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SC1406GCTSTR Datasheet(PDF) 9 Page - Semtech Corporation |
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SC1406GCTSTR Datasheet(HTML) 9 Page - Semtech Corporation |
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9 / 28 page  9 ã 2000 Semtech Corp. www.semtech.com POWER MANAGEMENT SC1406G Functional Description SUPPLY The chip is optimized to operate from a 3.3V + 5% rail but is also designed to work up to 6V maximum supply voltage. UNDER VOLTAGE LOCK-OUT CIRCUIT The under voltage lockout (UVLO) circuit consists of two com- parators, the low battery and low V CC (low supply voltage) comparators. The output of the comparator, gated with the Enable signal, turns on or off the internal bias, enables or disables the CO output, and initiates or resets the soft start timers. POWER GOOD GENERATOR If the chip is enabled but not in UVLO condition, and the core voltage gets within +10% of the VID programmed value, then a high level Power Good signal is generated on the PWRGD pin to trigger the CPU power up sequence. If the chip is either disabled or enabled in UVLO condition, then PWRGD stays low. This condition is satisfied by the presence of an internal 200kW pull- down resistor connected from PWRGD to ground. During soft start, PWRGD stays low independently from the status of Vcore voltage. PWRGD is high when all of the following conditions are true: 1 EN is high 2 Soft-start has completed 3 LBIN and VCC are above their under-voltage trip levels. BAND GAP REFERENCE A better than +1% precision band-gap reference acts as the internal reference voltage standard of the chip, which all critical biasing voltages and currents are derived from. All references to VREF in the equations to follow will assume V REF = 1.7V. CORE CONVERTER CONTROLLER Precision VID DAC Reference The 5-bit digital to analog converter (DAC) serves as the pro- grammable reference source of the core comparator. Program- ming is accomplished by CMOS logic level VID code applied to the DAC inputs. The VID code vs. the DAC output is shown in the Output Voltage Table. The accuracy of the VID DAC is main- tained on the same level as the band gap reference. There is a 10µA pull-up current on each DAC input when EN is high. Core Comparator This is an ultra-fast hysteretic comparator with a typical propaga- tion delay of approximately 20ns at a 20mV overdrive. This chip can be used in a standard hysteretic mode controller configuration and in an IMVP hysteretic controller scheme. Detailed instructions for the IMVP solution are found in the PowerStep solution design procedure section of this datasheet. Current Limit Comparator The current limit comparator monitors the core converter output current and turns the high side switch off when the current exceeds the upper current limit threshold, VHCL and re-enable only if the load current drops below the lower current limit threshold, VLCL. The current is sensed by monitoring the voltage drop across the current sense resistor, R CS, connected in series with the core converter main inductor (the same resistor used for IMVP input signal generation). The thresholds have the following relationships: Core Converter Soft Start Timer This circuit controls the ramp-up time of the core voltage in order to reduce the initial inrush current on the core input voltage (battery) rail. The soft-start circuit consists of an internal current source, external soft-start timing capacitor, internal discharge switch across the capacitor, and a comparator monitoring the capacitor voltage. LINEAR REGULATOR CONTROLLERS 1.5V Linear Regulator This block is a low drop-out (LDO) linear-regulator controller, which drives an external PNP bipolar transistor as a pass element. The linear regulator is capable of delivering 500mA steady-state DC current and can support transient currents of greater than 1A, depending on pass element and output capacitor selection. 2.5V Linear Regulator This block is a low drop-out (LDO) linear regulator controller, which drives an external PNP bipolar transistor as a pass element. The LDO linear regulator is capable of delivering 100mA steady-state DC current and can support transient currents greater than 200mA, depending on pass element and output capacitor selection. Linear Regulator Soft-Start The soft-start circuit of the linear regulators is similar to that of the core converter, and is used to control the ramp-up time of the linear regulator output voltages. For maximum flexibility in controlling the start-up sequence, the soft-start function of the linear regulators is separated from that of the core converter. REF CLSET CLOH HCL V R R V • • = 3 REF CLSET CLOH LCL V R R V • • = 2 REF CLSET CLOH HYSCL V R R V • = |
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