Datasheet LT8304, LT8304-1 (Analog Devices) - 16

LT8304/LT8304-1
APPLICATIONS INFORMATION Undervoltage Lockout (UVLO)
during light load conditions to ensure accurate output volt- A resistive divider from V age information. The minimum energy delivery creates a IN to the EN/UVLO pin imple- ments undervoltage lockout (UVLO). The EN/UVLO enable minimum load requirement, which can be approximately falling threshold is set at 1.214V with 14mV hysteresis. In estimated as: addition, the EN/UVLO pin sinks 2.5µA when the voltage LP 2 RI •I • f SW(MIN) MIN on the pin is below 1.214V. This current provides user ILOAD(MIN) = programmable hysteresis based on the value of R1. The 2• VOUT programmable UVLO thresholds are: LPRI = Transformer primary inductance 1.228V • R ( 1+R2) I V = +2.5µA •R1 SW(MIN) = Minimum switch current limit = 0.53A (MAX) IN(UVLO+) R2 fMIN = Minimum switching frequency = 14kHz (MAX) 1.214V • R ( 1+R2) V = The LT8304 typically needs less than 0.5% of its full IN(UVLO– ) R2 output power as minimum load. Alternatively, a Zener diode with its breakdown of 10% higher than the output Figure 7 shows the implementation of external shutdown voltage can serve as a minimum load if pre-loading is not control while still using the UVLO function. The NMOS acceptable. For a 5V output, use a 5.6V Zener with cathode grounds the EN/UVLO pin when turned on, and puts the connected to the output. The LT8304-1 requires slightly LT8304 in shutdown with quiescent current less than 3µA. higher minimum load, typically 2% of full load. VIN
Output Short Protection
R1 When the output is heavily overloaded or shorted to ground, EN/UVLO the reflected SW pin waveform rings longer than the in- RUN/STOP ternal blanking time. After the 350ns minimum switch-off LT8304 R2 CONTROL time, the excessive ringing falsely triggers the boundary (OPTIONAL) mode detector and turns the power switch back on again GND 8304 F07 before the secondary current falls to zero. Under this condition, the LT8304 runs into continuous conduction
Figure 7. Undervoltage Lockout (UVLO)
mode at 350kHz (TYP) maximum switching frequency. If the sampled RREF voltage is still less than 0.6V after 11ms
Minimum Load Requirement
(typ) soft-start timer, the LT8304 initiates a new soft-start cycle. If the sampled RREF voltage is larger than 0.6V after The LT8304 samples the isolated output voltage from 11ms, the switch current may run away and exceed the the primary-side flyback pulse waveform. The flyback 2.4A maximum current limit. Once the switch current hits pulse occurs once the primary switch turns off and the 3.6A over current limit, the LT8304 also initiates a new secondary winding conducts current. In order to sample soft-start cycle. Under either condition, the new soft-start the output voltage, the LT8304 has to turn on and off for a cycle throttles back both the switch current limit and switch minimum amount of time and with a minimum frequency. frequency. The output short-circuit protection prevents the The LT8304 delivers a minimum amount of energy even switch current from running away and limits the average output diode current. 8304fa 16 For more information www.linear.com/LT8304 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts 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 Revision History Typical Application Related Parts