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LM21212-2 Datenblatt(PDF) 10 Page - Texas Instruments |
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LM21212-2 Datenblatt(HTML) 10 Page - Texas Instruments |
10 / 31 page LM21212-2 SNVS715A – MARCH 2011 – REVISED MARCH 2013 www.ti.com OPERATION DESCRIPTION GENERAL The LM21212-2 switching regulator features all of the functions necessary to implement an efficient low voltage buck regulator using a minimum number of external components. This easy to use regulator features two integrated switches and is capable of supplying up to 12A of continuous output current. The regulator utilizes voltage mode control with trailing edge modulation to optimize stability and transient response over the entire output voltage range. The device can operate at high switching frequency allowing use of a small inductor while still achieving high efficiency. The precision internal voltage reference allows the output to be set as low as 0.6V. Fault protection features include: current limiting, thermal shutdown, over voltage protection, and shutdown capability. The device is available in the HTSSOP-20 package featuring an exposed pad to aid thermal dissipation. The LM21212-2 can be used in numerous applications to efficiently step-down from a 5V or 3.3V bus. PRECISION ENABLE The enable (EN) pin allows the output of the device to be enabled or disabled with an external control signal. This pin is a precision analog input that enables the device when the voltage exceeds 1.35V (typical). The EN pin has 110 mV of hysteresis and will disable the output when the enable voltage falls below 1.24V (typical). If the EN pin is not used, it can be left open, and will be pulled high by an internal 2 µA current source. Since the enable pin has a precise turn-on threshold it can be used along with an external resistor divider network from VIN to configure the device to turn-on at a precise input voltage. UVLO The LM21212-2 has a built-in under-voltage lockout protection circuit that keeps the device from switching until the input voltage reaches 2.7V (typical). The UVLO threshold has 200 mV of hysteresis that keeps the device from responding to power-on glitches during start up. If desired the turn-on point of the supply can be changed by using the precision enable pin and a resistor divider network connected to VIN as shown in Figure 27 in the design guide. CURRENT LIMIT The LM21212-2 has current limit protection to avoid dangerous current levels on the power FETs and inductor. A current limit condition is met when the current through the high side FET exceeds the rising current limit level (ICLR). The control circuitry will respond to this event by turning off the high side FET and turning on the low side FET. This forces a negative voltage on the inductor, thereby causing the inductor current to decrease. The high side FET will not conduct again until the lower current limit level (ICLF) is sensed on the low side FET. At this point, the device will resume normal switching. A current limit condition will cause the internal soft-start voltage to ramp downward. After the internal soft-start ramps below the Feedback (FB) pin voltage, (nominally 0.6 V), FB will begin to ramp downward, as well. This voltage foldback will limit the power consumption in the device, thereby protecting the device from continuously supplying power to the load under a condition that does not fall within the device SOA. After the current limit condition is cleared, the internal soft-start voltage will ramp up again. Figure 23 shows current limit behavior with VSS, VFB, VOUT and VSW. SHORT-CIRCUIT PROTECTION In the unfortunate event that the output is shorted with a low impedance to ground, the LM21212-2 will limit the current into the short by resetting the device. A short-circuit condition is sensed by a current-limit condition coinciding with a voltage on the FB pin that is lower than 100 mV. When this condition occurs, the device will begin its reset sequence, turning off both power FETs and discharging the soft-start capacitor after tRESETSS (nominally 110 µs). The device will then attempt to restart. If the short-circuit condition still exists, it will reset again, and repeat until the short-circuit is cleared. The reset prevents excess current flowing through the FETs in a highly inefficient manner, potentially causing thermal damage to the device or the bus supply. 10 Submit Documentation Feedback Copyright © 2011–2013, Texas Instruments Incorporated Product Folder Links: LM21212-2 |
Ähnliche Teilenummer - LM21212-2_15 |
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Ähnliche Beschreibung - LM21212-2_15 |
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