MIC5315 Datasheet, PDF (9/13 Pages) Micrel Semiconductor – Low Voltage Dual 300mA LDO with Voltage Select

Micrel, Inc.
Application Information
The MIC5315 is a high performance, dual low input
voltage, ultra-low dropout regulator designed for low
applications requiring very fast transient response. The
MIC5315 utilizes two input supplies (V_IN and V_BIAS),
significantly reducing the dropout voltage.
The MIC5315 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 MIC5315. Bypassing on the
bias pin is recommended to improve performance of the
regulator during line and load transients. A 1µF ceramic
capacitor from V_BIAS-to-ground is recommended to help
reduce the high frequency noise from being injected into
the control circuitry.
Input Supply Voltage
V_IN1 and V_IN2, provide the supply to power the LDOs
independently. The minimum input voltage is 1.7V
allowing conversion from low voltage supplies. The low
input voltage provides high efficiency by reducing the
input to output voltage step which minimizes the
regulator power loss.
Input Capacitor
The MIC5315 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.
X5R or X7R dielectrics are recommended for the input
capacitor. Y5V dielectrics lose most of their capacitance
over temperature and are therefore, not recommended.
Output Capacitor
The MIC5315 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
recommended 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
July 2008
MIC5315
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.
Bypass Capacitor
A capacitor can be placed from the bypass pin-to-ground
to reduce the 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 the bypass
capacitance. A unique, quick-start circuit allows the
MIC5315 to drive a large capacitor on the bypass pin
without significantly slowing turn-on time.
No-Load Stability
Unlike many other voltage regulators, the MIC5315 will
remain stable and in regulation with no load. This is
especially important in CMOS RAM keep-alive
applications.
Enable/Shutdown
The MIC5315 is provided with dual active-high enable
pins that allow each regulator to be disabled
independently. 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.
Voltage Select
The MIC5315 incorporates voltage select technology to
set LDO2’s voltage output to a preset lower level. The
/VSC2 pin is an active low input. A logic high signal sets
V_OUT2 to the full output voltage; while a logic low signal
sets V_OUT2 to the lower output voltage.
Thermal Considerations
The MIC5315 is designed to provide 300mA of
continuous current for both outputs in a very small
package. Maximum ambient operating temperature can
be calculated based upon the output current and the
voltage drop across the part. Given that the input voltage
is 1.8V, the output voltage is 1.5V for V_OUT1, 1.0V for
V_OUT2 and the output current = 300mA for each output.
The actual power dissipation of the regulator circuit can
be determined using the equation:
P_D = (V_IN – V_OUT1) I_OUT1 + (V_IN – V_OUT2) I _OUT2 + V_BIAS I_GND
Because this device is CMOS and the ground current is
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M9999-070208-A