Datasheet LTC4365 (Analog Devices) - 11

link to page 11 link to page 11 link to page 10 link to page 10 LTC4365
applicaTions inForMaTion
Figure 5 shows the timing associated with the UV pin.
Procedure for Selecting UV/OV External Resistor Values
Once a UV fault propagates through the UV comparator The following 3-step procedure helps select the resistor (tFAULT), the FAULT output is asserted low and a 50mA values for the resistive divider of Figure 4. This procedure current sink discharges the GATE pin. As VOUT falls, the minimizes UV and OV offset errors caused by leakage GATE pin tracks VOUT. currents at the respective pins. 1. Choose maximum tolerable offset at the UV pin, VUV VUV + VUVHYST UV VOS(UV). Divide by the worst case leakage current at tFAULT tFAULT the UV pin, IUV (10nA). Set the sum of R1 + R2 equal to VOS(UV) divided by 10nA. Note that due to the FAULT presence of R3, the actual offset at UV will be slightly tGATE(FAST) tRECOVERY lower: GATE EXTERNAL N-CHANNEL MOSFET V TURNS OFF 4365 F05 R1+ R2 = OS(UV) I UV
Figure 5. UV Timing (OV < (VOV – VOVHYST), SHDN > 1.2V)
2. Select the desired VIN UV trip threshold, UVTH. Find the value of R3: Figure 6 shows the timing associated with the OV pin. Once an OV fault propagates through the OV comparator V  UV  R3 = OS(UV) • TH – 0.5V (tFAULT), the FAULT output is asserted low and a 50mA IUV  0.5V  current sink discharges the GATE pin. As V OUT falls, the GATE pin tracks VOUT. 3. Select the desired VIN OV trip threshold, OVTH. Find the values of R1 and R2: VOV VOV – VOVHYST OV  VOS(UV)  tFAULT tFAULT  I  +R3   R1 FAULT = UV • 0.5V OVTH tGATE(FAST) tRECOVERY V GATE EXTERNAL N-CHANNEL MOSFET TURNS OFF R2 = OS(UV) – R1 4365 F06 I UV
Figure 6. OV Timing (UV > (VUV + VUVHYST), SHDN > 1.2V)
The example of Figure 4 uses standard 1% resistor values. The following parameters were selected: When both the UV and OV faults are removed, the external MOSFET is not immediately turned on. The input supply VOS(UV) = 3mV must remain within the user selected power good window IUV = 10nA for at least 36ms (tRECOVERY) before the load is again connected to the supply. This recovery timeout period UVTH = 3.5V filters noise (including line noise) at the input supply and OVTH = 18V prevents chattering of power at the load. For applications that require faster turn-on after a fault, the LTC4365-1 provides a 1ms recovery timeout period. 4365fa For more information www.linear.com/LTC4365 11 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 Revision History Related Parts