MIC5258_06 Datasheet, PDF (7/9 Pages) Micrel Semiconductor – 150mA μCap CMOS LDO Regulator w/Power Good

MIC5258
Applications Information
Enable/Shutdown
The MIC5258 comes with an active-high enable pin that al-
lows the regulator to be disabled. Forcing the enable pin low
disables the regulator and sends it into a “zero” off-mode-cur-
rent 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
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 MIC5258 requires an output capacitor for stability. The
design requires 1µF or greater on the output to maintain stabil-
ity. The capacitor can be a low-ESR ceramic chip capacitor.
The MIC5258 has been designed to work specifically 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 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 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 ca-
pacitor 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.
Power Good
The power good output is an open-drain output. It is designed
essentially to work as a power-on reset generator once
the regulated voltage was up and/or a fault condition. The
output of the power good drives low when a fault condition
AND an undervoltage detection occurs. The power good
output comes back up once the output has reached 97% of
its nominal value and a 1ms to 5ms delay has passed. See
Timing Diagram.
The MIC5258’s internal circuit intelligently monitors overcur-
rent, overtemperature and dropout conditions and ORs thes
outputs together ti indicate some fault condition. this output
is fed into an on-board delay circuitry that drives the open
drain transistor to indicate a fault.
Micrel, Inc.
Transient Response
The MIC5258 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 response by
significantly decreasing transient response recovery time
during the transition from heavy load (100mA) to light load
(100µA).
Active Shutdown
The MIC5258 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 MIC5258 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 dis-
sipation of the package, use the junction-to-ambient thermal
resistance of the device and the following basic equation:
P_D(max) =
T_J(max) - T_A
θ_JA
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 MIC5258.
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
The actual power dissipation of the regulator circuit can be
determined using the equation:
P_D = (V_IN – V_OUT) I_OUT + V_IN I_GND
Substituting P_D(max) for P_D and solving for the operating
conditions that are critical to the application will give the
maximum operating conditions for the regulator circuit. For
example, when operating the MIC5258-1.2BM5 at 50°C with
a minimum footprint layout, the maximum input voltage for a
set output current can be determined as follows:
P_D
(max)
=
125°C - 50°C
235°C/W
P_D(max) = 315mW
The junction-to-ambient thermal resistance for the minimum
footprint is 235°C/W, from Table 1. The maximum power dis-
sipation must not be exceeded for proper operation. Using
the output voltage of 1.2V and an output current of 150mA,
the maximum input voltage can be determined. Because this
device is CMOS and the ground current is typically 100µA over
the load range, the power dissipation contributed by the ground
current is < 1% and can be ignored for this calculation.
April 2006
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MIC5258