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SC4502 Datasheet(PDF) 7 Page - Semtech Corporation |
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SC4502 Datasheet(HTML) 7 Page - Semtech Corporation |
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7 / 19 page  7 2005 Semtech Corp. www.semtech.com POWER MANAGEMENT SC4502/SC4502H Operation The SC4502/SC4502H is a programmable constant- frequency peak current-mode step-up switching regulator with an integrated power transistor. As shown in the block diagram in Figure 2, the power transistor is turned on at the trailing edge of the clock. Switch current is sensed with an integrated sense resistor. The sensed current signal 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 2A current-limit threshold. ILIM therefore provides cycle-by-cycle current limit. Current-limit is not affected by slope compensation because the current limit comparator ILIM is not in the PWM signal path. Current-mode switching regulators utiilize a dual-loop feedback control system. In the SC4502/SC4502H 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 SC4502/SC4502H can be programmed up to 2MHz with an external resistor from the ROSC pin to the ground. For converters requiring extremely low or high duty cycles, the operating frequency can be lowered to maintain the necessary minimum on time or the minimum off time. The SC4502/SC4502H 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 SC4502/SC4502H. 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 R2 with its center tap tied to the FB pin (Figure 3) sets the output voltage. − = 1 1.242V V R R OUT 2 1 (1) The input bias current of the error amplifier will introduce an error of: () % 1.242V 100 //R R 40nA V V 2 1 OUT OUT ⋅ ⋅ = ∆ (2) The percentage error of a V OUT = 5V converter with R1 = 100K Ω and R 2 = 301KΩ is Operating Frequency and Efficiency Switching frequency of SC4502/SC4502H 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, 1.4MHz and 2MHz are plotted in Figure 1(b) for SC4502. Duty Cycle The duty cycle D of a boost converter in continuous conduction mode is: Figure 3. The Output Voltage is set with a Resistive Divider VOUT R2 R1 40nA FB 2 SC4502/SC4502H Applications Information () 0.24% 1.242V 100 K 301 // K 100 40nA V V OUT OUT = ⋅ Ω Ω ⋅ = ∆ |
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Similar Description - SC4502 |
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