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MAX4766CETA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX4766CETA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page 0.075A to 1.5A, Programmable Current-Limit Switch _______________________________________________________________________________________ 9 Autoretry (MAX4766A) When the forward current-limit threshold is exceeded, the tBLANK timer begins counting (Figure 3). The timer resets if the overcurrent condition disappears before tBLANK has elapsed. A retry time delay (tRETRY) starts immediately after tBLANK has elapsed and during that time, the switch turns off. At the end of tRETRY, the switch turns on again. If the fault still exists, the cycle repeats. If the fault has been removed, the switch stays on. The autoretry feature saves system power in the case of an overcurrent or short-circuit condition. During tBLANK, when the switch is on, the supply current is at the current limit. During tRETRY, when the switch is off, the current through the switch is zero. Instead of observing the full load current, the switch sees the equivalent load current times duty cycle or: With a typical tBLANK = 15ms and typical tRETRY = 150ms, the duty cycle is 10%, which results in a 90% power savings over the switch being on the entire time. Continuous Current Limit In the continuous current-limit version of the MAX4766, the output stays at the current limit until the output load reduces. In this case, FLAG asserts as soon as there is current limit without waiting for blanking time and deasserts as soon as a short-circuit condition is removed. In this current mode, if the thermal power rat- ing of the package is exceeded, the device will go into thermal shutdown and will not operate until the die tem- perature has cooled. Latch-Off When the forward current-limit threshold is exceeded, tBLANK timer begins counting. The timer resets if the overcurrent condition disappears before tBLANK has elapsed. The switch turns off and FLAG asserts if the overcurrent condition continues beyond the blanking time. Reset the switch by either toggling ON (Figure 2a) or cycling the input voltage (Figure 2b). Reverse-Current Protection The MAX4766 features a reverse-current protection cir- cuitry to limit the current back flow to 1µA when the out- put voltage exceeds the input voltage by a 100mV. During this period, the switch turns off and FLAG asserts (MAX4766B/C) without waiting for tBLANK time to elapse. If at this point the output voltage drops below input +100mV, the switch turns on and FLAG deasserts (MAX4766B/C). Thermal Shutdown A thermal-shutdown circuitry protects the devices from overheating. The switch turns off and FLAG goes (MAX4766B/C) low immediately (no fault blanking) when the junction temperature exceeds +150°C. The MAX4766B (latch-off) does not turn on again until ON is cycled from high to low to high. The MAX4766A (autoretry) and the MAX4766C (continuous) turn the switch on again after the device temperature drops approximately +15°C. F FL LA AG G Indicator FLAG is an open-drain fault indicator output and requires an external pullup resistor to a DC supply (usually IN). FLAG goes low when any of the following conditions occur: • The device is in current limit for more than the fault- blanking period in the case of MAX4766B. • The device is in current limit in the case of MAX4766C (no blanking time). • The switch is in autoretry mode. • The OUT voltage is above the IN voltage by more than 100mV. • The die temperature exceeds the thermal-shutdown temperature limit of the +15°C. II SUPPLY LOAD t tt BLANK BLANK RETRY = + ⎛ ⎝⎜ ⎞ ⎠⎟ Figure 3. Autoretry Fault-Blanking Diagram LOAD CURRENT SWITCH STATUS tBLANK OFF ON tRETRY CURRENT LIMIT SHORT REMOVED tBLANK tBLANK |
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