Micrel, Inc.

MIC2155/2156

November 2009

2

M9999-111209-B

Contents

Ordering Information ............................................................................................................................................................... 3

Typical Application .................................................................................................................................................................. 3

Pin Configuration..................................................................................................................................................................... 4

Pin Description ........................................................................................................................................................................ 4

Absolute Maximum Ratings .................................................................................................................................................... 6

Operating Ratings ................................................................................................................................................................... 6

Electrical Characteristics......................................................................................................................................................... 6

Typical Characteristics ............................................................................................................................................................ 9

Functional Diagram ............................................................................................................................................................... 11

Functional Description........................................................................................................................................................... 12

Startup .............................................................................................................................................................................. 12

Soft Start ........................................................................................................................................................................... 12

Enable............................................................................................................................................................................... 13

Supply Voltages and Internal References ........................................................................................................................ 13

UVLO ................................................................................................................................................................................ 14

Power Good ...................................................................................................................................................................... 14

Oscillator and Frequency Synchronization ....................................................................................................................... 15

MOSFET Gate Drive Circuitry .......................................................................................................................................... 15

dv/dt Induced Turn On of the Low-Side MOSFET............................................................................................................ 16

Remote Sense .................................................................................................................................................................. 16

Setting the Output Voltage................................................................................................................................................ 17

Current Limit and Overcurrent Protection......................................................................................................................... 17

Current Limit Setting ......................................................................................................................................................... 18

The simple method ....................................................................................................................................................... 18

Accurate method .......................................................................................................................................................... 18

Example: ........................................................................................................................................................................... 18

Inductor Current Sensing.................................................................................................................................................. 19

Current Sharing................................................................................................................................................................. 19

Startup into a Pre-Biased Output...................................................................................................................................... 20

Separate Input Supplies ................................................................................................................................................... 20

Component Selection, Guidelines and Design Example.................................................................................................. 21

Output Filter Selection ...................................................................................................................................................... 21

Output Capacitor Selection............................................................................................................................................... 23

Inductor Current Sense Components ............................................................................................................................... 24

Input Capacitor Selection.................................................................................................................................................. 24

MOSFET Selection ........................................................................................................................................................... 24

RMS Current and MOSFET Power Dissipation Calculation............................................................................................. 25

External Schottky Diode ................................................................................................................................................... 26

Snubber Design ................................................................................................................................................................ 26

Compensation of the Output Voltage Loop ...................................................................................................................... 28

Error Amplifier Design Procedure ......................................................................................................................................... 30

Step 1: Decide on the crossover frequency...................................................................................................................... 30

Step 2: Determine the gain required at the crossover frequency ..................................................................................... 30

Step 3: Determine the gain boost needed at the crossover frequency (fc) ...................................................................... 30

Step 4: Determine the frequencies fz2 and fp1 ................................................................................................................ 30

Step 5: Determine the frequency for fz1........................................................................................................................... 30

Step 6: Determine the frequency for fp2........................................................................................................................... 31

Calculating Error Amplifier Component Values ................................................................................................................ 31

Compensation of the Current Sharing Loop ..................................................................................................................... 31

General Layout and Component Placement .................................................................................................................... 33

Design and Layout Checklist ............................................................................................................................................ 34

Package Information ............................................................................................................................................................. 35