MIC5219

Micrel

MIC5219

8

July 2000

Applications Information

The MIC5219 is designed for 150mA to 200mA output current

applications where a high current spike (500mA) is needed

for short, startup conditions. Basic application of the device

will be discussed initially followed by a more detailed discus-

sion of higher current applications.

Enable/Shutdown

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

lator. EN is compatible with CMOS logic. If the enable/

shutdown feature is not required, connect EN to IN (supply

input). See Figure 5.

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.

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

µF minimum is recommended when C

(see Figure 5). 2.2

µF minimum is recommended when C

BYP

is 470pF (see Figure 6). For applications <3V, the output

capacitor should be increased to 22

µF minimum to reduce

start-up overshoot. Larger values improve the regulator’s

transient response. The output capacitor value may be in-

creased without limit.

The output capacitor should have an ESR (equivalent series

resistance) of about 5

Ω or less and a resonant frequency

above 1MHz. Ultra-low-ESR capacitors could cause oscilla-

tion and/or underdamped transient response. Most tantalum

or aluminum electrolytic capacitors are adequate; film types

will work, but are more expensive. Many aluminum electrolyt-

ics have electrolytes that freeze at about –30

°C, so solid

tantalums are recommended for operation below –25

°C.

At lower values of output current, less output capacitance is

needed for stability. The capacitor can be reduced to 0.47

µF

for current below 10mA or 0.33

µF for currents below 1mA.

No-Load Stability

The MIC5219 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.

Reference Bypass Capacitor

BYP is connected to the internal voltage reference. A 470pF

capacitor (C

reference, providing a significant reduction in output noise

(ultra-low-noise performance). C

phase margin; when using C

or greater are generally required to maintain stability.

The start-up speed of the MIC5219 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

Thermal Considerations

The MIC5219 is designed to provide 200mA of continuous

current in two very small profile packages. 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 thermal resistance,

junction-to-ambient, 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 thermal resis-

tance, junction-to-ambient, for the MIC5219.

Package

θθθθθ

θθθθθ

θθθθθ

JC

Minimum Footprint

2 oz. Copper

MM8™ (MM)

160

°C/W

70

°C/W

30

°C/W

SOT-23-5 (M5)

220

°C/W

170

°C/W

130

°C/W

Table 1. MIC5219 Thermal Resistance

The actual power dissipation of the regulator circuit can be

determined using one simple equation.

P

Substituting P

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, if we are operating the MIC5219-3.3BM5 at room

temperature, with a minimum footprint layout, we can deter-

mine the maximum input voltage for a set output current.

P

=

125 C – 25 C

C/W

D(max)

°°

()

°

220

P

The thermal resistance, junction-to-ambient, for the mini-

mum footprint is 220

°C/W, taken from table 1. The maximum

power dissipation number cannot be exceeded for proper

operation of the device. Using the output voltage of 3.3V, and

an output current of 150mA, we can determine the maximum

input voltage. Ground current, maximum of 3mA for 150mA

of output current, can be taken from the Electrical Character-

istics section of the data sheet.

455mW = (V

455mW = (150mA)

× V

950mW = 153mA

× V

IN

V

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.