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LTC4212CMS Datasheet(PDF) 10 Page - Linear Technology |
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LTC4212CMS Datasheet(HTML) 10 Page - Linear Technology |
10 / 24 page 10 LTC4212 4212f Figure 1. ON Pin Sets the Undervoltage Lockout Voltage Externally 3.3V R1 10k R2 10k ON PIN (a) VCC = 3.3V 5V R1 20k R2 10k ON PIN (b) VCC = 5V 12V R1 61.9k R2 10k ON PIN (c) VCC = 12V 4212 F01 Hot Circuit Insertion When circuit boards are inserted into or removed from live backplanes, the supply bypass capacitors can draw huge transient currents from the backplane power bus as they charge. The transient current can cause permanent dam- age to the connector pins as well as cause glitches on the system supply, causing other boards in the system to reset. The LTC4212 is designed to turn a printed circuit board’s supply voltages ON and OFF in a controlled manner, allow- ing the circuit board to be safely inserted or removed from a live backplane. Output Voltage Monitor Unlike other LTC Hot Swap controller products, the LTC4212 does not have an FB pin and monitors onboard DC/DC converters via an external power supply monitor IC such as the LTC1326-2.5 or the LTC1727. This allows several DC/DC converters to be monitored at the same time. The LTC4212’s PGI or power good input pin is used to monitor the RST or comparator outputs of the monitor IC and it can also be tied directly to the PGOOD pin of a DC/DC converter. Undervoltage Lockout The LTC4212’s internal power-on reset circuit initializes the start-up procedure and ensures the IC is in the proper state if the input supply voltage exceeds 2.34V. If the supply voltage falls below 2.23V, the LTC4212 is in undervoltage lockout (UVLO) mode, and the GATE pin is pulled low. Since the UVLO circuitry uses hysteresis, the LTC4212 restarts after the supply voltage rises above 2.34V and the ON pin goes high. In addition, users can utilize the ON comparator (COMP1) or the FAULT comparator (COMP6) to effectively set up a higher undervoltage lockout level. Figure 1 shows the external resistive divider for the ON pin to adjust the system’s undervoltage lockout voltage. The system will enter the plug-in cycle after the ON pin rises above 1.316V. The resistive divider sets the circuit to turn on when VCC reaches around 79% of its final value. If a different turn on VCC voltage is desired change the resistive divider ratio accordingly. The FAULT comparator can also be used to set a higher undervoltage lockout voltage. If the FAULT comparator is used for this purpose, the system will wait for the input voltage to increase above the level set by the user before starting the second timing cycle. Also, if the input voltage drops below the set level in normal operating mode, the electronic circuit breaker (ECB) trips and the user must cycle the ON pin or VCC to restart the system. OPERATIO System Timing System timing for the LTC4212 is generated by the TIMER circuitry (see the Block Diagram). If the LTC4212’s inter- nal timing circuit is off, an internal N-channel FET connects the TIMER pin to GND. If the timing circuit is enabled, an internal 2 µA current source is then connected to the TIMER pin to charge CTIMER at a rate given by Equation 1: C Charge -Up Rate TIMER = µ 2A C TIMER (1) When the TIMER pin voltage reaches COMP4’s threshold of 1.236V, the TIMER pin is reset to GND. Equation 2 gives an expression for the timer period: tV C A TIMER TIMER = µ 1 236 2 .• (2) As a design aid, the LTC4212’s timer period as a function of the CTIMER using standard values from 3.3nF to 0.33µF is shown in Table 1. The CTIMER value is vital to ensure a proper start-up and reliable operation. This timing period should not be exces- sive as an output short can occur at start-up causing the external MOSFET to overheat. A good starting point is to |
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