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MAX1513 Datasheet(PDF) 20 Page - Maxim Integrated Products |
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MAX1513 Datasheet(HTML) 20 Page - Maxim Integrated Products |
20 / 28 page TFT-LCD Power-Supply Controllers 20 ______________________________________________________________________________________ Because VSENSE would be greater than 100mV, the cir- cuit in Figure 9 should be used and the scale factor is: Calculate RS1 and RS2: The closest 1% standard values for RS1 and RS2 are 866 Ω and 1.13kΩ, respectively. If the 2.2µH inductor used in the typical operating cir- cuit (Figures 1 and 2) has a typical DCR of 10m Ω and a maximum DCR of 14m Ω, the RC time constant of the sense network is: Select CS = 0.1µF, RS is: Assuming ∆T is 40°C and TC is 0.5%, the worst-case high sense voltage over temperature is: Because VSENSE would be much less than 80mV, the circuit in Figure 10 can be used to improve the current- limit accuracy. Calculate RS3 and RS4: The closest 1% standard values for RS3 and RS4 are 2.61k Ω and 14.0Ω, respectively. Output-Capacitor Selection The output capacitor and its equivalent series resistance (ESR) affect the circuit’s stability, output voltage ripple, and transient response. The Output-Capacitor Stability Requirement section discusses the output capacitance requirement based on the loop stability. This section deals with how to determine the output capacitance according to the ripple voltage and load-transient requirements. The total output voltage ripple has two components: the ohmic ripple due to the capacitor’s equivalent series resistance (ESR), and the capacitive ripple caused by the charging and discharging of the output capacitance: where VMAIN is the output voltage of the step-up regu- lator, IMAIN is the output current, COUT is the output capacitance, RESR is the ESR of the output capacitor, fOSC is the switching frequency, and IPEAK is the peak inductor current (see the Inductor Selection section). VV V VI R V I C VV Vf RIPPLE RIPPLE ESR RIPPLE C RIPPLE ESR PEAK ESR RIPPLE C MAIN OUT MAIN IN MAIN OSC () ( ) () () =+ ≈× ≈× × ⎛ ⎝⎜ ⎞ ⎠⎟ - R VV V R S S 3 4 15 4 5 1 4 0 0 044 2600 2614 2614 2600 14 . . = + ×= == - 5V - .5V - .1V - ΩΩ ΩΩ Ω VA m C mV SENSE =× + × ° () = 2 6 14 1 0 005 40 44 . . Ω R s F S . == 220 01 2200 µ µ Ω τ µ µ . == 22 10 220 H m s Ω R R S S 1 2 489 0 571 856 856 0 571 1 0 571 1139 . . . == = × = Ω Ω Ω - SF mV mV . == 100 175 0 571 VA m C mV SENSE =× + × ° () = 2 6 56 1 0 005 40 175 . . Ω MAX1513 MAX1514 L RS3 CS VIN VMAIN CS+ CS- GATE GND FB RS4 Figure 10. Lossless Current Sensing with VSENSE < 80mV |
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