MIC5270-3.0BM5 Micrel Semiconductor, MIC5270-3.0BM5 Datasheet - Page 4

• Manufacturer Part Number: MIC5270-3.0BM5
• Description: IttyBitty Negative Low-Dropout Regulator Preliminary Information
• Manufacturer: Micrel Semiconductor

MIC5270
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 F 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 F 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
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:
MIC5270
V
V
I
T
I
OUT
GND
A
IN
OUT
= ambient operating temperature
= input voltage
= output current
= ground current
= output voltage
or greater at 100kHz may cause a high
286
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:
The actual power dissipation of the regulator circuit can be
determined using one simple equation.
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.
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.
P
P
P
P
425mW
425mW
925mW 101mA V
V
D(max)
D(max)
D(max)
D
IN
9.16Vmax
V
IN
D(max)
125 C 25 C
425mW
T
100mA V
V
J(max)
V
235 C/W
IN
OUT OUT
, determined above, for P
JA
5.0V 100mA V
I
T
IN
A
IN
A
, the maximum junction tem-
V
1mA V
IN GND
I
IN
IN
1mA
500mW
D
March 1999
and solving
Micrel