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SC1485 Datasheet(PDF) 8 Page - Semtech Corporation |
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SC1485 Datasheet(HTML) 8 Page - Semtech Corporation |
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8 / 17 page  8 2004 Semtech Corp. www.semtech.com SC1485 POWER MANAGEMENT Applications Information +5V Bias Supplies The SC1485 requires an external +5V bias supply in ad- dition to the battery. If stand-alone capability is required, the +5V supply can be generated with an external linear regulator such as the Semtech LP2951A. To minimize channel to channel crosstalk, each controller has 4 sup- ply pins, VDDP, PGND, VCCA and AGND. To avoid ground loops, separate AGND planes are rec- ommended. Each contoller requires its own AGND plane which should be tied by a single trace to the negative terminal of that controller’s output capacitor. All external components referenced to AGND in the schematic should then be connected to the appropriate AGND plane. The supply decoupling capacitor for controller 1 should be tied between VCCA1 and AGND1. Likewise, the supply decoupling capacitor for controller 2 should be tied be- tween VCCA2 and AGND2. A single 10 ohm resistor should be used to decouple the VCCA supplies from the main VDDP supplies. PGND can then be a separate plane which is not used for routing traces. All PGND connec- tions are connected directly to this plane with special attention given to avoiding indirect connections which may create ground loops. As mentioned above, the two AGND planes must be connected to the PGND plane at the negative terminal of the respective output capaci- tors. The VDDP1 and VDDP2 input provides power to the up- per and lower gate drivers. A decoupling capacitor for each supply is recommended. No series resistor between VDDP and the 5 volt bias is required. Pseudo-fixed Frequency Constant On-Time PWM Con- troller The PWM control architecture consists of a constant-on- time, pseudo fixed frequency PWM controller, (Figure 1). The output ripple voltage developed across the output filter capacitor’s ESR provides the PWM ramp signal elimi- nating 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 in- versely proportional to input voltage. A second one-shot sets the minimum off-time to 400ns typically. 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 proportional current is used to charge an in- ternal on-time capacitor. The TON time is the time re- quired 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 imple- mentation results in a nearly constant switching frequency without the need of a clock generator. • + • = − IN OUT 3 TON 12 ON V V ) 10 x 37 R ( 10 x 3 . 3 T +50ns R TON is a resistor connected from the input supply to the TON pin. The graph on page 16 shows the relationship between R TON and switching frequency. Enable & Psave The SC1485 combines the ENABLE and PSAVE functions into a single pin. When the pin is tied to ground the SMPS is disabled. When it is tied to +5V the SMPS is enabled and PSAVE is active. In order to enter PSAVE, The SC1485 PSAVE comparator will look for 8 consecutive inductor current reversals. When this happens, the controller will switch into PSAVE mode. At the same time, the SC1485 will increase the on-time by 1.5 times its set value. This will increase the ripple current and ripple voltage by 1.5 times their continuous conduction mode (CCM) values. This increase has two benefits. First, the reduction in switching frequency will improve efficiency. Second, hys- teresis is added to the PSAVE circuit. This is important because when in PSAVE, the very first time a current re- versal does not occur, the SC1485 will exit the PSAVE mode. This allows the device to rapidly respond to tran- sient load conditions, while adding hysteresis to elimi- nate false PSAVE exits. When the pin is left floating, the pin is internally pulled to 2V, enabling the SMPS in CCM. Output Voltage Selection The output voltage selection is set by the feedback re- sistors R2 & R3 of Figure 3. The internal reference is 1.5V. The internal feedback pin is multiplied by three to match the 1.5V reference. Therefore the output can be selected to a minimum of 0.5V. The equation for select- ing the output voltage based on Figure 3 is: 5 . 0 3 R 2 R 1 V OUT • + = |
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Similar Description - SC1485 |
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