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LM2706TL Fiches technique(PDF) 11 Page - National Semiconductor (TI) |
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LM2706TL Fiches technique(HTML) 11 Page - National Semiconductor (TI) |
11 / 15 page Bypass Operation (Continued) Automatic Bypass modes, the output voltage will follow the input voltage less the voltage drop across the resistance of the bypass PFET. These modes are more efficient than operating in PWM mode at 100% duty cycle because the resistance of the bypass PFET is less than the series resis- tance of the PWM PFET and inductor. This translates into higher voltage available on the output in bypass mode, for a given battery voltage. The part can be placed in bypass operation by sending BYP pin high for Forced Bypass mode. During Forced Bypass mode, the bypass switch remains on irrespective of the state of ABD pin. A second way to place the part in bypass operation is by setting ABD low for Automatic Bypass mode. In Automatic Bypass mode, the bypass switch turns on when the difference between the input voltage and programmed output voltage is less than 275 mV (typ) for more than the bypass delay time 10 µs (typ). The bypass switch turns off when the input voltage is higher than the programmed output voltage by 440 mV (typ) for longer than the bypass delay time. The bypass delay time is provided to prevent false triggering into Automatic Bypass Mode modes by either spikes or dips in V IN. Operating Mode Selection Controls The LM2706 is designed for digital control of the operating modes using BYP and ABD pins. The settings for these pins are outlined in Table 1. Setting BYP high (>1.2V) places the device in Forced Bypass mode. Setting BYP low and ABD high forces operation in PWM mode. Setting both BYP and ABD low (<0.5V) or leaving them floating places the device in Automatic Bypass mode. The BYP and ABD pins have 5 µA (typ) pull down currents for default operation in Auto- matic Bypass mode for automatic switchover between PWM and Bypass operation in systems where digital mode control is not required. TABLE 1. Operating Modes Mode Logic Level ABD BYP Forced Bypass 0 or 1 1 Auto-Bypass / PWM 0 0 PWM 0or1 0 Overvoltage Protection The LM2706 has an over-voltage comparator that prevents the output voltage from rising too high, when the device is left in PWM mode under low-load conditions. When the output voltage rises by 350 mV over its regulation threshold, the OVP comparator inhibits PWM operation to skip pulses until the output voltage returns to the regulation threshold. In over voltage protection, output voltage and ripple increases. At light loads when the required on time to regulate becomes less than the minimum on time of the LM2706 (around 200ns) then the output voltage will increase till it hits the overvoltage threshold and the part will be in overvoltage protection mode. Shutdown Mode Setting the EN digital input pin low (<0.5V) places the LM2706 in a 0.1 µA (typ) shutdown mode. During shutdown, the PFET switch, NFET synchronous rectifier, reference, control and bias circuitry of the LM2706 are turned off. Setting EN high enables normal operation. While turning on, soft start is activated. The device takes around 600 µs to complete the soft-start interval and come into regulation during turn-on. EN should be set low to turn off the LM2706 during power-up and under voltage conditions when the supply is less than the 2.7V minimum operating voltage. The LM2706 is de- signed for compact portable applications, such as mobile phones. In such applications, the system controller deter- mines power supply sequencing and requirements for small package size outweigh the additional size required for inclu- sion of UVLO (Under Voltage Lock-Out) circuitry. Internal Synchronous Rectification While in PWM mode, the LM2706 uses an internal NFET as a synchronous rectifier to reduce rectifier forward voltage drop and associated power loss. Synchronous rectification provides a significant improvement in efficiency whenever the output voltage is relatively low compared to the voltage drop across an ordinary rectifier diode. During moderate and heavy loads, the internal NFET syn- chronous rectifier is turned on during the inductor current down slope during the second part of each cycle. The syn- chronous rectifier is turned off prior to the next cycle, or when the inductor current ramps to zero at light loads. The NFET is designed to conduct through it’s intrinsic body diode dur- ing transient intervals before it turns on, eliminating the need for an external diode. Current Limiting A current limit feature allows the LM2706 to protect itself and external components during overload conditions. In PWM mode a 750 mA max cycle-by-cycle current limit is normally used. In the Bypass mode the bypass FET peak current limit is 650 mA (typ). During overload conditions the LM2706 limits output current to this value. If the output current reaches the bypass FET peak current limit then the current folds back to the fold back current value of around 450 mA. A reduction in the output current below the reset value of around 300 mA resumes normal bypass operation. In cases where the bypass FET is turned on with a big differential between the input and output voltages the bypass FET may see a peak current exceeding its peak current limit due to the charging of the output capacitor plus the load current and will go to the foldback current value thus acting as a constant current source. As the output capacitor gets charged the bypass FET current will go below the reset limit and the bypass switch will be fully on thus acting as a switch again. Dynamically Adjustable Output Voltage The LM2706 features dynamically adjustable output voltage to eliminate the need for external feedback resistors. The output can be set from 1.5V to 3.25V by changing the voltage on the analog V CONTROL pin. This feature is useful in PA applications where peak power is needed only when the handset is far away from the base station or when data is being transmitted. In other instances the transmitting power can be reduced and hence the supply voltage to the PA can www.national.com 11 |
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