MIC5270

Micrel

MIC5270

286

March 1999

Applications Information

The MIC5270 is a general-purpose negative regulator that

can be used in any system that requires a clean negative

voltage from a negative output. This includes post regulating

of dc-dc converters (transformer based or charge pump

based voltage converters). These negative voltages typically

require a negative low-dropout voltage regulator to provide a

clean output from typically noisy lines.

Input Capacitor

A 1

µF input capacitor should be placed from IN to GND if

there is more than 2 inches of wire or trace between the input

and the ac filter capacitor, or if a battery is used as the input.

Output Capacitor

The MIC5270 requires an output capacitor for stable opera-

tion. A minimum of 1

µF of output capacitance is required. The

output capacitor can be increased without limitation to im-

prove transient response. The output does not require ESR

to maintain stability, therefore a ceramic capacitor can be

used. High-ESR capacitors may cause instability. Capacitors

with an ESR of 3

Ω or greater at 100kHz may cause a high

frequency oscillation.

Low-ESR tantalums are recommended due to the tight ca-

pacitance tolerance over temperature.

Ceramic chip capacitors have a much greater dependence

on temperature, depending upon the dielectric. The X7R is

recommended for ceramic capacitors because the dielectric

will change capacitance value by approximately 15% over

temperature. The Z5U dielectric can change capacitance

value by as much 50% over temperature, and the Y5V

dielectric can change capacitance value by as much as 60%

over temperature. To use a ceramic chip capacitor with the

Y5V dielectric, the value must be much higher than a tanta-

lum to ensure the same minimum capacitor value over

temperature.

No-Load Stability

The MIC5270 does not require a load for stability.

Thermal Considerations

Absolute values will be used for thermal calculations to clarify

what is meant by power dissipation and voltage drops across

the part.

Proper thermal design for the MIC5270-5.0BM5 can be

accomplished with some basic design criteria and some

simple equations. The following information must be known

to implement your regulator design:

V

V

I

T

I

Maximum power dissipation can be determined by knowing

the ambient temperature, T

perature, 125

°C, and the thermal resistance, junction to

ambient. The thermal resistance for this part, assuming a

minimum footprint board layout, is 235

°C/W. The maximum

power dissipation at an ambient temperature of 25

°C can be

determined with the following equation:

P

TT

D(max)

J(max)

A

JA

=

−

θ

P

125 C

25 C

235 C/W

°−

°

°

P

425mW

The actual power dissipation of the regulator circuit can be

determined using one simple equation.

PV

V

I

V

I

DIN

OUT

OUT

IN

GND

=−

()

+⋅

Substituting P

for the operating conditions that are critical to the application

will give the maximum operating conditions for the regulator

circuit. The maximum power dissipation number cannot be

exceeded for proper operation of the device. The maximum

input voltage can be determined using the output voltage of

5.0V and an output current of 100mA. Ground current, of 1mA

for 100mA of output current, can be taken from the Electrical

Characteristics section of the data sheet.

425mW

V

5.0V 100mA

V

1mA

IN

IN

=−

()

+⋅

425mW

100mA V

1mA V

500mW

IN

IN

=⋅

+

⋅

925mW

101mA V

IN

=⋅

V

9.16Vmax

Therefore, a –5.0V application at 100mA of output current

can accept a maximum input voltage of –9.16V in a SOT-23-5

package. For a full discussion of heat sinking and thermal

effects on voltage regulators, refer to Regulator Thermals

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

lators handbook.