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KB3302 Datasheet(PDF) 6 Page - Kingbor Technology Co |
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KB3302 Datasheet(HTML) 6 Page - Kingbor Technology Co |
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6 / 17 page  6 KB3302 Kingbor Technology Co.,Ltd TEL:(86)0755-26508846 FAX:(86)0755-26509052 OPERATION The KB3302 is a programmable constant-frequency peak current-mode step-up switching regulator with an integrated 2A power transistor. Referring to the block diagrams in Figures 2 and 3, the power transistor is switched on at the trailing edge of the clock. Switch current is sensed with an integrated sense resistor. The sensed current is summed with the slope-compensating ramp before compared to the output of the error amplifier EA. The PWM comparator trip point determines the switch turn-on pulse width. The current-limit comparator ILIM turns off the power switch when the switch current exceeds the 2.8A current-limit threshold. ILIM therefore provides cycle-by-cycle current limit. Current-limit is not affected by slope compensation because the current comparator ILIM is not in the PWM signal path. Current-mode switching regulators utilize a dual-loop feedback control system. In the KB3302 the amplifier output COMP controls the peak inductor current. This is the inner current loop. The double reactive poles of the output LC filter are reduced to a single real pole by the inner current loop, easing loop compensation. Fast transient response can be obtained with a simple Type-2 compensation network. In the outer loop, the error amplifier regulates the output voltage. The switching frequency of the KB3302 can be programmed up to 2MHz with an external resistor from the ROSC pin to the ground. For converters requiring extreme duty cycles, the operating frequency can be lowered to maintain the necessary minimum on time or the minimum off time. The KB3302 requires a minimum input of 1.4V to operate. A voltage higher than 1.1V at the shutdown pin enables the internal linear regulator REG in the KB3302. After V REG becomes valid, the soft-start capacitor is charged with a 1.5 µA current source. A PNP transistor clamps the output of the error amplifier as the soft-start capacitor voltage rises. Since the COMP voltage controls the peak inductor current, the inductor current is ramped gradually during soft-start, preventing high input start-up current. Under fault conditions (V IN <1.4V or over temperature) or when the shutdown pin is pulled below 1.1V, the soft-start capacitor is discharged to ground. Pulling the shutdown pin below 0.1V reduces the total supply current to 10 µA. Setting the Output Voltage An external resistive divider R 1 and R 2 with its center tap tied to the FB pin (Figure 4) sets the output voltage. ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = 1 V 242 . 1 V R R OUT 2 1 (1) VOUT R2 R1 40nA FB 2 KB3302 The input bias current of the error amplifier will introduce an error of: () % V 242 . 1 100 R // R nA 40 V V 2 1 OUT OUT = ∆ (2) The percentage error of a V OUT = 5V converter with R 1 = 100K Ω and R 2 = 301K Ω is () % 24 . 0 V 242 . 1 100 K 301 // K 100 nA 40 V V OUT OUT = = ∆ Operating Frequency and Efficiency Switching frequency of KB3302 is set with an external resistor from the ROSC pin to the ground. A graph showing the relationship between R OSC and switching frequency is given in the “Typical Characteristics”. High frequency operation reduces the size of passive components but switching losses are higher. The efficiencies of 5V to 12V converters operating at 700KHz, 1.35MHz and 2MHz are shown in Figure 1(b). The peak efficiency of the KB3302 appears to decrease 0.5% for every 100KHz increase in frequency. Figure 4. The Output Voltage is set with a Resistive Divider APPLICATIONS INFORMATION |
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