MIC5325_11 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Low VIN/VOUT 400mA ULDO™ with Ultra-Low IQ

Micrel, Inc.
Application Information
The MIC5325 is a high performance, low-dropout linear
regulator designed for low current applications requiring
fast transient response. The MIC5325 utilizes two input
supplies, significantly reducing dropout voltage, perfect for
low-voltage, DC-to-DC conversion. The MIC5325 requires
a minimum of external components.
The MIC5325 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 MIC5325. 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 MIC5325 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 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 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 MIC5325 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 performance 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.
March 2011
MIC5325
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 MIC5325 to drive a
large capacitor on the bypass pin without significantly
slowing turn-on time.
Minimum Load Current
The MIC5325, unlike most other regulators, does not
require a minimum load to maintain output voltage
regulation.
Enable/Shutdown
The MIC5325 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 MIC5325 is designed to provide 400mA 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 = 400mA. 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) × 400mA
P_D = 0.18W
To determine the maximum ambient operating
temperature 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) = 125°C, the maximum junction temperature of the
die θ_JA thermal resistance = 90°C/W.
8
M9999-030811-C