Datasheet LT3001 (Analog Devices) - 6

link to page 6 LT3001
APPLICATIONS INFORMATION Output Voltage
must also be followed in selecting primary-side magnetiz- The R ing inductance: FB resistor is the only external resistor used to pro- gram the output voltage. t L ON(MIN) • VIN(MAX) The output voltage is set by: PRI ≥ I SW(MIN) ⎛ R ⎞ V FB ⎜ ⎟− V tON(MIN) = Minimum switch-on time = 170ns OUT = 100µA • F ⎝NPS ⎠
Undervoltage Lockout (UVLO)
VF = Output diode forward voltage A resistive divider from V N IN to the EN/UVLO pin imple- PS = Transformer effective primary-to-secondary ments undervoltage lockout (UVLO). The EN/UVLO pin turns ratio falling threshold is set at 1.228V with 14mV hysteresis.
Primary Inductance Requirement
In addition, the EN/UVLO pin sinks 2.5µA when the volt- age at the pin is below 1.228V. This current provides user The LT3001 obtains output voltage information from the programmable hysteresis based on the value of R1. The reflected output voltage on the SW pin. The conduction of programmable UVLO thresholds are: secondary current reflects the output voltage on the pri- mary SW pin. The sample-and-hold error amplifier needs 1.242V •(R1 V +R2) IN(UVLO+) = + 2.5µA •R1 a minimum 450ns to settle and sample the reflected out- R2 put voltage. In order to ensure proper sampling, the sec- 1.228V •(R1+R2) ondary winding needs to conduct current for a minimum VIN(UVLO−) = R2 of 450ns. The following equation gives the minimum value for primary-side magnetizing inductance: Figure 1 shows the implementation of external shutdown control while still using the UVLO function. The NMOS t ( ) L OFF(MIN) • N PS • VOUT + VF grounds the EN/UVLO pin when turned on, and puts the PRI ≥ I LT3001 in shutdown with quiescent current less than 2µA. SW(MIN) tOFF(MIN) = Minimum switch-off time = 450ns VIN ISW(MIN) = Minimum switch current limit = 290mA (typ) R1 In addition to the primary inductance requirement for EN/UVLO the minimum switch-off time, the LT3001 has minimum RUN/STOP switch-on time that prevents the chip from turning on LT3001 R2 CONTROL (OPTIONAL) the power switch shorter than approximately 170ns. This minimum switch-on time is mainly for leading-edge GND blanking the initial switch turn-on current spike. If the 3001 F01 inductor current exceeds the desired current limit during
Figure 1. Undervoltage Lockout (UVLO)
that time, oscillation may occur at the output as the cur- rent control loop will lose its ability to regulate. Therefore, the following equation relating to maximum input voltage Rev. 0 6 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Operation Applications Information Package Description