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AN-6861 Datasheet(PDF) 4 Page - Fairchild Semiconductor |
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AN-6861 Datasheet(HTML) 4 Page - Fairchild Semiconductor |
4 / 12 page AN-6861 APPLICATION NOTE © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 6/9/09 4 I Δ 2 RF EDC I K I Δ = S PK D I Figure 4. MOSFET Current and Ripple Factor (KRF) (Design Example) Determining the ripple factor as 0.57 2 2 3 () (90 0.53) 2 2 61 65 10 0.57 503 MIN INP MAX M INP SW RF VD L Pf K H μ ⋅ ⋅ == ⋅⋅ × ⋅ = 61 1.28 90 0.53 INP EDC MIN INP MAX P I A VD == = ⋅ ⋅ 63 90 0.53 1.46 503 10 65 10 MIN INP MAX MSW VD I A Lf − ⋅ Δ= = = ×⋅ × DS 1.28 0.73 2.01 2 PK EDC I I IA Δ =+ = + = 22 22 3( ) ( ) 23 0.53 3(1.28) (0.73) 0.98 3 RMS MAX DS EDC D I II A Δ ⎡⎤ =+ ⎢⎥ ⎣⎦ ⎡⎤ =+ = ⎣⎦ [STEP-5] Determine the Sensing Resistor Value The current sensing resistor value should be determined considering the over-current protection threshold and the pulse-by-pulse current limit threshold, as shown in Figure 5. The peak value of current sensing voltage (VCS) should be lower than the pulse-by-pulse current limit level for peak load condition. It should be lower than the OCP threshold for nominal load conditions to prevent false triggering of OCP protection during normal operation. CS DS CS VI R =⋅ Figure 5. Determining Current Sensing Resistor The peak drain current at minimum input voltage and peak load condition was obtained from Equation (9) in Step-4. The peak drain current at minimum input voltage and nominal load condition is given as: . () : 2( ) MIN PK INN IN RO DS N MIN INN RO MIN INN RO MIN MSW INN RO PV V I VV VV CCM Lf V V ⋅+ = ⋅ ⋅ + ⋅+ (13) . 2 : PK INN DS N SW M P I DCM fL ⋅ = ⋅ (14) Whether the converter operates in CCM or DCM at minimum input voltage and nominal load condition is determined by: () 21 : () 21 : MIN INN RO INN M SW MIN INN RO MIN INN RO INN M SW MIN INN RO VV P L f CCM VV VV P Lf DCM VV + ⋅> ⋅ + ⋅< ⋅ (15) The condition for the sensing resistor is given as: . 0.5 0.89 CS PK DS N CS PK DS R I R I < < (16) (Design Example) For minimum input voltage and nominal load condition, the operation mode is DCM as: 63 () 2 (115 100) 2 23 503 10 65 10 1 115 100 MIN INN RO INN M SW MIN INN RO VV PL f VV − + ⋅ ⋅ + = ⋅⋅ × ⋅ × ⋅ < ⋅ The peak drain current at minimum input voltage and nominal power condition is given as: DS.N 36 2 223 1.19 65 10 503 10 PK INN SW M P I A fL − ⋅ ⋅ == = ⋅× ⋅ × The conditions for the sensing resistor are given as: . .2 0.5 0.5 0.42 1.19 0.89 0.89 0.44 2.01 CS PK DS N CS PK DS P R I R I < == Ω < == Ω Thus, a 0.39 Ω resistor is selected for the current-sensing resistor |
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