MIC2536-2YMM

MIC2536 Micrel MIC2536 Dual USB Power Distribution Switch Final Information General Description Features The MIC2536 is a cost-effective high-side power switch, with two independently controlled channels, optimized for buspowered Universal Serial Bus (USB) applications. Few external components are necessary to satisfy USB requirements. Each switch channel of the MIC2536 will supply up to 100mA as required for USB bus-powered downstream devices. Fault current is limited to typically 275mA by fast-acting currentlimit circuitry which minimizes voltage droop on the upstream port during fault conditions. A flag output with transient filter indicates fault conditions to the local USB controller but will ignore short flag signals resulting from inrush current during hot plug-in events. Soft start eliminates the momentary voltage droop on other ports that may occur when the switch is enabled in buspowered applications. Additional features include thermal shutdown to prevent catastrophic switch failure from highcurrent loads and 3.3V and 5V logic compatible enable inputs. The MIC2536 is available in active-high and active-low versions in 8-lead SOP and MSOP. • • • • • • • • • • • • • • Compliant to USB specifications 2.7V to 5.5V operating range 150mA minimum continuous load current per channel 400mΩ typical on-resistance Fast-acting short circuit protection with thermal shutdown Integrated filter eliminates false overcurrent flag assertions Individual open-drain fault flag pins with transient filter 3V/5V-compatible enable inputs Active-high (-1) and active-low (-2) versions Reverse-current blocking in off mode (no “body diode”) Soft-start circuit 100µA maximum on-state supply current VIN) when the output is disabled. In this situation, the MIC2536 prevents reverse current flow. Thermal Shutdown Each output MOSFET has its own thermal sensor. If either or both channels reach 135°C, affected channel(s) will be shut down and flag(s) asserted. 10°C of hysteresis prevents the switches from turning on until the die temperature drops to 125°C. Overtemperature detection functions only when at least one switch is enabled. The MIC2536 will automatically reset its output when the die temperature cools to approximately 125°C. The MIC2536 output and FLG signal will continue to cycle on and off until the device is disabled or the fault is removed. Depending on PCB layout, package, ambient temperature, etc., it may take several hundred milliseconds from the occurrence of the fault to the output MOSFET being shut off. Delay to reach thermal shutdown will be shortest with a dead short on the output. Current-Limit Induced Thermal Shutdown Internal circuitry increases the output MOSFET on-resistance until the series combination of the MOSFET on-resistance and the load impedance limits output current to approximately 275mA. The resulting increase in power dissipation may cause the shorted channel to go into thermal shutdown. In addition, even though individual channels are thermally isolated, it is possible they may shut down when an adjacent channel is shorted. When this is undesirable, thermal shutdown can be avoided by externally responding to the fault and disabling the current-limited channel before the shutdown temperature is reached. The delay between the flag indication of a current-limit fault and thermal shutdown will vary with ambient temperature, board layout, and load impedance, but is typically several seconds. The USB controller must therefore recognize a fault and disable the appropriate channel within this time. Power Dissipation Power dissipation depends on several factors such as the load, PCB layout, ambient temperature and package type. March 2000 Total power dissipation of the device will be the summation of PD for both channels. To relate this to junction temperature, the following equation can be used: Tj = PD × θJA + TA where: Tj = junction temperature TA = ambient temperature θJA = is the thermal resistance of the package Current Sensing and Limiting The current-limit threshold is preset internally. The preset level prevents damage to the output MOSFET and external load but allows a minimum current of 150mA through the output MOSFET of each channel. The current-limit circuit senses a portion of the output FET switch current. The current sense resistor shown in the block diagram is virtual and has no voltage drop. The reaction to an overcurrent condition varies with the following three scenarios: Switch Enabled into Short Circuit If a switch is enabled into a heavy load or short circuit, the switch immediately goes into a constant-current mode, reducing the output voltage. The FLG is asserted indicating an overcurrent condition. Short Circuit Applied to Output When a heavy load or short circuit is applied to an enabled switch, a large transient current may flow until the currentlimit circuitry responds. Once this occurs, the device limits current to less than the maximum short-circuit current-limit specification. Current-Limit Response Ramped Load The MIC2536 current-limit profile exhibits a small foldback effect of approximately 100mA. Once this current-limit threshold is exceeded the device enters constant-current mode. This constant current is specified as the short-circuit currentlimit in the “Electrical Characteristics” table. It is important to note that the MIC2536 will deliver load current up to the current-limit threshold before entering current-limited operation. Fault Flag FLGx is an open-drain N-channel MOSFET output. Fault flags are active (low) for current-limit or thermal shutdown. In the case where an overcurrent condition occurs, FLG will be asserted only after the flag response delay time, tD has elapsed. This ensures that FLG is asserted only upon valid overcurrent conditions and that erroneous error reporting is eliminated. False overcurrent conditions can occur during hot-plug events when a highly capacitive load is connected and causes a high transient inrush current that exceeds the current-limit threshold. The flag response delay time is typically 12ms. 9 MIC2536 MIC2536 Micrel Enable Input EN must be driven logic high or logic low for a clearly defined input. Floating the input may cause unpredictable operation. EN should not be allowed to go negative with respect to GND. Printed Circuit Board Hot-Plug The MIC2536 is an ideal inrush current-limiter for hot-plug applications. Due to the integrated charge pump, the MIC2536 presents a high impedance when off and slowly becomes a low impedance as it turns on. This “soft-start” feature effectively isolates power supplies from highly capacitive loads by reducing inrush current. Figure 2 shows how the MIC2536 may be used in a hot-plug card application. Overcurrent Transients The MIC2536 incorporates an internal circuit designed to prevent FLG from being asserted due to transient inrush current. Overcurrent events 430µF), the length of the transient due to inrush current may exceed the delay provided by the integrated filter. Since this inrush current exceeds the current-limit delay specification, FLG will be asserted during this time. To prevent the logic controller from responding to FLG being asserted, an external RC filter, as shown in Figure 3, can be used to filter out transient FLG assertion. The value of the RC time constant should be selected to match the length of the transient, minus flag tD. Applications Information Supply Filtering A 0.1µF to 1µF bypass capacitor from IN to GND, located at the device, is strongly recommended to control supply transients. Without a bypass capacitor, an output short may cause sufficient ringing on the input (from supply lead inductance) to damage internal control circuitry. Input or output transients must not exceed the absolute maximum supply voltage (VIN(max) = 6V) even for a short duration. VIN 2.7V to 5.5V MIC2536 ENA OUTA FLGA IN FLGB GND ENB OUTB 0.1µF to 1µF Figure 1. Supply Bypassing MIC2536-2 1 VCC 0.1 µF to "Hot" Receptacle OUTA EN 8 2 FLGA IN 3 7 FLGB GND 6 4 ENB OUTB 5 Backend Function CBULK GND Adaptor Card Figure 2. Hot-Plug Card Application V+ Logic Controller OVERCURRENT MIC2536 10k 1 R C 2 3 4 EN OUTA FLGA IN FLGB GND ENB OUTB 8 7 6 5 Figure 3. Transient Filter MIC2536 10 March 2000 MIC2536 Micrel may not consume more than 500µA. In a nonconfigured state all downstream devices will be switched off. In most cases, a nonconfigured hub is not a practical state for the system. Therefore, the 2.5mA specification is the applicable target specification for the suspend state. In a bus-powered hub with less than 4 ports, the hub may use the additional current for internal functions. The 500µA worst case suspend current must be further divided among the data port termination resistors and internal functions. The termination resistors will consume 3.6V ÷ (16.5KΩ – 5%) = 230µA. This leaves only 270µA for internal functions. Assuming 100µA as the maximum USB controller suspend current, 170µA remains for the rest of the system. The MIC2536 will consume 100µA maximum, leaving a margin of 70µA. USB Voltage Regulation USB specifications require a minimum downstream voltage supply of 4.40V from a bus-powered hub port (See Application Note 17 for details). The USB specification allows for a 100mV drop across the hub, leaving 250mV for PCB, upstream cable, and connector resistance. Therefore, the onresistance of the switch for each port, not including PCB resistance, must be