advertisement Versatile Voltage Monitors Simplify Detection of Overvoltage and Undervoltage Faults – Design Note 408 Scott Jackson Introduction Many modern electronic systems have strict power supply operating ranges—requiring accurate monitoring of each supply. Some systems must know that all supplies are present and stable before startup and some must know if the supplies deviate from safe operating conditions. to externally disable both outputs. The LTC2912 has a third option with latching capability and a non-inverted overvoltage output. Each part has an internal 6.5V shunt regulator allowing the device to be used in a system with any supply level. The LTC®2912, LTC2913, and LTC2914 supervisors respectively monitor single, dual, and quad power supplies for undervoltage and overvoltage with tight 1.5% threshold accuracy over temperature. All monitors in the multiple-monitor devices share a common undervoltage output and a common overvoltage output with a timeout period that is adjustable or disabled. Each monitor has input glitch rejection to ensure reliable reset operation without false or noisy triggering. Basic Operation Figure 1 shows a typical application for the LTC2914. Each monitored input is compared to a 0.5V threshold. Any channel can be configured to monitor both undervoltage and overvoltage conditions using a 3-resistor divider. When monitoring a positive voltage, the VH input of the channel is connected to the high-side tap of the resistive divider and triggers an undervoltage condition while the VL input is connected to the low-side tap of the resistive divider and triggers an overvoltage condition. When an Each part has at least two options: one with capability to latch the overvoltage output and one with capability , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. P0WER SUPPLIES 5V 3.3V 2.5V 1.8V 0.1µF 44.2k 1k 4.53k VCC VH1 SEL VL1 VH2 OV 27.4k VL2 4.53k SYSTEM LTC2914-1 1k 19.6k VH3 1k UV REF LATCH 12.4k VL3 VH4 4.53k 1k VL4 GND 4.53k TMR CTMR 22nF Figure 1. Quad UV/OV Supply Monitor 02/07/408 DN408 F01 TIMEOUT = 200ms undervoltage condition is detected, the ⎯U⎯V output asserts low. Once all undervoltage conditions clear, the ⎯U⎯V output remains asserted until a timeout period has elapsed. This timeout period is set by a capacitor between the TMR and GND pins. The timeout period can be disabled by tying the TMR pin to VCC. Figure 2 shows the timeout period versus TMR capacitance. The ⎯O⎯V output behaves in a similar manner. On parts with latching capability, the ⎯O⎯V output latches when asserted until cleared by the ⎯L⎯A⎯T⎯C⎯H pin. Holding the ⎯L⎯A⎯T⎯C⎯H pin high bypasses the overvoltage latch. Minimum Fault Length Monitor The LTC2912-3 can be used to detect an undervoltage condition with a minimum duration by using the VL input UV/OV TIMEOUT PERIOD, tUOTO (ms) 10000 1000 100 10 1 0.1 1 10 100 TMR PIN CAPACITANCE, CTMR (nF) 1000 DN408 F02 Figure 2. Timeout Period vs Capacitance VMONITOR 12V 9.6V FAULT and the non-inverted OV output. For example, an automobile system may need to monitor the 12V power supply during a power up condition. During the initial cranking of the automobile, the power supply droops. If the supply droop exists for an extended period, the system may need to disconnect various circuits from the supply for protection or disconnect circuits to reduce the load. This is accomplished by the circuit shown in Figure 3. The timeout function of the LTC2912 typically starts when a fault clears. However, because the VL input is used in this case to monitor an undervoltage instead of an overvoltage, the timeout function occurs at the beginning of an undervoltage condition and the OV output remains high until the period has elapsed. If the fault still exists ⎯ U ⎯ L⎯ T⎯ ) when this timeout period elapses, the OV output ( F⎯ A pulls low until the fault clears. Choosing a 0.47µF timing capacitor produces a 4.1s timeout delay. Therefore, any supply droop lower than 9.6V and longer than 4.1s asserts FAULT. Figure 4 shows the resulting waveform of a supply fault condition of 9V for 5 seconds. Conclusion The LTC2912, LTC2913, and LTC2914 simplify power supply monitoring of any voltage level by offering superior performance and flexibility. Only a few resistors are needed to configure monitoring of multiple voltages for both undervoltage and overvoltage conditions. The LTC2914 offers these features in a 16-lead SSOP and 16-lead (5mm × 3mm) DFN package, the LTC2913 in a 10-lead MSOP and 10-lead (3mm × 3mm) DFN package, and the LTC2912 in a tiny 8-lead ThinSOTTM and 8-lead (3mm × 2mm) DFN package. VCC 5V CBYP 0.1µF RB 10k VCC LATCH ROV 10k VMONITOR 12V 9V UV VH LTC2912-3 OV VL RA 549k TMR CTMR 0.47µF GND FAULT 4.1s 5V 0V FAULT TMR DN408 F03 TFAULT = 4.1s Figure 3. Fault Detection Circuit for a 4.1s Undervoltage Condition 1s/DIV DN408 F04 Figure 4. Fault Detection Waveform of a 4.1s Undervoltage Condition Data Sheet Download For applications help, call (408) 432-1900, Ext. 2452 www.linear.com Linear Technology Corporation dn408f LT 0207 305K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●