MIC5250_03 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – Dual 150mA μCap CMOS LDO Regulator

MIC5250
Applications Information
Enable/Shutdown
The MIC5250 comes with active-high enable pins that allows
either regulator to be disabled. Forcing an enable pin low
disables the respective regulator and places it into a “zero”
off-mode-current state. In this state, current consumed by the
regulator goes nearly to zero. Forcing an enable pin high
enables the output voltage. This part is CMOS therefore the
enable pin cannot be left floating; a floating enable pin may
cause an indeterminate state on the output.
Input Capacitor
Input capacitors are not required for stability. A 1µF input
capacitor is recommended for either regulator when the bulk
ac supply capacitance is more than 10 inches away from the
device, or when the supply is a battery.
Output Capacitor
The MIC5250 requires output capacitors for stability. The
design requires 1µF or greater on each output to maintain
stability. Capacitors can be low-ESR ceramic chip capaci-
tors. The MIC5250 has been designed to work specifically
with low-cost, small chip capacitors. Tantalum capacitors can
also be used for improved capacitance over the operating
temperature range. The value of the capacitor can be in-
creased without bounds.
Bypass Capacitor
Capacitors can be placed from each noise bypass pin to their
respective ground to reduce output voltage noise. These
capacitors bypass the internal references. A 0.01µF capaci-
tor is recommended for applications that require low-noise
outputs.
Transient Response
The MIC5250 implements a unique output stage design
which dramatically improves 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 re-
sponse recovery time during the transition from heavy load
(100mA) to light load (100µA).
Active Shutdown
Each regulator 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.
Cross Talk
When a load transient occurs on one output of the MIC5250,
the second output may couple a small amount of ripple to its
output. This typically comes from a common input source or
from poor grounding. Using proper grounding techniques
such as star grounding as well as good bypassing directly at
the inputs of each regulator will help to reduce the magnitude
of the cross talk. See “Functional Characteristics” for an
example of cross talk performance.
Micrel
Thermal Considerations
The MIC5250 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 )} −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 of the die.
θ_JA is layout dependent. Table 1 shows the typical thermal
resistance for a minimum footprint layout for the MIC5250.
Package
MSOP-10
θ_JA at Recommended
Minimum Footprint
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(reg 2)}
Each output is rated for 150mA of output current, but the
application may limit the amount of output current based on
the total power dissipation and the ambient temperature.
A typical application may call for two 3.0V outputs from a
single Li-ion battery input. This input can be as high as 4.2V.
When operating at high ambient temperatures, the output
current may be limited. When operating at an ambient of
60°C, the maximum power dissipation of the package is
calculated as follows:
P_D(max)
=
 125°C − 60°C
 200°C/W 
P_D(max) = 325mW
For the application mentioned above, if regulator 1 is sourcing
150mA, it contributes the following to the overall power
dissipation:
( ) ^P_D(reg1) ^{= V}_IN ^−V_OUT ^I_OUT ^+V_IN ^I_GND
P_D(reg1) = (4.2V − 3.0V)150mA + 4.2V × 100µA
P_D(reg1) = 180.4mW
MIC5250
8
June 2003