MIC39500_05 Datasheet, PDF (7/10 Pages) Micrel Semiconductor – 5A μCap Low-Voltage Low-Dropout Regulator

MIC39500/39501
Applications Information
The MIC39500/1 is a high-performance low-dropout voltage
regulator suitable for moderate to high-current voltage regula-
tor applications. Its 400mV dropout voltage at full load makes
it especially valuable in battery-powered systems and as a
high-efficiency noise filter in post-regulator applications. Un-
like older NPN-pass transistor designs, where the minimum
dropout voltage is limited by the base-to-emitter voltage drop
and collector-to-emitter saturation voltage, dropout perfor-
mance of the PNP output of these devices is limited only by
the low V_CE saturation voltage.
A trade-off for the low dropout voltage is a varying base drive
requirement. Micrel’s Super βeta PNP™ process reduces this
drive requirement to only 2% to 5% of the load current.
The MIC39500/1 regulator is fully protected from damage due
to fault conditions. Current limiting is provided. This limiting is
linear; output current during overload conditions is constant.
Thermal shutdown disables the device when the die tempera-
ture exceeds the maximum safe operating temperature. Tran-
sient protection allows device (and load) survival even when
the input voltage spikes above and below nominal. The output
structure of these regulators allows voltages in excess of the de-
sired output voltage to be applied without reverse current flow.
V_IN
MIC39500-x.x
IN OUT
C_IN
GND
V_OUT
C_OUT
Figure 1. Capacitor Requirements
Thermal Design
Linear regulators are simple to use. The most complicated
design parameters to consider are thermal characteristics.
Thermal design requires four application-specific param-
eters:
• Maximum ambient temperature (T_A)
• Output Current (I_OUT)
• Output Voltage (V_OUT)
• Input Voltage (V_IN)
• Ground Current (I_GND)
Calculate the power dissipation of the regulator from these
numbers and the device parameters from this datasheet,
where the ground current is taken from data sheet.
P_D = (V_IN – V_OUT) × I_OUT + V_IN × I_GND
The heat sink thermal resistance is determined by:
where:
T_{J (max)} ≤ 125°C and θ_CS is between 0° and 2°C/W.
The heat sink may be significantly reduced in applications
where the minimum input voltage is known and is large com-
Micrel
pared with the dropout voltage. Use a series input resistor
to drop excessive voltage and distribute the heat between
this resistor and the regulator. The low-dropout properties of
Micrel Super βeta PNP regulators allow significant reductions
in regulator power dissipation and the associated heat sink
without compromising performance. When this technique
is employed, a capacitor of at least 1µF is needed directly
between the input and regulator ground.
Refer to Application Note 9 for further details and examples
on thermal design and heat sink specification.
Output capacitor
The MIC39500/1 requires an output capacitor to maintain
stability and improve transient response. Proper capaci-
tor selection is important to ensure proper operation. The
MIC39500/1 output capacitor selection is dependent upon the
ESR (equivalent series resistance) of the output capacitor to
maintain stability. When the output capacitor is 47µF or greater,
the output capacitor should have less than 1Ω of ESR. This
will improve transient response as well as promote stability.
Ultra-low-ESR capacitors, such as ceramic chip capacitors
may promote instability. These very low ESR levels may
cause an oscillation and/or underdamped transient response.
When larger capacitors are used, the ESR requirement ap-
proaches zero. A 100µF ceramic capacitor can be used on
the output while maintaining stability. A low-ESR 47µF solid
tantalum capacitor works extremely well and provides good
transient response and stability over temperature. Aluminum
electrolytics can also be used, as long as the ESR of the
capacitor is < 1Ω.
The value of the output capacitor can be increased without
limit. Higher capacitance values help to improve transient
response and ripple rejection and reduce output noise.
Input capacitor
An input capacitor of 1µF or greater is recommended when
the device is more than 4 inches away from the bulk ac supply
capacitance, or when the supply is a battery. Small surface-
mount ceramic chip capacitors can be used for bypassing.
Larger values will help to improve ripple rejection by bypass-
ing the input to the regulator, further improving the integrity
of the output voltage.
Transient Response and 3.3V to 2.5V or 2.5V to 1.8V
Conversion
The MIC39500/1 has excellent transient response to varia-
tions in input voltage and load current. The device has been
designed to respond quickly to load current variations and
input voltage variations. Large output capacitors are not re-
quired to obtain this performance. A standard 47µF output
capacitor, preferably tantalum, is all that is required. Larger
values improve performance even further.
By virtue of its low-dropout voltage, this device does not
saturate into dropout as readily as similar NPN-based de-
signs. When converting from 3.3V to 2.5V, or 2.5V to 1.8V,
the NPN-based regulators are already operating in dropout,
with typical dropout requirements of 1.2V or greater. To con-
vert down to 2.5V without operating in dropout, NPN-based
regulators require an input voltage of 3.7V at the very least.
The MIC39500/1 regulator provides excellent performance
August 2005
7
M9999-082605-B