MIC5252_04 Datasheet, PDF (8/9 Pages) Micrel Semiconductor – 150mA High PSRR, Low Noise μCap CMOS LDO

MIC5252
Applications Information
Enable/Shutdown
The MIC5252 comes with an active-high enable pin that al-
lows the regulator to be disabled. Forcing the enable pin low
disables the regulator and sends it into a “zero” off-mode-cur-
rent state. In this state, current consumed by the regulator
goes nearly to zero. Forcing the enable pin high enables
the output voltage. This part is CMOS and the enable pin
cannot be left floating; a floating enable pin may cause an
indeterminate state on the output.
Input Capacitor
The MIC5252 is a high performance, high bandwidth device.
Therefore, it requires a well-bypassed input supply for opti-
mal performance. A 1µF capacitor is required from the input
to ground to provide stability. Low-ESR ceramic capacitors
provide optimal performance at a minimum of space. Addi-
tional high-frequency capacitors, such as small valued NPO
dielectric type capacitors, help filter out high frequency noise
and are good practice in any RF based circuit.
Output Capacitor
The MIC5252 requires an output capacitor for stability. The
design requires 1µF or greater on the output to maintain stabil-
ity. 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 300mΩ.
The output capacitor can be increased, but performance has
been optimized for a 1µF ceramic output capacitor and does
not improve significantly with larger capacitance.
X7R/X5R dielectric-type ceramic capacitors are recom-
mended because of their temperature performance. X7R-type
capacitors change capacitance by 15% over their operating
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 operat-
ing temperature ranges. To use a ceramic chip capacitor with
Y5V dielectric, the value must be much higher than an X7R
ceramic capacitor to ensure the same minimum capacitance
over the equivalent operating temperature range.
Bypass Capacitor
A capacitor is required from the noise bypass pin to ground
to reduce output voltage noise. The capacitor bypasses
the internal reference. A 0.01µF capacitor is recommended
for applications that require low-noise outputs. The bypass
capacitor can be increased, further reducing noise and im-
proving PSRR. Turn-on time increases slightly with respect
to bypass capacitance. A unique quick-start circuit allows
the MIC5252 to drive a large capacitor on the bypass pin
without significantly slowing turn-on time. Refer to the “Typi-
cal Characteristics” section for performance with different
bypass capacitors.
Active Shutdown
The MIC5252 also features an active shutdown clamp, which
is an N-Channel MOSFET that turns on when the device is
disabled. This allows the output capacitor and load to dis-
charge, de-energizing the load.
Micrel, Inc.
No-Load Stability
The MIC5252 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 MIC5252 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 dis-
sipation of the package, use the junction-to-ambient thermal
resistance of the device and the following basic equation:
P_D (max)
=


T_J (max)
θ_JA
−
T_A


T_J(max) is the maximum junction temperature of the die,
125°C, and T_A is the ambient operating temperature. θ_JA is
layout dependent; Table 1 shows examples of junction-to-
ambient thermal resistance for the MIC5252.
Package θ_JA Recommended θ_JA 1” Square θ_JC
Minimum Footprint Copper Clad
SOT-23-5
(M5 or D5)
235°C/W
185°C/W 145°C/W
Table 1. SOT-23-5 Thermal Resistance
The actual power dissipation of the regulator circuit can be
determined using the equation:
P_D = (V_IN – V_OUT) I_OUT + V_IN I_GND
Substituting P_D(max) 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. For
example, when operating the MIC5252-2.8BM5 at 50°C with
a minimum footprint layout, the maximum input voltage for a
set output current can be determined as follows:
P_D (max)
=
 125 °C − 50°C 
 235 °C/W 
P_D(max) = 315mW
The junction-to-ambient thermal resistance for the minimum
footprint is 235°C/W, from Table 1. The maximum power dis-
sipation must not be exceeded for proper operation. Using
the output voltage of 2.8V and an output current of 150mA,
the maximum input voltage can be determined. Because this
device is CMOS and the ground current is typically 100µA over
the load range, the power dissipation contributed by the ground
current is < 1% and can be ignored for this calculation.
315mW = (V_IN – 2.8V) 150mA
315mW = V_IN × 150mA – 420mW
735mW = V_IN × 150mA
V_IN(max) = 4.9V
Therefore, a 2.8V application at 150mA of output current
can accept a maximum input voltage of 4.9V in a SOT-23-5
package. For a full discussion of heat sinking and thermal
effects on voltage regulators, refer to the “Regulator Ther-
mals” section of Micrel’s Designing with Low-Dropout Voltage
Regulators handbook.
M9999-020305
8
February 2005