MIC5321 datasheet(9/26 Pages) MICROCHIP

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DS20006678A-page 9

MIC5321

4.0

APPLICATION INFORMATION

4.1

Enable/Shutdown

The MIC5321 comes with a single active-high enable

pin that allows both regulators to be disabled

simultaneously. 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.

4.2

Input Capacitor

The MIC5321 is a high-performance, high-bandwidth

device. Therefore, it requires a well-bypassed input

supply for optimal performance. A 1 µF 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.

4.3

Output Capacitor

The MIC5321 requires an output capacitor of 1 µF 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 1 µF ceramic output capacitor

and does not improve significantly with larger

capacitance.

X7R/X5R dielectric-type ceramic capacitors are

recommended

because

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.

4.4

Bypass Capacitor

A capacitor can be placed from the noise bypass pin to

ground to reduce output voltage noise. The capacitor

bypasses the internal reference. A 0.1 µF capacitor is

recommended for applications that require low-noise

outputs. The bypass capacitor can be increased,

further reducing noise and improving PSRR. Turn-on

time increases slightly with respect to bypass

capacitance. A unique, quick-start circuit allows the

MIC5321 to drive a large capacitor on the bypass pin

without significantly slowing turn-on time. Refer to the

Typical Performance Curves section for performance

with different bypass capacitors.

4.5

No-Load Stability

Unlike many other voltage regulators, the MIC5321 will

remain stable and in regulation with no load. This is

especially important in CMOS RAM keep-alive

applications.

4.6

Thermal Considerations

The MIC5321 is designed to provide 150 mA of

continuous current for both outputs 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 3.3V, the output voltage is 2.8V for VOUT1,

2.5V for VOUT2 and the output current equals 150 mA.

The actual power dissipation of the regulator circuit can

be determined using the equation:

EQUATION 4-1:

Because this device is CMOS and the ground current

is typically <150 µA over the load range, the power

dissipation contributed by the ground current is less

than 1% and can be ignored for this calculation.

EQUATION 4-2:

To determine the maximum ambient operating

temperature

the

package,

use

the

junction-to-ambient thermal resistance of the device

and the following basic equation:

EQUATION 4-3:

The table below shows the junction-to-ambient thermal

resistance for the UDFN package option.

Package

θJA Rec. Min.

Footprint

θJC

6-Lead UDFN

100°C/W

2°C/W

VIN VOUT1

–





IOUT1



VIN VOUT2

–





IOUT2



VIN

IGND



3.3V 2.8V

–





150mA



3.3V 1.5V

–





150mA



0.345W

PD MAX





TJ MAX





–

JA

-------------------------------

Where:

TJ(MAX) = 125°C, the max. junction temp. of the die.

θJA = Thermal resistance of 100°C/W.

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