Micrel, Inc.

MIC47100

February 2008

10

M9999-022108-A

(408) 944-0800

Applications Information

The MIC47100 is a high speed, dual supply NMOS LDO

designed to take advantage of point-of-load applications that

use multiple supply rails to generate a low voltage, high

current power supply. The MIC47100 can source 1A of

output current while only requiring a 1µF ceramic output

capacitor for stability.

The MIC47100 regulator is fully protected from damage due

to fault conditions, offering linear current limiting and thermal

shutdown.

Bias Supply Voltage

to the control portion of the MIC47100. Bypassing on

the bias pin is recommended to improve performance

of the regulator during line and load transients. Small

high frequency noise from being injected into the

control circuitry from the bias rail and are good design

practice.

Input Supply Voltage

minimum input voltage is 1V, allowing conversion from

low voltage supplies.

Output Capacitor

The MIC47100 requires an output capacitor of 1µF or

greater to maintain stability. The design is optimized

for use with low-ESR ceramic chip capacitors. High

ESR capacitors may cause high frequency oscillation.

The

output

capacitor

can

be

increased,

but

performance has been optimized for a 1µF ceramic

output capacitor and does not improve significantly

with larger capacitance.

X7R/X5R dielectric-type 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 as 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 capacitor to ensure

the same minimum capacitance over the equivalent

operating temperature range.

Input Capacitor

The MIC47100 is a high-performance, high bandwidth

device. Therefore, it requires a well-bypassed input

supply for optimal performance. A 1µF capacitor is

required from the input to ground to provide stability.

Low-ESR

ceramic

capacitors

provide

optimal

performance at a minimum of space. Additional high-

frequency capacitors, such as small-valued NPO

dielectric-type

capacitors,

help

filter

out

high-

frequency noise and are good practice in any RF-

based circuit.

Minimum Load Current

The MIC47100, unlike most other regulators, does not

require a minimum load to maintain output voltage

regulation.

Adjustable Regulator Design

The

MIC47100

adjustable

version

allows

programming the output voltage anywhere between

0.8V and 2.0V. Two resistors are used. The R1

not exceed 10kΩ. Larger values can cause instability.

R2 connects between the adjust pin and ground. The

resistor values are calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

=

1

0.7

V

R2

R1

OUT

Enable/Shutdown

The MIC47100 comes with a single active-high enable

pin that allows the regulator to be disabled. Forcing

the enable pin low disables the regulator and sends it

into a “zero” off-mode-current state. In this state,

current consumed by the regulator goes nearly to

zero. Forcing the enable pin high enables the output

voltage. The active-high enable pin uses CMOS

technology and the enable pin cannot be left floating;

a floating enable pin may cause an indeterminate

state on the output.

Thermal Considerations

The MIC47100 is designed to provide 1A of

continuous current in a very small package. Maximum

ambient operating temperature can be calculated

based on the output current and the voltage drop

across the part. Given that the input voltage is 1.8V,

the output voltage is 1.2V and the output current is

1A. The actual power dissipation of the regulator

circuit can be determined using the equation:

Because this device is CMOS, the ground current is

insignificant for power dissipation and can be ignored

for this calculation.