MIC5207

Micrel

January 2000

7

MIC5207

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:

P

=

T

– T

D(max)

J(max)

A

JA

()

θ

T

125

°C, and T

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

ambient thermal resistance for the MIC5207.

Package

θθθθθ

θθθθθ

θθθθθ

JC

Minimum Footprint

Copper Clad

SOT-23-5 (M5)

235

°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:

P

Substituting P

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, when operating the MIC5207-3.3BM5 at room

temperature with a minimum footprint layout, the maximum

input voltage for a set output current can be determined as

follows:

P

125˚C25˚C

235

−

P

425mW

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 3000

µA or 3mA.

455mW = (V

455mW = V

920mW = V

V

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

accept a maximum input voltage of 6V in a SOT-23-5 pack-

age. For a full discussion of heat sinking and thermal effects

on voltage regulators, refer to the Regulator Thermals sec-

tion of Micrel’s

Designing with Low-Dropout Voltage Regula-

tors handbook.

Low-Voltage Operation

The MIC5207-1.8 and MIC5207-2.5 require special consid-

eration when used in voltage-sensitive systems. They may

momentarily overshoot their nominal output voltages unless

appropriate output and bypass capacitor values are chosen.

During regulator power up, the pass transistor is fully satu-

rated for a short time, while the error amplifier and voltage

reference are being powered up more slowly from the output

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 1

µF 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 (C

GND quiets this reference, providing a significant reduction in

output noise. C

when using C

generally required to maintain stability.

The start-up speed of the MIC5207 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 C

consider omitting C

If output noise is not a major concern, omit C

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. 1.0

µF minimum is recommended when C

used (see Figure 2). 2.2

µF minimum is recommended when

C

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 5

Ω 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

0.47

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

1mA.

No-Load Stability

The MIC5207 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.

Thermal Considerations

The MIC5207 is designed to provide 180mA of continuous

current in a very small package. Maximum power dissipation

can be calculated based on the output current and the voltage