MIC5254_07 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – Dual 150mA μCap LDO with Error Flag Outputs

Micrel, Inc.
Application Information
Enable/Shutdown
The MIC5254 comes with an active-high enable pin for
each regulator 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.
This part is CMOS and the enable pin cannot be left
floating; a floating enable pin may cause an indeterm-
inate state on the output.
Input Capacitor
The MIC5254 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.
Output capacitor
The MIC5254 requires an output capacitor for stability.
The design requires 1µF or greater on the output to
maintain stability. 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 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 capaci-
tance over the equivalent operating temperature range.
Error Flag
The error flag output is an active-low, open-drain output
that drives low when a fault condition AND an under-
voltage detection occurs. Internal circuitry intelligently
monitors overcurrent, overtemperature and dropout
conditions and ORs these outputs together to indicate
some fault condition. The output of that OR gate is
ANDed with an output voltage monitor that detects an
undervoltage condition. That output drives the open-
drain transistor to indicate a fault. This prevents
December 2007
MIC5254
chattering or inadvertent triggering of the error flag. The
error flag must be pulled-up using a resistor from the flag
pin to either the input or the output.
The error flag circuit was designed essentially to work
with a capacitor to ground to act as a power-on reset
generator, signaling a power-good situation once the
regulated voltage was up and/or out of a fault condition.
This capacitor delays the error signal from pulling high,
allowing the downstream circuits time to stabilize. When
the error flag is pulled-up to the input without using a
pull-down capacitor, there can be a glitch on the error
flag upon start up of the device. This is due to the
response time of the error flag circuit as the device starts
up. When the device comes out of the “zero” off mode
current state, all the various nodes of the circuit power
up before the device begins supplying full current to the
output capacitor. The error flag drives low immediately
and then releases after a few microseconds. The
intelligent circuit that triggers an error detects the output
going into current limit AND the output being low while
charging the output capacitor. The error output then pulls
low for the duration of the turn-on time. A capacitor from
the error flag to ground will filter out this glitch. The glitch
does not occur if the error flag pulled up to the output.
Active Shutdown
The MIC5254 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 discharge, de-energizing the load.
No Load Stability
The MIC5254 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 MIC5254 is a dual LDO voltage regulator designed
to provide two output voltages from one package. Both
regulator outputs are capable of sourcing 150mA of
output current. Proper thermal evaluation needs to be
done to ensure that the junction temperature does not
exceed its maximum value, 125°C. Maximum power
dissipation can be calculated based on the output
current and the voltage drop across each regulator. The
sum of the power dissipation of each regulator
determines the total power dissipation. The maximum
power dissipation that this package is capable of
handling can be determined using thermal resistance,
junction to ambient, 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,
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