MIC5238

Micrel

MIC5238

8

August 2003

Applications Information

Enable/Shutdown

The MIC5238 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 Bias Capacitor

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. Larger values may be

required if the source supply has high ripple.

Output Capacitor

The MIC5238 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 a

X7R ceramic capacitor to ensure the same minimum capaci-

tance over the equivalent operating temperature range.

No-Load Stability

The MIC5238 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 Considerations

The MIC5238 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:

P

TT

D(MAX)

J(MAX)

A

JA

=

−









θ

T

125

°C, and T

layout dependent; Table 1 shows the junction-to-ambient

thermal resistance for the MIC5238.

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 MIC5238-1.0BM5 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. With very high input-to-output volt-

age differentials, the output current is limited by the total

power dissipation. Total power dissipation is calculated using

the following equation:

P

Since the bias supply draws only 18

µA, that contribution can

be ignored for this calculation.

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.

469mW = V

V

For higher current outputs only a lower input voltage will work

for higher ambient temperatures.

Assuming a lower output current of 20mA, the maximum input

voltage can be recalculated:

319mW = (V

339mW = V

V

Maximum input voltage for a 20mA load current at 50

°C

ambient temperature is 16.8V. Since the device has a 6V

rating, it will operate over the whole input range.

Dual Suppy Mode Efficiency

By utilizing a bias supply the conversion efficiency can be

greatly enhanced. This can be realized as the higher bias

supply will only consume a few

µA’s while the input supply will

require a few mA’s! This equates to higher efficiency saving

valuable power in the system. As an example, consider an

output voltage of 1V with an input supply of 2.5V at a load