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SC1470ITSTR Datasheet(PDF) 7 Page - Semtech Corporation |
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SC1470ITSTR Datasheet(HTML) 7 Page - Semtech Corporation |
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7 / 25 page  7 2005 Semtech Corp. www.semtech.com SC1470 POWER MANAGEMENT +5V Bias Supplies The SC1470 requires an external +5V bias supply in addition to the battery. If stand-alone capability is required, the +5V supply can be generated with an external linear regulator such as the Semtech LP2951. For optimal operation, the controller has its own ground reference, VSSA, which should be tied by a single trace to PGND at the negative terminal of the output capacitor (see Layout Guidelines). All external components referenced to VSSA in the Typical Applications Circuit on Page 1 should be connected to VSSA. The supply decoupling capacitor should be tied directly between the VCCA and VSSA pins. A 10 Ω resistor should be used to decouple VCCA from the main VDDP supply. PGND can then be a separate plane which is not used for routing traces. All PGND connections are connected directly to the ground plane with special attention given to avoiding indirect connections which may create ground loops. As mentioned above, VSSA must be connected to the PGND plane at the negative terminal of the output capacitor only. The VDDP input provides power to the upper and lower gate drivers. A decoupling capacitor is required. No series resistor between VDDP and 5V is required. See Layout Guidelines for more details. Pseudo-fixed Frequency Constant On-Time PWM Controller The PWM control architecture consists of a constant on- time, pseudo fixed frequency PWM controller (see Figure 1, SC1470 Block Diagram). The output ripple voltage developed across the output filter capacitor’s ESR provides the PWM ramp signal eliminating the need for a current sense resistor. The high-side switch on-time is determined by a one-shot whose period is directly proportional to output voltage and inversely proportional to input voltage. A second one-shot sets the minimum off-time which is typically 400ns. On-Time One-Shot (t ON) The on-time one-shot comparator has two inputs. One input looks at the output voltage, while the other input samples the input voltage and converts it to a current. This input voltage-proportional current is used to charge an internal on-time capacitor. The on-time is the time required for the voltage on this capacitor to charge from zero volts to VOUT, thereby making the on-time of the high-side switch directly proportional to output voltage and inversely proportional to input voltage. This implementation results in a nearly constant switching frequency without the need for a clock generator. For VOUT < 3.3V: ns 50 V V ) 10 x 37 R ( 10 x 3 . 3 t BAT OUT 3 TON 12 ON + • + • = − For 3.3V ≤ VOUT ≤ 5V: ns 50 V V ) 10 x 37 R ( 10 x 3 . 3 85 . 0 t BAT OUT 3 TON 12 ON + • + • • = − R TON is a resistor connected from the input supply (VBAT) to the TON pin. Due to the high impedance of this resistor, the TON pin should always be bypassed to VSSA using a 1nF ceramic capacitor. Enable & Psave The EN/PSV pin enables the supply. When EN/PSV is tied to VCCA the controller is enabled and power save will also be enabled. If PSAVE is enabled, the SC1470 PSAVE comparator will look for the inductor current to cross zero on eight consecutive switching cycles by comparing the phase node (LX) to PGND. Once observed, the controller will enter power save and turn off the low side MOSFET when the current crosses zero. To improve light-load efficiency and add hysteresis, the on-time is increased by 50% in power save. The efficiency improvement at light-loads more than offsets the disadvantage of slightly higher output ripple. If the inductor current does not cross zero on any switching cycle, the controller will immediately exit power save. Since the controller counts zero crossings, the converter can sink current as long as the current does not cross zero on eight consecutive cycles. This allows the output voltage to recover quickly in response to negative load steps even when psave is enabled. When the EN/PSV pin is tri-stated, an internal pull-up will activate the controller and power save will be disabled. When the EN/PSV pin is pulled low, the supply is disabled and the MOSFET drivers are tri-stated. Application Information |
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