MIC5271 Datasheet, PDF (8/10 Pages) Micrel Semiconductor – μCap Negative Low-Dropout Regulator

MIC5271
Applications Information
The MIC5271 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 1µF 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 MIC5271 requires an output capacitor for stable opera-
tion. A minimum of 1µF 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 3Ω 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 MIC5271 does not require a load for stability.
Enable Input
The MIC5271 comes with an enable pin that allows the
regulator to be disabled. Forcing the enable pin higher than
the negative threshold and lower than the positive threshold
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. The MIC5271 will be in the on mode when
the voltage applied to the enable pin is either greater than the
positive threshold or less than the negative threshold.
Enable Input
5
4
3
Regulator On
2
1
0
Regulator Off
-1
-2
-3
Regulator On
-4
-5-3 _{-5 -7 -9 -11 -13 -15}
SUPPLY VOLTAGE (V)
Micrel
Thermal Considerations
Absolute values will be used for thermal calculations to clarify
the meaning of power dissipation and voltage drops across
the part.
Proper thermal design for the MIC5271-5.0BM5 can be
accomplished with some basic design criteria and some
simple equations. The following information must be known
to implement your regulator design:
V_IN = input voltage
V_OUT = output voltage
I_OUT = output current
T_A = ambient operating temperature
I_GND = ground current
Maximum power dissipation can be determined by knowing
the ambient temperature, T_A, the maximum junction tem-
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_D(max)
=
T_J(max) −
θ_JA
T_A
P_D(max)
=
125°C − 25°C
235°C/W
P_D(max) = 425mW
The actual power dissipation of the regulator circuit can be
determined using one simple equation.
( ) P_D = V_IN – V_OUT I_OUT + V_IN × I_GND
Substituting P_D(max), determined above, for P_D and solving
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}_IN ^{− 5.0V 100mA + V}_IN ^{× 1mA}
( ) ^{425mW = 100mA × V}_IN ^{+ 1mA × V}_IN ^{− 500mW}
925mW = 101mA × V_IN
V_IN = 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
Regulators” handbook.
MIC5271
8
October 2003