Fujitsu MB39A104 manual Selection of Components, CH1

Page 24
PS (ON)

MB39A104

SELECTION OF COMPONENTS

• Pch MOS FET

The P-ch MOSFET for switching use should be rated for at least 20% more than the maximum input voltage. To minimize continuity loss, use a FET with low RDS(ON) between the drain and source. For high input voltage and high frequency operation, on/off-cycle switching loss will be higher so that power dissipation must be considered. In this application, the Toshiba TPC8102 is used. Continuity loss, on/off switching loss, and total loss are deter- mined by the following formulas. The selection must ensure that peak drain current does not exceed rated values, and also must be in accordance with overcurrent detection levels.

Continuity loss : PC

PC = ID 2 RDS (ON) Duty

On-cycle switching loss : PS (ON)

= VD (Max) ID tr fOSC

6

Off-cycle switching loss : PS (OFF)

PS (OFF) =

VD (Max) ID (Max) tf fOSC

6

 

Total loss : PT

PT = PC + PS (ON) + PS (OFF)

Example: Using the Toshiba TPC8102

CH1

Input voltage VIN (Max) = 19 V, output voltage VO = 5 V, drain current ID = 3 A, Oscillation frequency fOSC = 500 kHz, L = 15 H, drain-source on resistance RDS (ON) =: 50 mΩ, tr = tf =: 100 ns.

Drain current (Max) : ID (Max)

ID (Max)

= IO +

VIN VO

 

ton

 

 

 

2L

 

 

 

 

 

 

 

 

 

 

 

 

= 3 +

19 5

1

0.263

 

2 15 106

500 103

 

=: 3.25 (A)

 

 

 

 

 

Drain current (Min) : ID (Min)

 

 

 

ID (Min)

= IO

VIN VO

ton

 

 

 

2L

 

 

 

 

 

 

 

 

 

 

 

= 3

19 5

1

0.263

 

2 15 106

500 103

 

=: 2.75 (A)

 

 

 

 

 

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Contents Features DescriptionPackage MB39A104 PIN AssignmentsPin No Symbol Descriptions PIN DescriptionBlock Diagram Parameter Symbol Condition Rating Unit Min Max Parameter Symbol Condition Value Unit Min TypAbsolute Maximum Ratings Recommended Operating ConditionsElectrical Characteristics ParameterPin No Conditions Min Typ MaxParameter Symbol Pin No Conditions Value Unit MinTypical Characteristics Reference Voltage vs. Ambient TemperatureTriangular Wave Oscillation Frequency Power Dissipation vs. Ambient Temperature Functions Timer-latch overcurrent protection circuit block OCP Under voltage lockout protection circuit UvloProtection circuit operating function table Operating circuitSetting the Output Voltage Setting the Triangular Oscillation FrequencySetting the SOFT-START and Discharge Times MB39A104 Treatment Without Using CS Terminal Without Setting Soft-Start TimeAbout TIMER-LATCH Protection Circuit Overcurrent detection circuitOvercurrent Protection Circuit Range of Operation Method to detect by mean currentTimer-latch short-circuit protection circuit Treatment Without Using Cscp Terminal Resetting the Latch of Each Protection CircuitEquivalent Circuit On Power on OFF Standby mode Parts List Component Specification VendorSelection of Components CH1CH2 = 3 2 ⋅ 0.05 ⋅ 0.174 = 0.078 W Example Peak-to-peak value ΔIL Di ≥ I O ⋅ 1 − ESR ≤ ΠfC L 050 491 ESR ≤ ΠfC L 033 364 Reference Data Total Efficiency vs. Input VoltageSwitching Wave Form CH1 Usage Precaution Ordering InformationDo not apply negative voltages Part number Package RemarksPackage Dimensions IndexFujitsu Limited