August 2003

7

MIC5233

MIC5233

Micrel

Applications Information

Enable/Shutdown

The MIC5233 comes with an active-high enable pin that

allows the regulator to be disabled. Forcing the enable pin low

disables the regulator and sends it into a “zero” off-mode-

current state. In this state, current consumed by the regulator

goes nearly to zero. Forcing the enable pin high enables the

output voltage.

Input Capacitor

The MIC5233 has high input voltage capability up to 36V. The

input capacitor must be rated to sustain voltages that may be

used on the input. An input capacitor may be required when

the device is not near the source power supply or when

supplied by a battery. Small, surface mount, ceramic capaci-

tors can be used for bypassing. A larger value may be

required if the source supply has high ripple.

Output Capacitor

The MIC5233 requires an output capacitor for stability. The

design requires 2.2

µF or greater on the output to maintain

stability. The design is optimized for use with low-ESR

ceramic chip capacitors. High-ESR capacitors may cause

high frequency oscillation. The maximum recommended

ESR is 3

Ω. The output capacitor can be increased without

limit. Larger valued capacitors help to improve transient

response.

X7R/X5R dielectric-type ceramic capacitors are recom-

mended because of their temperature performance. X7R-

type capacitors change capacitance by 15% over their oper-

ating temperature range and are the most stable type of

ceramic capacitors. Z5U and Y5V dielectric capacitors change

value by as much as 50% and 60% respectively over their

operating temperature ranges. To use a ceramic chip capaci-

tor with Y5V dielectric, the value must be much higher than an

X7R ceramic capacitor to ensure the same minimum capaci-

tance over the equivalent operating temperature range.

No-Load Stability

The MIC5233 will remain stable and in regulation with no load

unlike many other voltage regulators. This is especially

important in CMOS RAM keep-alive applications.

Thermal Consideration

The MIC5233 is designed to provide 100mA 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:

P

TT

D(MAX)

J(MAX)

A

JA

=

−









θ

T

125

°C, and T

layout dependent; Table 1 shows examples of the junction-

to-ambient thermal resistance for the MIC5233.

Package

θθθθθ

Minimum Footprint

SOT-23-5

235

°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 MIC5233-3.0BM5 at 50

°C with

a minimum footprint layout, the maximum input voltage for a

set output current can be determined as follows:

P

125 C

50 C

235 C/W

°−

°

°









P

The junction-to-ambient (

θ

minimum footprint is 235

°C/W, from Table 1. It is important

that the maximum power dissipation not be exceeded to

ensure proper operation. Since the MIC5233 was designed

to operate with high input voltages, careful consideration

must be given so as not to overheat the device. With very high

input-to-output voltage differentials, the output current is

limited by the total power dissipation. Total power dissipation

is calculated using the following equation:

P

Due to the potential for input voltages up to 36V, ground

current must be taken into consideration.

If we know the maximum load current, we can solve for the

maximum input voltage using the maximum power dissipa-

tion calculated for a 50

°C ambient, 319mV.

P

319mW = (V

Ground pin current is estimated using the typical character-

istics of the device.

619mW = V

V

For higher current outputs only a lower input voltage will work

for higher ambient temperatures.

Assuming a lower output current of 10mA, the maximum input

voltage can be recalculated:

319mW = (V

349mW = V

V

Maximum input voltage for a 10mA load current at 50

°C

ambient temperature is 34.9V, utilizing virtually the entire

operating voltage range of the device.