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LM2655MTCX-ADJ Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM2655MTCX-ADJ Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Operation The LM2655 is a constant frequency (300kHz), current- mode PWM switcher that can be operated synchronously or asynchronously. SYNCHRONOUS OPERATION A converter is said to be in synchronous operation when a MOSFET is used in place of the catch diode. In the case of the buck converter, this MOSFET is known as the low-side MOSFET (the MOSFET connected between the input source and the low-side MOSFET is the high-side MOS- FET). Converters in synchronous operation exhibit higher efficiencies compared to asynchronous operation because the I 2R losses are reduced with the use of a MOSFET . Operation of the LM2655 in synchronous mode is identical to its operation in asynchronous mode, except that internal logic drives the low-side MOSFET. At the beginning of a switching cycle, the high-side MOSFET is on and current from the input source flows through the inductor and to the load. The current from the high-side MOSFET is sensed and compared with the output of the error amplifier (COMP pin). When the sensed current reaches the COMP pin voltage level, the high-side switch is turned off. After a 30ns delay (deadtime), the low-side driver goes high and turns the low-side MOSFET on. The current now flows through the low-side MOSFET, through the inductor and on to the load. A 30ns delay is necessary to insure that the MOSFETs are never on at the same time. During the 30ns deadtime, the current is forced to flow through the low-side MOSFET’s body diode. It is recommended that a low forward drop schottky diode be placed in parallel to the low-side MOSFET so that current will be more efficiently conducted during this 30ns deadtime. This Schottky diode should be placed within 5mm of the switch pin so that current limit is not effected (see External Schottky Diode section). At the end of the switching cycle, the low-side switch is turned off and after another 30ns delay, the cycle is repeated. Current through the high-side MOSFET is sensed by pat- ented circuitry that does not require an external sense resis- tor. As a result, system cost and size are reduced, efficiency is increased, and noise immunity of the sensed current is improved. A feedforward from the input voltage is added to reduce the variation of the current limit over the input voltage range. ASYNCHRONOUS OPERATION A unique feature of the LM2655 is that it can be operated in either synchronous or asynchronous mode. When operating in asynchronous mode, a small amount of efficiency is sac- rificed for a less expensive solution. Any diode may be used, but it is recommended that a low forward drop schottky diode be use to maximize efficiency. When operating the LM2655 in asynchronous mode, the LDR pin should be terminated with a large resistor (>1 Meg Ω), or left floating. Operation in asynchronous mode is similar to that of synchronous mode, except the internal low-side MOSFET logic is not used. At the beginning of a switching cycle, the high-side MOSFET is on and current from the input source flows through the inductor and to the load. The current from the high-side MOSFET is sensed and compared with the output of the error amplifier (COMP pin). When the sensed current reaches the COMP pin voltage level, the high-side switch is turned off. At this instant, the load current is commutated through the catch diode. The current now flows through the diode and the inductor and on to the load. At the end of the switching cycle, the high-side switch is turned on and the cycle is repeated. PROTECTIONS The peak current in the system is monitored by cycle-by- cycle current limit circuitry. This circuitry will turn the high- side MOSFET off whenever the current through the high- side MOSFET reaches a preset limit (see plots). A second level current limit is accomplished by the undervoltage pro- tection: if the load pulls the output voltage down below 80% of its nominal value, the undervoltage latch protection will wait for a period of time (set by the capacitor at the LDELAY pin, see LDELAY CAPACITOR section for more information). If the output voltage is still below 80% of its nominal after the waiting period, the latch protection will be enabled. In the latch protection mode, the low-side MOSFET is on and the high-side MOSFET is off. The latch protection will also be enabled immediately whenever the output voltage exceeds the overvoltage threshold (110% of its nominal). Both pro- tections are disabled during start-up.(See SOFT-START CA- PACITOR section and LDELAY CAPACITOR section for more information.) Toggling the input supply voltage or the shutdown pin can reset the device from the latched protec- tion mode. Design Procedure This section presents guidelines for selecting external com- ponents. INPUT CAPACITOR A low ESR aluminum, tantalum, ceramic, or any other type of capacitor is needed between the input pin and power ground. This capacitor prevents large voltage transients from appearing at the input. The capacitor is selected based on the RMS current and voltage requirements. The RMS cur- rent is given by: The RMS current reaches its maximum (I OUT/2) when V IN equals 2VOUT. For an aluminum or ceramic capacitor, the voltage rating should be at least 25% higher than the maximum input voltage. If a tantalum capacitor is used, the voltage rating required is about twice the maximum input voltage. The tantalum capacitor should be surge current tested by the manufacturer to prevent damage by the inrush current. It is also recommended to put a small ceramic capacitor (0.1 µF) between the input pin and ground pin to reduce high frequency noise. INDUCTOR The most critical parameters for the inductor are the induc- tance, peak current and the DC resistance. The inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages: A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, current stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage ripple require- www.national.com 9 |
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