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LM3525M-H bảng dữ liệu(PDF) 7 Page - National Semiconductor (TI) |
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LM3525M-H bảng dữ liệu(HTML) 7 Page - National Semiconductor (TI) |
7 / 10 page Functional Description The LM3525-H and LM3525-L are high side P-Channel switches with active-high and active-low enable inputs, re- spectively. Fault conditions turn off and inhibit turn-on of the output transistor and activate the open-drain error flag tran- sistor sinking current to the ground. INPUT AND OUTPUT IN (Input) is the power supply connection to the control cir- cuitry and the source of the output MOSFET. OUT (Output) is the drain of the output MOSFET. In a typical application circuit, current flows through the switch from IN to OUT towards the load. If V OUT is greater than VIN when the switch is enabled, cur- rent will flow from OUT to IN since the MOSFET is bidirec- tional. THERMAL SHUTDOWN LM3525 is internally protected against excessive power dis- sipation. In the event of a shorted output or heavy loads that could elevate the die temperature to above approximately 135˚C, the thermal shutdown circuit of the LM3525 will be activated and the power switch turned off. The switch is turned on after die temperature drops by 10˚C. This built-in temperature hysteresis prevents undesirable os- cillation of the thermal protection circuit and allows the de- vice to reset itself after it is cooled down. UNDERVOLTAGE LOCKOUT UVLO prevents the MOSFET switch from turning on until in- put voltage exceeds 1.9V. UVLO shuts off the MOSFET switch and signals the fault flag if input voltage drops below 1.8V. UVLO functions only when device is enabled. CURRENT LIMIT The current limit circuit is designed to protect the system supply, the MOSFET switches and the load from damage caused by excessive currents. The current limit threshold is set internally to allow a minimum of 500 mA through the MOSFET but limit the maximum current to 1.0A typical. FAULT FLAG The fault flag is an open-drain output capable of sinking 10 mA load current to typically 60 mV above ground. The fault flag is active (pulled low) when any of the following conditions are present: undervoltage, current limit, or ther- mal shutdown. A 1ms delay in reporting fault condition prevents erroneous fault flags and eliminates the need for external RC delay net- work. Application Information FILTERING The USB specification indicates that “no less than 120 µF tantalum capacitors” must be used on the output of each downstream port. This bulk capacitance provides the short-term transient current needed during a hot plug-in. Current surges caused by the input capacitance of the down stream device could generate undesirable EMI signals. Fer- rite beads in series with all power and ground lines are rec- ommended to eliminate or significantly reduce EMI. In selecting a ferrite bead, the DC resistance of the wire used must be kept to minimum to reduce the voltage drop. A 0.01 µF ceramic capacitor is recommended on each port directly between the V bus and ground pins to prevent EMI damage to other components during the hot - detachment. Adequate capacitance must be connected to the input of the device to limit the input voltage drop during a hot-plug event to less than 330 mV. For a few tens of µs, the host must sup- ply the inrush current to the peripheral, charging its bulk ca- pacitance to V bus. This current is initially supplied by the in- put capacitor. A 33 µF 16V tantalum capacitor is recommended. In choosing the capacitors, special attention must be paid to the Effective Series Resistance, ESR, of the capacitors to minimize the IR drop across the capacitor’s ESR. SOFT START To eliminate the upstream voltage droop caused by the high in-rush current drawn by the output capacitors, the inrush current is internally limited to 1.0A. TRANSIENT OVER-CURRENT DELAY In USB applications, it is required that output bulk capaci- tance is utilized to support hot-plug events. During hot-plug events, inrush currents may also cause the flag to go active. Since these conditions are not valid over-current faults, the USB controller must ignore the flag during these events. High transient current is also generated when switch is en- abled and large values of capacitance at the output have to be rapidly charged. The inrush currents created could ex- ceed the short circuit current limit threshold of the device forcing it into the current limit mode. The capacitor is charged with the maximum available short circuit current set by the LM3525. The duration of the inrush current depends on the size of the output capacitance and load current. Since this is not a valid fault condition, the LM3525 delays the gen- eration of the fault flag for 1 ms. If condition persists due to other causes such as a short, a fault flag is generated after a 1 ms delay has elapsed. The LM3525’s 1 ms delay in issuing the fault flag is adequate for most applications. If longer delays are required, an RC fil- ter as shown in Figure 1 may be used. PCB LAYOUT CONSIDERATIONS In order to meet the USB requirements for voltage drop, droop and EMI, each component used in this circuit must be evaluated for its contribution to the circuit performance as shown in Figure 2. The PCB layout rules and guidelines must be followed. • Place the switch as close to the USB connector as pos- sible. Keep all V bus traces as short as possible and use at least 50-mil, 1 ounce copper for all V bus traces. Solder plating the traces will reduce the trace resistance. DS101055-24 FIGURE 1. Transient Fitter www.national.com 7 |
Số phần tương tự - LM3525M-H |
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Mô tả tương tự - LM3525M-H |
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