MIC5211 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – Dual μCap 80mA LDO Regulator Preliminary Information

MIC5211
Applications Information
Enable/Shutdown
ENA and ENB (enable/shutdown) may be controlled sepa-
rately. Forcing ENA/B high (>2V) enables the regulator. The
enable inputs typically draw only 15µA.
While the logic threshold is TTL/CMOS compatible, ENA/B
may be forced as high as 20V, independent of V_IN. ENA/B
may be connected to the supply if the function is not required.
Input Capacitor
A 0.1µF capacitor should be placed from IN to GND if there
is more than 10 inches of wire between the input and the ac
filter capacitor or when a battery is used as the input.
Output Capacitor
Typical PNP based regulators require an output capacitor to
prevent oscillation. The MIC5211 is ultrastable, requiring only
0.1µF of output capacitance per regulator for stability. The
regulator is stable with all types of capacitors, including the
tiny, low-ESR ceramic chip capacitors. The output capacitor
value can be increased without limit to improve transient
response.
The capacitor should have a resonant frequency above
500kHz. Ceramic capacitors work, but some dielectrics have
poor temperature coefficients, which will affect the value of
the output capacitor over temperature. Tantalum capacitors
are much more stable over temperature, but typically are
larger and more expensive. Aluminum electrolytic capacitors
will also work, but they have electrolytes that freeze at about
–30°C. Tantalum or ceramic capacitors are recommended
for operation below –25°C.
No-Load Stability
The MIC5211 will remain stable and in regulation with no load
(other than the internal voltage divider) unlike many other
voltage regulators. This is especially important in CMOS
RAM keep-alive applications.
Thermal Shutdown
Thermal shutdown is independent on both halves of the dual
MIC5211, however, an overtemperature condition in one half
may affect the other half because of proximity.
Thermal Considerations
When designing with a dual low-dropout regulator, both
sections must be considered for proper operation. The part is
designed with thermal shutdown, therefore, the maximum
junction temperature must not be exceeded. Since the dual
regulators share the same substrate, the total power dissipa-
tion must be considered to avoid thermal shutdown. Simple
thermal calculations based on the power dissipation of both
regulators will allow the user to determine the conditions for
proper operation.
The maximum power dissipation for the total regulator sys-
tem can be determined using the operating temperatures and
the thermal resistance of the package. In a minimum footprint
configuration, the SOT-23-6 junction-to-ambient thermal re-
sistance (θ_JA) is 220°C/W. Since the maximum junction
temperature for this device is 125°C, at an operating tem-
perature of 25°C the maximum power dissipation is:
Micrel
P_D(max)
=
T_J(max) −
θ_JA
T_A
P_D(max)
=
125°C − 25°C
220°C/W
P_D(max) = 455mW
The MIC5211-3.0 can supply 3V to two different loads inde-
pendently from the same supply voltage. If one of the regu-
lators is supplying 50mA at 3V from an input voltage of 4V, the
total power dissipation in this portion of the regulator is:
( ) ^P_D1 ^{= V}_IN ^{− V}_OUT ^I_OUT ^{+ V}_IN ^⋅I_GND
P_D1 = (4V − 3V) 50mA + 4V ⋅ 0.85mA
P_D1 = 53.4mW
Up to approximately 400mW can be dissipated by the remain-
ing regulator (455mW – 53.4mW) before reaching the ther-
mal shutdown temperature, allowing up to 50mA of current.
( ) ^P_D2 ^{= V}_IN ^{− V}_OUT ^I_OUT ^{+ V}_IN ^⋅I_GND
P_D2 = (4V − 3V) 50mA + 4V ⋅ 0.85mA
P_D2 = 53.4mW
The total power dissipation is:
P_D1 + P_D2 = 53.4mW + 53.4mW
P_D1 + P_D2 = 106.8mW
Therefore, with a supply voltage of 4V, both outputs can
operate safely at room temperature and full load (50mA).
V_IN
MIC5211
IN OUTA
ENA OUTB
V_OUTA
V_OUTB
ENB GND
1µF 1µF
Figure 1. Thermal Conditions Circuit
In many applications, the ambient temperature is much
higher. By recalculating the maximum power dissipation at
70°C ambient, it can be determined if both outputs can supply
full load when powered by a 4V supply.
P_D(max)
=
T_J(max) −
θ_JA
T_A
P_D(max)
=
125°C − 70°C
220°C/W
P_D(max) = 250mW
At 70°C, the device can provide 250mW of power dissipation,
suitable for the above application.
When using supply voltages higher than 4V, do not exceed
the maximum power dissipation for the device. If the device
November 2000
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MIC5211