Datasheet LTC3421 (Analog Devices) - 9

LTC3421
OPERATION LOW VOLTAGE START-UP Oscillator
The LTC3421 includes an independent start-up oscillator The frequency of operation is set through a resistor designed to start-up at input voltages of 0.85V typical. from the RT pin to ground. An internally trimmed tim- The frequency and peak current limit during start-up are ing capacitor resides inside the IC. The oscillator can be internally controlled. The device can start-up under some synchronized with an external clock applied to the SYNC load (see graph of Start-Up Current vs Input Voltage). pin. When synchronizing the oscillator, the free running Soft-start and inrush current limiting are provided dur- frequency must be set to an approximately 30% lower ing start-up as well as normal mode. The same soft-start frequency than the desired synchronized frequency. capacitor is used for each operating mode.
Current Sensing
When either VIN or VOUT exceeds 2.25V, the IC enters nor- mal operating mode. Once the output voltage exceeds the Lossless current sensing converts the peak current signal input by 0.3V, the IC powers itself from VOUT instead of to a voltage to sum in with the internal slope compensa- VIN. At this point the internal circuitry has no dependency tion. This summed signal is compared to the error ampli- on the VIN input voltage, eliminating the requirement for fier output to provide a peak current control command for a large input capacitor. The input voltage can drop as low the PWM. The slope compensation in the IC is adaptive to as 0.5V without affecting circuit operation. The limiting the input voltage and output voltage. Therefore, the con- factor for the application becomes the availability of the verter provides the proper amount of slope compensation power source to supply sufficient energy to the output at to ensure stability, but not an excess to cause a loss of the low voltages and the maximum duty cycle, which is phase margin in the converter. clamped at 91% typical.
Error Amplifier LOW NOISE FIXED FREQUENCY OPERATION
The error amplifier is a transconductance amplifier, with its positive input internally connected to the 1.22V refer-
Shutdown
ence and its negative input connected to FB. A simple The part is shut down by pulling SHDN below 0.25V, and compensation network is placed from COMP to ground. activated by pulling the pin initially above 1V and main- Internal clamps limit the minimum and maximum error taining a high state down to 0.5V. Note that the SHDN pin amplifier output voltage for improved large-signal tran- can be driven above V sient response. During sleep (in Burst Mode operation), IN or VOUT as long as it is limited to less than the absolute maximum rating. the compensation pin is high impedance; however, clamps limit the voltage on the external compensation network,
Soft-Start
preventing the compensation capacitor from discharging to zero during the sleep time. The soft-start time is programmed with an external capac- itor to ground on the SS pin. An internal current source
Current Limit
charges it with a nominal 2.5µA. The voltage on the SS pin (in conjunction with the external resistor on the I The programmable current limit circuit sets the maxi- LIM pin) is used to control the peak current limit until the voltage mum peak current. This clamp level is programmed with on the capacitor exceeds 1.6V, at which point the external a resistor from ILIM to ground. In Burst Mode operation, resistor sets the peak current. In the event of a com- the current limit is automatically set to a nominal value manded shutdown or a thermal shutdown, the capacitor is of 0.6A peak for optimal efficiency. discharged automatically. Note that Burst Mode operation 150 is inhibited during the soft-start time. I LIM = R t(ms) = CSS(µF) • 320 where I is in amps and R is in kΩ. Rev. A For more information www.analog.com 9 Document Outline Description Typical Application Absolute Maximum Ratings Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Package Description Typical Application Related Parts Features Applications Typical Application Description Absolute Maximum Ratings Order Information Electrical Characteristics Package/Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Low Voltage Start-Up Low Noise Fixed Frequency Operation Burst Mode Operation Output Disconnect and Inrush Limiting Applications Information Component Selection Typical Application Package Description Revision History Typical Application Related Parts