Datasheet LT8316 (Analog Devices) - 19

LT8316
APPLICATIONS INFORMATION
This is a simplified equation; the actual ripple will depend
Output Current Regulation and Soft-Start
on load current, duty cycle and capacitor ESR. Using duty cycle information and the current limit set by The LT8316 samples the output voltage only when the VC pin, the LT8316 estimates the output current and switching. As a result, when it is operating at its minimum regulates it to a setpoint determined by the voltage on the frequency, a load transient may discharge the output IREG/SS pin. The output current is regulated according capacitor before the device can respond. The output to the equation: capacitor must be sufficient to prevent the load from brown-out in this event. Additional bulk capacitance may N I PS • VIREG/SS OUT = be desirable for this purpose. 25 • RSNS
Operation Under Light Output Loads
where The LT8316 detects the output voltage from the flyback VIREG/SS = Voltage on IREG/SS pin. pulse appearing on the tertiary winding, which requires A trimmed 10μA current flows out of the IREG/SS pin, delivering power to the output. Thus, the LT8316 deliv- so that a resistor tied from this pin to GND programs the ers a minimum amount of energy even during light load output current according to the equation: conditions to ensure accurate output voltage information. The minimum operating frequency at minimum load is 2.5M R Ω • IOUT • RSNS IREG/SS = approximately 3.5kHz. The minimum delivery of energy NPS creates a minimum load requirement on the output of Example: For an application with R approximately 1% of the maximum load power. SNS = 120mΩ, NPS = 10, and a desired regulated output current IOUT = 2A, an A Zener diode sufficiently rated to handle the minimum IREG/SS resistor is selected RIREG/SS = 60.4kΩ. load power can be used to provide a minimum load with- Circuit parasitics, especially transformer capacitance, will out decreasing efficiency in normal operation. In selecting influence the accuracy of output current regulation due to a Zener diode for this purpose, the Zener voltage should energy delivery to the parasitics. Although this effect is be high enough that the diode does not become the load usually small, some iteration may be necessary if accu- path during transient conditions but the voltage must still racy better than 5% is required. In this case, R be low enough that the MOSFET and output voltage rat- IREG/SS can be implemented with a rheostat and adjusted until ings are not exceeded when the Zener functions as the the desired output current is realized, and then replaced minimum load. with a fixed-value resistor for production. When the
Standby Mode Operation
rheostat is present, a small bypass capacitor is helpful to attenuate switching interference pickup by the rheostat. For extremely low no-load power dissipation, the LT8316 Additionally, an RC snubber placed across the secondary features a standby mode which is enabled by tying the rectifier can improve current regulation accuracy. SMODE pin to INTVCC. When the load current has dropped to zero, the LT8316 reduces its minimum switching fre- Soft-start functionality can also be implemented by con- quency by a factor of 16 from 3.5kHz to 220Hz. necting a capacitor from the IREG/SS pin to GND. The 10μA current will act to charge the external soft-start This reduces the minimum load current by a factor of 16, capacitor. At startup, the regulated output current wil rise at the cost of slower transient response. Because the monotonically until reaching voltage regulation. The soft- output voltage is sampled only once every 4.6ms, the start capacitor then charges entirely and the output current LT8316 will be unable to respond to load steps for up to regulation loop will not interfere with voltage regulation. this period. Rev. 0 For more information www.analog.com 19 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts