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LM3526-H bảng dữ liệu(PDF) 8 Page - National Semiconductor (TI) |
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LM3526-H bảng dữ liệu(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Functional Description The LM3526-H and LM3526-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 connection to 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 The LM3526 is internally protected against excessive power dissipation by a two-stage thermal protection circuit. If the device temperature rises to approximately 145˚C, the ther- mal shutdown circuitry turns off any switch that is current lim- ited. Non-overloaded switches continue to function normally. If the die temperature rises above 150˚C, both switches are turned off and both fault flag outputs are activated. Hyster- esis ensures that a switch turned off by thermal shutdown will not be turned on again until the die temperature is re- duced to 135˚C. Shorted switches will continue to cycle off and on, due to the rising and falling die temperature, until the short is removed. UNDERVOLTAGE LOCKOUT UVLO prevents the MOSFET switch from turning on until in- put voltage exceeds 1.8V (typical). If input voltage drops below 1.8V (typical), UVLO shuts off the MOSFET switch and signals the fault flag. UVLO func- tions 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 limits the output current to approximately 1.0A typical. FAULT FLAG The fault flag is an open-drain output capable of sinking 10 mA load current to typically 100 mV above ground. A parasitic diode exists between the flag pins and V IN pin. Pulling the flag pins to voltages higher than V IN will forward bias this diode and will cause an increase in supply current. This diode will also clamp the voltage on the flag pins to a di- ode drop above V IN. The fault flag is active (pulled low) when any of the following conditions are present: under-voltage, current limit, or ther- mal shutdown. A 1ms (typ.) delay in reporting the fault condition prevents er- roneous fault flags and eliminates the need for an external RC delay network. 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 a 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 in-rush 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 maximum in-rush current is internally limited to 1.5A. TRANSIENT OVER-CURRENT DELAY High transient current is also generated when the switch is enabled and large values of capacitance at the output have to be rapidly charged. The in-rush 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 LM3526. The duration of the in-rush current depends on the size of the output capacitance and load current. Since this is not a valid fault condition, the LM3526 delays the gen- eration of the fault flag for 1 ms. If the condition persists due to other causes such as a short, a fault flag is generated af- tera1ms delay has elapsed. The LM3526’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. DS101097-24 FIGURE 1. www.national.com 8 |
Số phần tương tự - LM3526-H |
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Mô tả tương tự - LM3526-H |
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