MIC2155_0911 Datasheet, PDF (2/35 Pages) Micrel Semiconductor – Two-Phase, Single-Output, PWM Synchronous Buck Control IC

Micrel, Inc.
MIC2155/2156
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
November 2009
2
M9999-111209-B