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MIC5205

150mA Low-Noise LDO Regulator

Micrel 

MIC5205

Applications Information

Enable/Shutdown

Forcing EN (enable/shutdown) high (> 2V) enables the regu-

lator. 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.

Input Capacitor

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.

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 output voltage should consider

larger values of CBYP. Likewise, if rapid turn-on is necessary,

consider omitting CBYP.

If output noise is not a major concern, omit CBYP and leave

BYP open.

Output Capacitor

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.

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 electro-

lytes that freeze at about –30°C, solid tantalums are recom-

mended for operation below –25°C.

At lower values of output current, less output capacitance is

required for output stability. The capacitor can be reduced to

0.47µF for current below 10mA or 0.33µF for currents below

1mA.

No-Load Stability

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.

Micrel

Thermal Considerations

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 ther-

mal resistance of the device and the following basic equation:

( ) PD(max) =

TJ(max) – TA

θJA

TJ(max) is the maximum junction temperature of the die,

125°C, and TA is the ambient operating temperature. θJA is

layout dependent; Table 1 shows examples of junction-to-

ambient thermal resistance for the MIC5205.

Package

θJA Recommended θJA 1" Square θJC

Minimum Footprint Copper Clad

SOT-23-5 (M5)

220°C/W

170°C/W 130°C/W

Table 1. SOT-23-5 Thermal Resistance

The actual power dissipation of the regulator circuit can be

determined using the equation:

PD = (VIN – VOUT) IOUT + VIN IGND

Substituting PD(max) for PD and solving for the operating

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:

PD(max) =

(125°C – 25°C)

220°C/W

PD(max) = 455mW

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

VIN(max) = 6.23V

Therefore, a 3.3V application at 150mA of output current can

accept a maximum input voltage of 6.2V in a SOT-23-5

package. For a full discussion of heat sinking and thermal

effects on voltage regulators, refer to the Regulator Thermals

section of Micrel’s Designing with Low-Dropout Voltage Regu-

lators handbook.

June 2000

7

MIC5205

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