the following basic equation:
A 1µF capacitor should be placed from IN to GND if
there is more than 10 inches of wire between the input
and the ac filter capacitor or if a battery is used as the
input.
Reference Bypass Capacitor
BYP (reference bypass) is connected to the internal
voltage reference. A 470pF capacitor (CBYP) connected
from BYP to GND quiets this reference, providing a
significant reduction in output noise. CBYP reduces the
regulator phase margin; when using CBYP, output
capacitors of 2.2µF or greater are generally required to
maintain stability.
TJ(max) − TA
PD(max)
=
θJA
T
J(max) is the maximum junction temperature of the die,
125°C, and T is the ambient operating temperature. θJA
A
is layout dependent; Table 1 shows examples of
junction-toambient thermal resistance for the MIC5205.
The start-up speed of the MIC5205 is inversely
proportional to the size of the reference bypass
capacitor. Applications requiring a slow ramp-up of
Package
θJA
Recommended
Minimum
θJA Square
Copper Clad
θJC
output voltage should consider larger values of CBYP
Likewise, if rapid turn-on is necessary, consider omitting
CBYP
.
Footprint
SOT-23-5(M5)
220°C/W
170°C/W
130°C/W
.
Table 1. SOT-23-5 Thermal Resistance
If output noise is not a major concern, omit CBYP and
leave BYP open.
The actual power dissipation of the regulator circuit can
be determined using the equation:
PD = (VIN – VOUT) IOUT + VIN IGND
Output Capacitor
Substituting PD(max) for PD and solving for the operating
An output capacitor is required between OUT and GND
to prevent oscillation. The minimum size of the output
capacitor is dependent upon whether a reference bypass
capacitor is used. 1.0µF minimum is recommended
when CBYP is not used (see Figure 2). 2.2µF minimum is
recommended when CBYP is 470pF (see Figure 1).
Larger values improve the regulator’s transient
response. The output capacitor value may be increased
without limit.
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit.
For example, when operating the MIC5205-3.3BM5 at
room temperature with a minimum footprint layout, the
maximum input voltage for a set output current can be
determined as follows:
125°C − 25°C
)
PD(max)
PD(max) = 455mW
=
220°C/W
The output capacitor should have an ESR (effective
series resistance) of about 5Ω or less and a resonant
frequency above 1MHz. Ultra-low-ESR capacitors can
cause a low amplitude oscillation on the output and/or
underdamped transient response. Most tantalum or
aluminum electrolytic capacitors are adequate; film types
will work, but are more expensive. Since many aluminum
electrolytics have electrolytes that freeze at about –
30°C, solid tantalums are recommended for operation
below –25°C.
The junction-to-ambient thermal resistance for the
minimum footprint is 220°C/W, from Table 1. The
maximum power dissipation must not be exceeded for
proper operation. Using the output voltage of 3.3V and
an output current of 150mA, the maximum input voltage
can be determined. From the Electrical Characteristics
table, the maximum ground current for 150mA output
current is 2500µA or 2.5mA.
455mW = (VIN – 3.3V) 150mA + VIN·2.5mA
455mW = VIN×150mA – 495mW + VIN·2.5mA
950mW = VIN×152.5mA
At lower values of output current, less output
capacitance is required for output stability. The capacitor
can be reduced to 0.47⎧F for current below 10mA or
M9999-020806
(408) 955-1690
February 2006
8
MICROCHIP [ MICROCHIP ]
