Datasheet LTC3458 (Analog Devices) - 9

LTC3458
U U W U APPLICATIO S I FOR ATIO Error Amp.
The error amplifier is a transconductance type,
Current Sensing.
Lossless current sensing converts the with its positive input internally connected to the 1.23V peak current signal to a voltage to sum in with the internal reference, and its negative input connected to FB. A simple slope compensation. This summed signal is compared to compensation network is placed from COMP to ground. the error amplifier output to provide a peak current control Internal clamps limit the minimum and maximum error command for the PWM. The slope compensation in the IC amp output voltage for improved large signal transient is adaptive to the input and output voltage, therefore the response. During sleep (in Burst Mode), the compensa- converter provides the proper amount of slope compensa- tion pin is high impedance, however clamps limit the tion to ensure stability, but not an excess to cause a loss voltage on the external compensation network, preventing of phase margin in the converter. the compensation capacitor from discharging to zero
Output Disconnect and Inrush Limiting.
The LTC3458 is during the sleep time. designed to allow true output disconnect by eliminating
Oscillator.
The frequency of operation is set through a body diode conduction of the internal P-channel MOSFET resistor from R rectifier. This allows V T to ground. An internally trimmed timing 0UT to go to zero volts during capacitor resides inside the IC. The oscillator frequency is shutdown, drawing no current from the input source. It calculated using the following formula: also allows for inrush current limiting at turn-on, minimiz- ing surge currents seen by the input supply. Note that to 1 obtain the advantages of output disconnect, there must be fOSC = 0 2.+ 0 0.04•R no external Schottky diodes connected between SW and T VOUT. where fOSC is in MHz and RT is in kΩ
Shutdown.
The part is shut down by pulling SHDN below The oscillator can be synchronized with an external clock 0.3V, and made active by pulling the pin above 1.25V. Note applied to the SYNC pin. When synchronizing the oscilla- that SHDN can be driven above VIN or VOUT, as long as it tor, the free running frequency must be set to approxi- is limited to less than 8V. mately 30% lower than the desired synchronized fre-
Synchronous Rectifier.
To prevent the inductor current quency. from running away, the P-channel MOSFET synchronous
Soft-Start.
The soft-start time is programmed with an rectifier is only enabled when VOUT > (VIN + 0.25V). external capacitor to ground on SS. An internal current
Thermal Shutdown.
If the die temperature reaches ap- source charges it with a nominal 5μA. The voltage on the proximately 150 SS pin (in conjunction with the external resistor on I °C, the part will go into thermal shutdown LIM) is and all switches will be turned off and the soft-start used to control the peak current limit until the voltage on capacitor will be reset. The part will be enabled again when the capacitor exceeds ~1V, at which point the external the die temperature has dropped by 10 resistor sets the peak current. In the event of a com- °C (nominal). manded shutdown, severe short-circuit, or a thermal
Zero Current Amplifier.
The zero current amplifier moni- shutdown, the capacitor is discharged automatically. tors the inductor current to the output and shuts off the synchronous rectifier once the current is below 50mA t(msec) = CSS (μF) • 200 typical, preventing negative inductor current.
Other LTC3458 Features and Functions Burst Mode Operation Antiringing Control.
The antiringing control places a Burst Mode operation can be automatic or user controlled. resistor across the inductor to damp the ringing on SW pin In automatic operation, the IC will automatically enter discontinuous conduction mode. The LC ringing Burst Mode operation at light load and return to fixed (L = inductor, CSW = Capacitance on SW pin) is low energy, frequency PWM mode for heavier loads. The user can but can cause EMI radiation. program the average load current at which the mode 3458fa 9