MIC29311 Datasheet, PDF (8/12 Pages) Micrel Semiconductor – 3A Fast-Response LDO Regulator for USB Preliminary Information

MIC29311
Applications Information
The MIC29311 is a high-performance low-dropout voltage
regulator suitable for all moderate to high-current voltage
regulator applications. The 600mV dropout voltage at full
load makes it especially valuable in battery-powered systems
and as high-efficiency noise filters in “post-regulator” applica-
tions. Its unique output voltage makes the MIC39311 ideal for
Universal Serical Bus (USB) power switching applications.
Unlike older NPN-pass transistor designs, where the mini-
mum dropout voltage is limited by the base-emitter voltage
drop and collector-emitter saturation voltage, dropout perfor-
mance of the PNP output of these devices is limited merely
by the low V_CE saturation voltage.
A trade-off for the low dropout voltage is a varying base drive
requirement. But Micrel’s Super βeta PNP™ process re-
duces this drive requirement to merely 1% to 5% of the load
current.
The MIC29311 regulator is fully protected from damage due
to fault conditions. Current limiting is provided. This limiting is
linear; output current under overload conditions is constant.
Thermal shutdown disables the device when the die tem-
perature exceeds the maximum safe operating temperature.
Transient 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 desired output voltage to be applied without reverse
current flow. The MIC29311 version offers a logic level on-off
control: when disabled, the device draws nearly zero current.
V_IN
Enable
Shutdown
MIC29311-5.1
IN
OUT
EN FLG
GND
V_OUT
Flag
Figure 1. Input and Output Capacitors
Thermal Design
Linear regulators are simple to use. The most complicated
design parameters to consider are thermal characteristics.
Thermal design requires the following application-specific
parameters:
Maximum ambient temperature, T_A
Output Current, I_OUT
Output Voltage, V_OUT
Input Voltage, V_IN
First, calculate the power dissipation of the regulator from
these numbers and the device parameters from this datasheet.
( ) ^P_D ^{= I}_OUT ^1.02V_IN ^{− V}_OUT
Where the ground current is approximated by 2% of I_OUT.
Then the heat sink thermal resistance is determined by:
θ_SA
=
T_J(max) − T_A
P_D
− θ_JC
+ θ_CS
Where T_{J (max)} ≤ 125°C and θ_CS is between 0 and 2°C/W.
Micrel
The heat sink may be significantly reduced in applications
where the minimum input voltage is known and is large
compared 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 ßeta PNP regulators allow very
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.
Please refer to Application Note 9 for further details and
examples on thermal design and heat sink specification.
Capacitor Requirements
For stability and minimum output noise, a capacitor on the
regulator output is necessary. The value of this capacitor is
dependent upon the output current; lower currents allow
smaller capacitors. The MIC29311 regulator is stable with a
minimum capacitor value of 10µF at full load.
This capacitor need not be an expensive low ESR type:
aluminum electrolytics are adequate. In fact, extremely low
ESR capacitors may contribute to instability. Tantalum ca-
pacitors are recommended for systems where fast load
transient response is important.
Where the regulator is powered from a source with a high AC
impedance, a 0.1µF capacitor connected between Input and
GND is recommended. This capacitor should have good
characteristics to above 250kHz. When the regulator is
located more than 3 inches from the ac bulk supply capaci-
tors, a 1µF or greater input capacitor is recommended.
Minimum Load Current
The MIC29311 regulator is specified between finite loads. If
the output current is too small, leakage currents dominate
and the output voltage rises. A 10mA minimum load current
is necessary for proper regulation.
Enable Input
The MIC29311 version features an enable (EN) input that
allows on-off control of the device. Special design allows
“zero” current drain when the device is disabled—only micro-
amperes of leakage current flows. The EN input has TTL/
CMOS compatible thresholds for simple interfacing with
logic, or may be directly tied to V_IN. Enabling the regulator
requires approximately 20µA of current into the EN pin.
Error Flag
The MIC29311 features an error flag which looks at the output
voltage and signals an error condition when this voltage
drops 5% below its expected value. The error flag is an open-
collector output that pulls low under fault conditions. It may
sink 10mA. Low output voltage signifies a number of possible
problems, including an overcurrent fault (the device is in
current limit) and low input voltage. The flag output is inopera-
tive during overtemperature shutdown conditions.
USB Applications
The main application of the MIC29311 is to control power
distribution in a self-powered Universal Serial Bus hub. For
self-powered hubs, the MIC29311 provides 5.1V ±3% to
downstream ports from an unregulated supply voltage (see
MIC29311
8
December 1998