MIC5254 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – DUAL 150MA UCAP LDO WITH ERROR FLAG OUTPUTS

MIC5254
Applications 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
indeterminate state on the output.
Input Capacitor
The MIC5254 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 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 oper-
ating 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 capaci-
tor 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 undervoltage
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 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
Micrel
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 dis-
charge, 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 regula-
tor 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 it’s 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,
125°C and T_A is the ambient operating temperature of the die.
θ_JA is layout dependent. Table 1 shows the typical thermal
resistance for a minimum footprint layout for the MIC5254.
Package θ_JA at Recommended Minimum Footprint
MSOP-10
200°C/W
Table 1. Thermal Resistance
The actual power dissipation of each regulator output can be
calculated using the following simple equation:
P_D = (V_IN – V_OUT)I_OUT + V_IN × I_GND
Each regulator contributes power dissipation to the overall
power dissipation of the package.
P_D(total) = P_D(reg1) + P_D(reg2)
September 2003
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MIC5254