MIC5249 Datasheet, PDF (5/7 Pages) Micrel Semiconductor – 300MA UCAP LDO WITH PROGRAMMABLE POWER ON RESET DELAY

MIC5249
Applications Information
Enable/Shutdown
The MIC5249 comes with an 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. This part is CMOS and the enable pin cannot
be left floating; a floating enable pin may cause an indetermi-
nate state on the output.
Input Capacitor
The MIC5249 is a high performance, high bandwidth device.
Therefore, it requires a well-bypassed input supply for opti-
mal performance. A 1.0µF 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 to filter out high frequency
noise and are good practice in any RF-based circuit.
Output Capacitor
The MIC5249 requires an output capacitor for stability. The
design requires 2.2µF 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 2.2µF 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.
Delay Pin Input
The power-on reset function can be implemented on the
MIC5249 by adding an external capacitor from the delay pin
to ground. This external capacitor sets the delay time (T_DELAY)
of the RESET output. The capacitor value required can be
easily calculated using the formula:
C_DELAY
=
 T_DELAY × I_DELAY 

V_DELAY

where I_DELAY is 0.55µA and the V_DELAY is 1.4V. When no
capacitor is used, there will be no delay and the POR output
acts like a standard error FLAG output.
Micrel
Reset Output
The RESET output of the MIC5249 provides the power-on
reset signal based on the capacitor from the DELAY pin to
ground when input power is applied to the regulator. The
reset signal stays low for a preset time period after power is
applied to the regulator, and then goes high (see “Timing
Diagram”).
The reset 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. The output
drives an open-drain transistor to indicate a fault. This pre-
vents chattering or inadvertent triggering of the reset. The
reset must be pulled up using a resistor form the RESET pin
to either the input or the output.
Bypass Pin Input
A bypass capacitor is required from the noise bypass pin to
ground to reduce output voltage noise. The capacitor by-
passes the internal reference. A 0.01µF capacitor is recom-
mended 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 MIC5249 to drive a large capacitor on the bypass
pin without significantly slowing the turn-on time.
Active Shutdown
The MIC5249 also features an active shutdown clamp, which
is a 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 MIC5249 will remain stable and in regulation with no load
unlike many other voltage regulators. This is especially
important in CMOS RAM keep-alive application.
March 2004
5
M0382-033004