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LTC1530C Datasheet(PDF) 10 Page - Linear Technology |
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LTC1530C Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  10 LTC1530 Figure 5b is derived based on the condition that ILMAX = ILOAD + IRIPPLE/2. Therefore, it only provides the minimum RIMAX value. It must be understood that during the initial power-up phase (VOUT = 0V), the initial start-up ILMAX can be much higher than the steady state condition ILMAX. Therefore, RIMAX must be selected with the start-up ILMAX in mind. In general, high output capacitance com- bined with a low value inductor increases the start-up ILMAX. Figures 6a and 6b plot the start-up ILMAX vs output capacitance and inductance for unloaded and loaded con- ditions with the current limit circuit disabled. Figures 6a and 6b are provided as examples. Actual ILMAX under start-up conditions must be measured for any application circuit so that RIMAX can be properly chosen. In order for the current limit circuit to operate properly and to obtain a reasonably accurate current limit threshold, the IMAX and IFB pins must be Kelvin sensed at Q1’s drain and source pins. A 0.1 µF decoupling capacitor can also be connected across RIMAX to filter switching noise. In addi- tion, LTC recommends that the voltage drop across the RIMAX resistor be set to ≥100mV. Otherwise, noise spikes or ringing at Q1’s source can cause the actual current limit to be greater than the desired current limit set point. MOSFET Gate Drive The PVCC supply must be greater than the input supply voltage, VIN, by at least one power MOSFET VGS(ON) for efficient operation. This higher voltage can be supplied with a separate supply, or it can be generated using a simple charge pump as shown in Figure 7. The 86% maximum duty cycle ensures sufficient off-time to refresh the charge pump during each cycle. As PVCC is powered up from 0V, the LTC1530 undervoltage lockout circuit prevents G1 and G2 from pulling high until PVCC reaches about 3.5V. To prevent Q1’s high RDS(ON) from triggering the current limit comparator while PVCC is slewing, the current limit circuit is disabled until PVCC is ≥8V. In addition, on start-up or recovery from thermal shutdown, the driver logic is designed to hold G2 low until G1 first goes high. ILMAX (A) 0 5500 4500 3500 2500 1500 500 16 18 1530 F05b 4 26 810 14 12 20 RIMAX ≥ 500Ω ILMAX = ILOAD + IRIPPLE/2 Q1 RDS(ON) = 0.05Ω 0.04 Ω 0.03 Ω 0.02 Ω 0.01 Ω Figure 5b. Minimum Required RIMAX vs ILMAX OUTPUT CAPACITANCE (mF) 0 25 20 15 10 5 0 2 46 8 1530 F06a 10 12 TA = 25°C VIN = 5V ILOAD = 0A L = 1.2 µH L = 4.7 µH L = 2.4 µH Figure 6a. Start-Up ILMAX vs Output Capacitance OUTPUT CAPACITANCE (mF) 0 30 25 20 15 10 5 0 2 46 8 1530 F06b 10 12 TA = 25°C VIN = 5V ILOAD = 10A L = 1.2 µH L = 4.7 µH L = 2.4 µH Figure 6b. Start-Up ILMAX vs Output Capacitance APPLICATIO S I FOR ATIO |
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