Datasheet LT3999 (Analog Devices) - 9

LT3999
APPLICATIONS INFORMATION Soft-Start and Current Limit
VIN VIN The LT3999 soft-start ramps the peak switch current over RA RA2 UVLO a time programmed by either a capacitor or a resistor and OR UVLO capacitor on the ILIM/SS pin. R OVLO/DC B RA1 OVLO/DC When programming the soft-start time with a capacitor only RB the soft-start time is calculated with the following formula: 3999 F01 tSS (ms) = CSS • 80
Figure 1. Precision UVLO and OVLO Resistor Divider
where CSS is in µF. Resistors are chosen by first selecting RB. Then calculate The current limit defaults to the internally set value because RA with the following formula: there is no resistor on the pin.  V   TH  When programming the soft-start time with a resistor RA =RB –1    1.25V  and capacitor on the ILIM/SS pin the soft-start time is calculated with the following formula: where VTH is the VIN referred voltage at which the supply τ = RC is enabled (UVLO) or disabled (OVLO/DC). where 3τ will be 95% of the maximum current.
Transformer Design
The cycle-by-cycle current limit of the LT3999 is set with Table 3 lists recommended center tapped transformers a resistor on the ILIM/SS pin. Use the following formula for a variety of input voltage, output voltage and power to calculate the value of the resistor: combinations. These transformers will yield slightly high R output voltages so that they can accommodate an LDO ILIM (kΩ) = ILIM • 86.4 regulator on the output.
OVLO/DC and UVLO
If your application is not listed, the LTC Applications group The UVLO pin has a precision voltage threshold with is available to assist in the choice and/or the design of the hysteresis to enable the LT3999. The pin is typically con- transformer. In the design/selection of the transformer nected to V the following characteristics are critical and should be IN through a resistor divider; however, it can be directly connected to V considered: IN. The OVLO/DC pin has a precision voltage threshold with
Table 3. Recommended Center Tapped Transformers
hysteresis to disable the LT3999 switching operation. The
NOMINAL NOMINAL
pin is typically connected to V
INPUT OUTPUT OUTPUT
IN through a resistor divider.
VOLTAGE (V) VOLTAGE (V) POWER (W) PART NUMBER
The OVLO/DC pin can be directly connected to GND to 5 5 5 Coilcraft PA6383 disable the function. It is possible to use two separate 5 12 1 Coilcraft PA6381 resistor divider strings for OVLO/DC and UVLO pins or combine them together and use one resistor divider string 5 12 3 Cooper Bussmann CTX02-19064 to drive both pins. See Figure 1. 12 12 10 Coilcraft PA6384 24 24 20 Cooper Bussmann CTX02-19061 3999fa For more information www.linear.com/LT3999 9 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