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MAX5914AEMH+T Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX5914AEMH+T Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 20 page +48V Quad Hot-Swap Controllers For Power-Over-LAN ______________________________________________________________________________________ 11 Detailed Description The MAX5913A/MAX5914A quadruple hot-swap con- trollers provide Power-Over-MDI, also known as Power- Over-LAN systems (Figure 5). The MAX5913A/ MAX5914A enable control of four external n-channel MOSFET switches from a single VCC ranging from +35V to +72V, with timing control and current-limiting functions built in. The MAX5913A/MAX5914A features include undervolt- age lockout (UVLO), 100mA relay drivers, dual-level cur- rent sense, foldback current limit, programmable overcurrent time and auto-retry periods, internal charge pumps to drive external MOSFET and soft-start, port sta- tus output indicating power-OK (POK) or open-circuit conditions (Figure 6). Switch and Relay Control Inputs The MAX5913A/MAX5914A ON_ inputs turn on the corre- sponding MOSFET switch. Driving ON_ high turns on the switch if the corresponding RLYON is driven high, and VCC > VUVLO for more than 25.6ms. Driving RLYON_ high immediately turns on the corresponding relay, and activates the 25.6ms delay after which the corresponding ON_ input is active. Driving RLYON_ low immediately turns off the switch and activates a 3.2ms delay, after which the relay is turned off. These internal delays safely allow driving ON_ and RLYON_ simultaneously. The relay is turned on while the switch is off so that there is no volt- age across the relay contacts. The relay is turned off while the switch is off so that there is no current flowing when the relay contacts are opened (see Figure 3). Input Voltage and UVLO The MAX5913A/MAX5914A operate from a +35V to +72V supply voltage. VCC powers the MAX5913A/MAX5914A analog circuitry and is monitored continuously during startup and normal operation. The MAX5913A/MAX5914A keep all MOSFET switches and relay drivers securely off before VCC rises above VUVLO. The MAX5913A/ MAX5914A turn off all MOSFET switches and relay drivers after VCC falls below VUVLO - VUVLO,H. Startup When the turn-on condition is met (see the Input Voltage and UVLO and Switch and Relay Control Inputs sections), the MAX5913A/MAX5914A slowly turn on the external MOSFET switch by charging its gate using a constant current source, IGATE (10µA typ). The gate voltage slope is determined by the total gate capaci- tance CGATE connected to this node. Since the output voltage follows the gate voltage, thus the output rises with a slope determined by: If a capacitor load is connected to the output, the total current through the FET is: where CL is the load capacitance and IL is the current required by any load connected to the output during the startup phase. If the current through the FET reaches the programmed current-limit value: the internal current-limit circuitry activates and regu- lates this FET current to be a value, ILIM, that depends on VOUT (IFLBK) (Figure 8). See the Current Sensing and Regulation section. In this case, the maximum rate of change of the output is determined by: The formula shows the necessity for ILIM to be larger than IL in order to allow the output voltage to rise. The foldback function is active as long as the circuit is in overcurrent condition. Should the overcurrent condition persist for a period longer than the maximum time tO, the switch is latched off and GATE_ is discharged to ground with a 1mA pulldown current. If auto-retry is enabled, the switch turns on again after a waiting period, tOFF, which is determined by the pro- grammed duty cycle. After the startup, the internal charge pumps provide (VCC + 9V) typical gate overdrive to fully turn on the switch. When the switch is fully on (voltage drop across the switch is ≤ 1.5V), and the switch is not in current limit, the POK signal is asserted. Current Sensing and Regulation The MAX5913A/MAX5914A control port current with using two voltage comparators (dual-level detection) that sense the voltage drop across an external current- sense resistor. Connect CSP_ to VCC and connect a current-sense resistor between CSP_ and DRAIN_. Kelvin sensing should be used as shown in Figure 7. ∆ ∆ V t II C OUT LIM L L = − I V R MAX SC SENSE = II C C I GATE L GATE L =+ ∆ ∆ V t I C OUT GATE GATE = |
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