MIC5309_08 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Low VIN/VOUT 300mA High PSRR ULDO with Ultra-Low IQ

Micrel, Inc.
Applications Information
The MIC5309 is a high performance, low-dropout linear
regulator designed for low current applications requiring
fast transient response. The MIC5309 utilizes two input
supplies, significantly reducing dropout voltage, perfect
for low-voltage, DC-to-DC conversion. The MIC5309
requires a minimum of external components.
The MIC5309 regulator is fully protected from damage
due to fault conditions, offering linear current limiting and
thermal shutdown.
Bias Supply Voltage
V_BIAS, requiring relatively light current, provides power to
the control portion of the MIC5309. Bypassing on the
bias pin is recommended to improve performance of the
regulator during line and load transients. 1µF ceramic
capacitor from V_BIAS to ground helps reduce high
frequency noise from being injected into the control
circuitry from the bias rail and is good design practice.
Input Supply Voltage
V_IN provides the supply to power the LDO. The minimum
input voltage is 1.7V, allowing conversion from low
voltage supplies.
Output Capacitor
The MIC5309 requires an output capacitor of 1µF or
greater to maintain stability. The design is optimized for
use with low-ESR ceramic chip capacitors. High ESR
capacitors may cause high frequency oscillation. 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 recomm-
ended 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 operating 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.
Input Capacitor
The MIC5309 is a high-performance, high bandwidth
device. Therefore, it requires a well-bypassed input
supply for optimal performance. A 1µF capacitor is
required from the input to ground to provide stability.
Low-ESR ceramic capacitors provide optimal perform-
ance at a minimum of space. Additional 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.
May 2008
MIC5309
Bypass Capacitor
A capacitor can be placed 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 improving PSRR. Turn-on time
increases slightly with respect to bypass capacitance. A
unique, quick-start circuit allows the MIC5309 to drive a
large capacitor on the bypass pin without significantly
slowing turn-on time.
Minimum Load Current
The MIC5309, unlike most other regulators, does not
require a minimum load to maintain output voltage
regulation.
Adjustable Regulator Design
The MIC5309 adjustable version allows programming
the output voltage anywhere between 0.8Vand 2V. Two
resistors are used. The resistor values are calculated by:
R1
=
R2
×
⎜⎜⎝⎛
V_OUT
0.775
− 1_{⎟⎟⎠⎞}
Where V_OUT is the desired output voltage.
Enable/Shutdown
The MIC5309 comes with a single active-high enable pin
that allows the regulator to be disabled. Forcing the
enable pin low 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. Forcing
the enable pin high enables the output voltage. The
active-high enable pin uses CMOS technology and the
enable pin cannot be left floating; a floating enable pin
may cause an indeterminate state on the output.
Thermal Considerations
The MIC5309 is designed to provide 300mA of
continuous current in a very small package. Maximum
ambient operating temperature can be calculated based
on the output current and the voltage drop across the
part. Given that the input voltage is 1.8V, the output
voltage is 1.2V and the output current = 300mA. The
actual power dissipation of the regulator circuit can be
determined using the equation:
P_D = (V_IN – V_OUT1) I _OUT + V_IN I_GND
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.
P_D = (1.8V – 1.2V) × 300mA
P_D = 0.18W
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M9999-051508-D