SLVS503F November 2003 – February 2020 TPS2490 , TPS2491
PRODUCTION DATA.
The TPS249x can be used with applications that require constant turn-on currents. The current is controlled by a single capacitor from the GATE terminal to ground with a series resistor. Q1 appears to operate as a source follower (following the gate voltage) in this implementation. Choose a time to charge, Δt, based on the output capacitor, input voltage VI, and desired charge current, ICHARGE. Select ICHARGE to be less than PLIM ÷ VVCC if the power limit feature is kept. See TPS2490/91 Design-in Calculator (SLVC033) for a calculation tool.
To select the gate capacitance:
where
This equation assumes that the MOSFET CGD is the controlling element as the gate and output voltage rise. CGD is non-linear with applied VDG. An averaged estimate may be made using the MOSFET VGS vs QG curve. Divide the charge accumulated during the plateau region by the plateau VGS to get CRS.
Because neither power nor current-limit faults are invoked during turnon, CTIMER can be chosen for fast transient turnoff response using the Q1 SOA curve. Choose the single pulse time conservatively from the Q1 SOA curve using maximum operating voltage and maximum trip current. A series resistor of about 1 kΩ should be used in conjunction with CG.