MIC5245

Micrel

MIC5245

8

August 2002

Applications Information

Enable/Shutdown

The MIC5245 comes with an 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. This part is CMOS and the enable pin cannot

be left floating; a floating enable pin may cause an indetermi-

nate state on the output.

Input Capacitor

An input capacitor is not required for stability. A 1

µF 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 MIC5245 requires an output capacitor for stability. The

design requires 1

µF or greater on the output to maintain

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

capacitor. The MIC5245 has been designed to work specifi-

cally with the 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.

X7R dielectric ceramic capacitors are recommended be-

cause of their temperature performance. X7R-type capaci-

tors change capacitance by 15% over their operating tem-

perature range and are the most stable type of ceramic

capacitors. Z5U and Y5V dielectric capacitors change value

by as much 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 or a tantalum capacitor to ensure the same minimum

capacitance value over the operating temperature range.

Tantalum capacitors have a very stable dielectric (10% over

their operating temperature range) and can also be used with

this device.

Bypass Capacitor

A capacitor can be placed from the noise bypass pin to

ground to reduce output voltage noise. The capacitor by-

passes the internal reference. A 0.01

µF capacitor is recom-

mended for applications that require low-noise outputs.

Transient Response

The MIC5245 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 re-

sponse by significantly decreasing transient response recov-

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

load (100

µA).

Active Shutdown

The MIC5245 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 MIC5245 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

dissipation of the package, use the junction-to-ambient ther-

mal resistance of the device and the following basic equation:

P

TT

D(max)

J(max)

A

JA

=

−









θ

T

125

°C, and T

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

ambient thermal resistance for the MIC5245.

Package

θθθθθ

θθθθθ

θθθθθ

JC

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

Substituting P

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, when operating the MIC5245-3.3BM5 at 50

°C with

a minimum footprint layout, the maximum input voltage for a

set output current can be determined as follows:

P

125 C

50 C

235 C/W

°−

°

°









P

The junction-to-ambient thermal resistance for the minimum

footprint is 235

°C/W, from Table 1. The maximum power

dissipation must not be exceeded for proper operation. Using

the output voltage of 3.3V and an output current of 150mA,

the maximum input voltage can be determined. Because this

device is CMOS and the ground current is typically 87

µA over

the load range, the power dissipation contributed by the

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

315mW = (V

315mW = V

810mW = V

V

Therefore, a 3.3V application at 150mA of output current can

accept a maximum input voltage of 5.4V in a SOT-23-5

package. For a full discussion of heat sinking and thermal