SLVS004I April 1979 – August 2016 TL317
PRODUCTION DATA.
NOTE
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The flexibility of the TL317 allows it to be configured to take on many different functions in DC power applications.
VO is calculated as shown in Equation 1. IADJ is typically 50 µA and negligible in most applications.
Power dissipation for linear regulators is calculated as shown in Equation 2. IADJ is typically 50 µA and negligible in most applications, so a typical way to calculate power dissipation for linear regulators is simplified to Equation 3.
VO is calculated as shown in Equation 4, where Vref equals the difference between OUTPUT and ADJUSTMENT voltages (approximately 1.25 V).
The use of the TL317 in this configuration limits the output current to Ilimit shown in Figure 8.
This application keeps a constant voltage across the second TL317 in the circuit.
The capacitor C1, in combination with the PNP transistor, helps the circuit to slowly start supplying voltage. In the beginning, the capacitor is not charged. Therefore, output voltage starts at VC1+ VBE + 1.25 V = 0 V + 0.65 V + 1.25 V = 1.9 V. As the capacitor voltage rises, VOUT also rises at the same rate. When the output voltage reaches the value determined by R1 and R2, the PNP is turned off.
The current limit operation mode can be used to trickle charge a battery at a fixed current. ICHG = 1.25 V / 24 Ω. VI must be greater than VBAT + 4.25 V (1.25 V [VREF] + 3 V [headroom]).
Power dissipation through resistor R1 is calculated as shown in Equation 5, so a resistor with the appropriate power rating must be chosen for this application.
As the charge current increases, the voltage at the bottom resistor increases until the NPN starts sinking current from the adjustment pin. The voltage at the adjustment pin drops, and consequently the output voltage decreases until the NPN stops conducting.
The NPNs at the top of the schematic allow higher currents at VOUT than the LM317 can provide, while still keeping the output voltage at levels determined by the adjustment pin resistor divider of the LM317.