MIC5247

Micrel, Inc.

M9999-022406

8

February 2006

Applications Information

Enable/Shutdown

The MIC5247 comes with an active-high enable pin that can

disable the regulator. 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 volt-

age. This part is CMOS and the enable pin cannot be left

floating; a floating enable pin may cause an indeterminate

state on the output.

Input Capacitor

An input capacitor is not required for stability. A 1µF input

capacitor is recommended when the bulk AC supply capaci-

tance is more than 10 inches away from the device, or when

the supply is a battery.

Output Capacitor

The MIC5247 requires an output capacitor for stability. The

designrequires1µForgreaterontheoutputtomaintainstabil-

ity. The capacitor can be a low-ESR ceramic chip capacitor.

The MIC5247 has been designed to work specifically with

low-cost, small chip capacitors. Tantalum capacitors can also

be used for improved capacitance over temperature. The

value of the capacitor can be increased without bound.

Bypass Capacitor

Acapacitor can be placed from the noise bypass pin to ground

to reduce output voltage noise. The capacitor bypasses the

internal reference. A 0.01µF capacitor is recommended for

applications that require low-noise outputs.

The bypass capacitor can be increased withoutbound,further

reducing noise and improving PSRR. Turn-on time remains

constant with respect to bypass capacitance. Refer to the

“Typical Characteristics” section for a graph of turn-on time

vs. bypass capacitor.

Transient Response

The MIC5247 implements a unique output stage to dramati-

cally improve 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 response recovery time

during the transition from heavy load (100mA) to light load

(85µA).

Active Shutdown

The MIC5247 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 dis-

charge, de-energizing the load.

Thermal Considerations

The MIC5247 is designed to provide 150mA of continuous

current in a very small package. Maximum power dissipation

can be calculated based on the output current and the voltage

drop across the part. To determine the maximum power dis-

sipation of the package, use the junction-to-ambient thermal

resistance of the device and the following basic equation:

P max

T max T

D

J

A

JA

(

)

(

)

θ

=

−













layout dependent; Table 1 shows examples of junction-to-

ambient thermal resistance for the MIC5247.

Package

Minimum Footprint

Copper Clad

SOT-23-5 (M5)

235°C/W

185°C/W

145°C/W

Table 1. SOT-23-5 Thermal Resistance

The actual power dissipation of the regulator circuit can be

determined using the equation:

P

V

V

I

V

I

D

IN

OUT

OUT

IN

GND

=

−

(

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, when operating the MIC5247-2.4BM5 at room

temperature with a minimum footprint layout, the maximum

input voltage for a set output current can be determined as

follows:

P max

125 C 25 C

235 C/W

P max

425mW

D

D

(

)

(

)

=

° −

°

°













=

The junction-to-ambient thermal resistance for the minimum

footprint is 235°C/W, from Table 1. The maximum power dis-

sipation must not be exceeded for proper operation. Using

the output voltage of 2.4V and an output current of 150mA,

the maximum input voltage can be determined. Because this

deviceisCMOSandthegroundcurrentistypically100µAover

theloadrange,thepowerdissipationcontributedbytheground

current is < 1% and can be ignored for this calculation.

Therefore, a 2.4V application at 150mA of output current

can accept a maximum input voltage of 5.2V in a SOT-23-5

package. For a full discussion of heat sinking and thermal

effects on voltage regulators, refer to the “Regulator Ther-

mals” section of Micrel’s Designing with Low-Dropout Voltage

Regulators handbook.

Fixed Regulator Applications

1

5

2

3

4

1µF

0.01µF

MIC5247-x.xBM5

Figure 1. Ultra-Low-Noise Fixed Voltage Application