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RT9259C Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT9259C Datasheet(HTML) 11 Page - Richtek Technology Corporation |
11 / 15 page RT9259C 11 DS9259C-03 August 2007 www.richtek.com ESR C 2 1 f OUT ESR × × = π The ESR zero is contributed by the ESR associated with the output capacitance. Note that this requires that the output capacitor should have enough ESR to satisfy stability requirements. The ESR zero of the output capacitor expressed as follows : 2) Compensation Frequency Equations The compensation network consists of the error amplifier and the impedance networks ZC and ZF as shown in Figure 9. Figure 9. Compensation Loop C2 x R2 x 2 1 fZ1 π = C2 C1 C2 x C1 x R2 x 2 1 fP1 + = π Figure 10 shows the DC-DC converter's gain vs. frequency. The compensation gain uses external impedance networks ZC and ZF to provide a stable, high bandwidth loop. High crossover frequency is desirable for fast transient response, + - FB V REF COMP EA Z C Z F C1 C2 R2 R1 R F V OUT Figure 10. Bode Plot Frequency 10Hz 100Hz 1.0KHz 10KHz 100KHz 1.0MHz vdb(vo) vdb(comp2) vdb(lo) -40 0 40 80 -60 10 100 1k 10k 100k 1M 80 40 0 20 60 -20 -40 -60 Loop Gain Compensation Gain Modulator Gain Frequency (Hz) but often jeopardize the system stability. In order to cancel one of the LC filter poles, place the zero before the LC filter resonant frequency. In the experience, place the zero at 75% LC filter resonant frequency. Crossover frequency should be higher than the ESR zero but less than 1/5 of the switching frequency. The second pole is placed at half the switching frequency. Thermal Considerations For continuous operation, do not exceed absolute maximum operation junction temperature 125 °C. The maximum power dissipation depends on the thermal resistance of IC package, PCB layout, the rate of surroundings airflow and temperature difference between junction to ambient. The maximum power dissipation can be calculated by following formula : PD(MAX) = ( TJ(MAX) − TA ) / θJA Where TJ(MAX) is the maximum operation junction temperature 125 °C, TAis the ambient temperature and the θJA is the junction to ambient thermal resistance. For recommended operating conditions specification of RT9259C, where TJ(MAX) is the maximum junction temperature of the die (125 °C) and TA is the maximum ambient temperature. The junction to ambient thermal resistance θJA is layout dependent. For VQFN-16L 4x4 packages, the thermal resistance θJA is 54°C/W on the standard JEDEC 51-7 four-layers thermal test board. The maximum power dissipation at TA = 25 °C can be calculated by following formula : PD(MAX) = ( 125 °C − 25°C ) / 54°C/W = 1.852 W for QFN-16L 4x4 packages PD(MAX) = ( 125 °C − 25°C) / 100°C/W = 1.000 W for SOP-14 packages PD(MAX) = ( 125 °C − 25°C ) / 110°C/W = 0.909 W for SSOP-16 packages The maximum power dissipation depends on operating ambient temperature for fixed TJ (MAX) and thermal resistance θJA. For RT9259C packages, the Figure 11 of derating curves allows the designer to see the effect of rising ambient temperature on the maximum power allowed. |
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