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LTC3458L Datasheet(PDF) 9 Page - Linear Technology |
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LTC3458L Datasheet(HTML) 9 Page - Linear Technology |
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9 / 16 page  LTC3458L 9 3458lfa APPLICATIO S I FOR ATIO Current Sensing. Lossless current sensing converts the peak current signal to a voltage to sum in with the internal slope compensation. This summed signal is compared to the error amplifier output to provide a peak current control command for the PWM. The slope compensation in the IC is adaptive to the input and output voltage, therefore the converter provides the proper amount of slope compensa- tion to ensure stability, but not an excess to cause a loss of phase margin in the converter. Output Disconnect and Inrush Limiting. The LTC3458L is designed to allow true output disconnect by eliminating body diode conduction of the internal P-channel MOSFET rectifier. This allows V0UT to go to zero volts during shutdown, drawing no current from the input source. It also allows for inrush current limiting at turn-on, minimiz- ing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, there must not be any external Schottky diodes connected between SW and VOUT. Shutdown. The part is shut down by pulling SHDN below 0.3V, and made active by pulling the pin above 1.25V. Note that SHDN can be driven above VIN or VOUT, as long as it is limited to less than 6V. Synchronous Rectifier. To prevent the inductor current from running away, the P-channel MOSFET synchronous rectifier is only enabled when VOUT > (VIN + 0.25V). Thermal Shutdown. If the die temperature reaches ap- proximately 150 °C, the part will go into thermal shutdown and all switches will be turned off and the soft-start capacitor will be reset. The part will be enabled again when the die temperature has dropped by 10 °C (nominal). Zero Current Amplifier (IZERO). The zero current amplifier monitors the inductor current to the output and shuts off the synchronous rectifier once the current is below 50mA typical, preventing negative inductor current. Burst Mode Operation Burst Mode operation can be automatic or user controlled. In automatic operation, the IC will automatically enter Burst Mode operation at light load and return to fixed frequency PWM mode for heavier loads. The user can program the average load current at which the mode Error Amp. The error amplifier is a transconductance type, with its positive input internally connected to the 1.225V reference, and its negative input connected to FB. A simple compensation network is placed from COMP to ground. Internal clamps limit the minimum and maximum error amp output voltage for improved large signal transient response. During sleep (in Burst Mode), the compensa- tion pin is high impedance, however clamps limit the voltage on the external compensation network, preventing the compensation capacitor from discharging to zero during the sleep time. Oscillator. The frequency of operation is set through a resistor from RT to ground. An internally trimmed timing capacitor resides inside the IC. The oscillator frequency is calculated using the following formula: f R OSC T = + 1 02 0004 .. • where fOSC is in MHz and RT is in kΩ The oscillator can be synchronized with an external clock applied to the SYNC pin. When synchronizing the oscilla- tor, the free running frequency must be set to approxi- mately 30% lower than the desired synchronized fre- quency. Soft-Start. The soft-start time is programmed with an external capacitor to ground on SS. An internal current source charges it with a nominal 5 μA. The voltage on the SS pin (in conjunction with the external resistor on ILIM) is used to control the peak current limit until the voltage on the capacitor exceeds ~1V, at which point the external resistor sets the peak current. In the event of a com- manded shutdown, severe short-circuit, or a thermal shutdown, the capacitor is discharged automatically. t(msec) = CSS (μF) • 200 Other LTC3458L Features and Functions Antiringing Control. The antiringing control places a resistor across the inductor to damp the ringing on SW pin discontinuous conduction mode. The LC ringing (L = inductor, CSW = Capacitance on SW pin) is low energy, but can cause EMI radiation. |
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Similar Description - LTC3458L |
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