MIC38300_13 Datasheet, PDF (11/15 Pages) Micrel Semiconductor – 3A High-Efficiency Low Dropout Regulator

Micrel, Inc.
Application Information
Enable Input
The MIC38300 features a TTL/CMOS compatible positive
logic enable input for on/off control of the device. High
enables the regulator while low disables the regulator. In
shutdown the regulator consumes very little current (only
a few microamperes of leakage). For simple applications
the enable (EN) can be connected to V_IN (IN).
Input Capacitor
PVIN provides power to the MOSFETs for the switch
mode regulator section and the gate drivers. Due to the
high switching speeds, a 10µF capacitor is recommended
close to PVIN and the power ground (PGND) pin for
bypassing.
Analog V_IN (AVIN) provides power to the analog supply
circuitry. Careful layout should be considered to ensure
high-frequency switching noise caused by PVIN is
reduced before reaching AVIN. A 1µF capacitor as close
to AVIN as possible is recommended.
Output Capacitor
The MIC38300 requires an output capacitor for stable
operation. As a µCap LDO, the MIC38300 can operate
with ceramic output capacitors of 10µF or greater. Values
of greater than 10µF improve transient response and
noise reduction at high frequency. X7R/X5R dielectric-
type ceramic capacitors are recommended because of
their superior temperature performance. X7R-type
capacitors change capacitance by 15% over their
operating temperature range and are the most stable
type of ceramic capacitors. Larger output capacitances
can be achieved by placing tantalum or aluminum
electrolytics in parallel with the ceramic capacitor. For
example, a 100µF electrolytic in parallel with a 10µF
ceramic can provide the transient and high frequency
noise performance of a 100µF ceramic at a significantly
lower cost. Specific undershoot/overshoot performance
will depend on both the values and ESR/ESL of the
capacitors.
For less than 5mV noise performance at higher current
loads, 20µF capacitors are recommended at LDOIN and
LDOOUT.
Low Pass Filter Pin
The MIC38300 features a Low Pass Filter (LPF) pin for
adjusting the switcher frequency. By tuning the
frequency, the user can further improve output ripple
without losing efficiency. Adjusting the frequency is
accomplished by connecting a resistor between the LPF
and SW pins. A small value resistor would increase the
frequency while a larger value resistor decreases the
frequency. Recommended R_LPF value is 25kΩ. Please
see Typical Characteristics section for more details.
MIC38300
Adjustable Regulator Design
The adjustable MIC38300 output voltage can be
programmed from 1V to 5.0V using a resistor divider from
output to the FB pin. Resistors can be quite large, up to
100kΩ because of the very high input impedance and low
bias current of the sense amplifier. For large value
resistors (>50kΩ) R1 should be bypassed by a small
capacitor (C_FF = 0.1µF bypass capacitor) to avoid
instability due to phase lag at the ADJ/SNS input.
Figure 1. Adjustable Regulator with Resistors
The output resistor divider values are calculated by
Equation 1:
V_OUT
= 1V ^R1
 ^R2
+ 1

Eq. 1
Efficiency Considerations
Efficiency is defined as the amount of useful output
power, divided by the amount of power supplied.
Efficiency
_
%
=

V_OUT
V_IN
× I_OUT
× I_IN

× 100
Eq. 2
Maintaining high efficiency serves two purposes. It
reduces power dissipation in the power supply, reducing
the need for heat sinks and thermal design
considerations and it reduces consumption of current for
battery-powered applications. Reduced current draw from
a battery increases the devices operating time and is
critical in handheld devices.
April 10, 2013
11
Revision 5.0