MIC5246_02 Datasheet, PDF (7/12 Pages) Micrel Semiconductor – 150mA μCap CMOS LDO Regulator

MIC5246
Applications Information
Enable/Shutdown
The MIC5246 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
An input capacitor is not required for stability. A 1µF input
capacitor is recommended when the bulk ac supply capaci-
tance is more than 10 inches away from the device, or when
the supply is a battery.
Output Capacitor
The MIC5246 requires an output capacitor for stability. The
design requires 1µF or greater on the output to maintain
stability. The capacitor can be a low-ESR ceramic chip
capacitor. The MIC5246 has been designed to work specifi-
cally with the low-cost, small chip capacitors. Tantalum
capacitors can also be used for improved capacitance over
temperature. The value of the capacitor can be increased
without bound.
X7R dielectric ceramic capacitors are recommended be-
cause of their temperature performance. X7R-type capaci-
tors change capacitance by 15% over their operating tem-
perature range and are the most stable type of ceramic
capacitors. Z5U and Y5V dielectric capacitors change value
by as much 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 or a tantalum capacitor to ensure the same minimum
capacitance value over the operating temperature range.
Tantalum capacitors have a very stable dielectric (10% over
their operating temperature range) and can also be used with
this device.
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.
Error Flag Circuit
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, then 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. This glitch is filtered by putting a capacitor from the error
flag to ground. The glitch does not occur if the error flag pulled
up to the output.
Transient Response
The MIC5246 implements a unique output stage to dramati-
cally improve transient response recovery time. The output is
a totem-pole configuration with a P-channel MOSFET pass
device and an N-channel MOSFET clamp. The N-channel
clamp is a significantly smaller device that prevents the
output voltage from overshooting when a heavy load is
removed. This feature helps to speed up the transient re-
sponse by significantly decreasing transient response recov-
ery time during the transition from heavy load (100mA) to light
load (85µA).
Active Shutdown
The MIC5246 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.
Thermal Considerations
The MIC5246 is designed to provide 150mA of continuous
current in a very small package. Maximum power dissipation
can be calculated based on the output current and the voltage
drop across the part. To determine the maximum power
dissipation of the package, use the junction-to-ambient ther-
mal resistance of the device 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. θ_JA is
layout dependent; Table 1 shows examples of junction-to-
ambient thermal resistance for the MIC5246.
Package
θ_JA Recommended θ_JA 1" Square θ_JC
Minimum Footprint Copper Clad
SOT-23-5 (M5)
235°C/W
185°C/W 145°C/W
Table 1. SOT-23-5 Thermal Resistance
June 2000
7
MIC5246