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MAX5947CESA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX5947CESA Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 15 page Positive High-Voltage, Hot-Swap Controllers ______________________________________________________________________________________ 11 TIMER TIMER provides a method for programming the maxi- mum time the device is allowed to operate in current limit. When the current-limit circuitry is not active, TIMER is pulled to GND by a 3µA current source. After the current-limit circuit becomes active, an 80µA pullup current source is connected to TIMER, and the voltage rises with a slope equal to 77µA/CTIMER, as long as the current-limit circuit remains active. Once the desired maximum current-limit time is chosen, the capacitor value is: C(nF) = 65 x t(ms) or TLIMIT = (CTIMER/80µA) x 1.233V When the current-limit circuit turns off, TIMER is dis- charged to GND by the 3µA current source. Whenever TIMER reaches 1.233V, the internal fault latch is set. GATE is immediately pulled to GND and TIMER is pulled back to GND by the 3µA current source. When TIMER falls below 0.5V, ON is pulsed low to reset the internal fault latch. The waveform in Figure 9 shows how the output latches off following a short circuit. The drop across the sense resistor is held at 12mV as the timer ramps up. Since the output did not rise, FB remains below 0.5V and the circuit latches off. For Figure 9, CT = 100nF. Undervoltage and Overvoltage Detection ON can be used to detect an undervoltage condition at the power-supply input. ON is internally connected to an analog comparator with 80mV of hysteresis. If ON falls below its threshold voltage (1.233V), GATE is pulled low and is held low until ON is high again. Figure 10 shows an overvoltage detection circuit. When the input voltage exceeds the Zener diode’s breakdown voltage, D1 turns on and starts to pull TIMER high. After TIMER is pulled higher than 1.233V, the fault latch is set and GATE is pulled to GND immediately, turning off transistor Q1 (see Figure 11). Operation is restored either by interrupting power or by pulsing ON low. Power-Good Detection The MAX5933_/MAX5947_ include a comparator for mon- itoring the output voltage. The noninverting input (FB) is compared against an internal 1.233V precision reference and exhibits 80mV hysteresis. The comparator’s output (PWRGD) is open drain and capable of operating from a pullup as high as 80V. The PWRGD is similar to PWRGD with an opposite polarity (active low) output. The PWRGD (PWRGD) can be used to directly enable/disable a power module with an active-high enable input. Figure 12 shows how to use PWRGD to control an active-low enable-input power module. Signal inversion is accomplished by transistor Q2 and R7. Supply Transient Protection The MAX5933_/MAX5947_ are 100% tested and guar- anteed to be safe from damage with supply voltages up to 80V. However, spikes above 85V may damage the device. During a short-circuit condition, the large change in currents flowing through the power-supply traces can cause inductive voltage spikes which could exceed 85V. To minimize the spikes, the power-trace parasitic inductance should be minimized by using wider traces or heavier trace plating and a 0.1µF bypass capacitor placed between VCC and GND. A transient voltage suppressor (TVS) at the input can also prevent damage from voltage surges. RESPONSE TIME TO OVERCURRENT VCC - VSENSE (mV) 200 100 2 4 6 8 10 12 14 0 0300 Figure 8. Response Time to Overcurrent SHORT-CIRCUIT WAVEFORMS ISENSE 1A/div TIMER 1V/div OUTPUT 50V/div GATE 50V/div 10ms/div Figure 9. Short-Circuit Waveforms |
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