Handy features of a USB current limit switch

USB, which is most commonly found in laptops and desktop PCs, has now found its way in every type of system available – home appliances, automotive products, and industrial and control systems. USB is the preferred way of communication between the host system and the USB device. When the USB device is connected to the host system, the latter is responsible for providing power to the USB device. Hence, the host system’s power supplies are designed by keeping in mind the USB device’s power requirement.

The USB organization mandates that the USB host system must provide at least 500mA of current for each USB connector. Thus, each USB device connected to the host system can draw a minimum of 500mA. As the current requirements of USB devices have increased, system design engineers are now providing more than 500mA of current per connector. The maximum amount of current drawn by the USB device must be limited so that the USB device can’t draw more current than what the host’s power supply can provide. A switch is used for the purpose of limiting the current to the USB device. Semiconductor manufacturers are making USB current limiting switches for 0.5A and higher currents. This article discusses the use of a current limit switch in USB applications and its commonly used features.

COMMON APPLICATIONS

In Figure 1, a USB host system is shown connected to the USB device using a cable. The USB cable is composed of power lines (Vbus and Gnd) and data lines (D+ and D-). The power lines are used to supply power from the host system’s power supply to the USB device. A current limit switch – e.g. MIC2004 from Micrel – is present between the host system’s power supply and the USB device. The MIC2004 switch provides easy operation and greater control of distributing power to the USB device. It allows or prevents current to the load, based on whether the switch is enabled or disabled.

The switch also features a current limit, which means that when the switch is enabled, the maximum current that the load can draw is equal to the current limit. This current limiting feature of the switch is essentially what will be used to limit the current to the USB devices.

Figure 2 shows a typical application of a current limit switch. Here the USB device is connected to the output of MIC2004-0.5. The MIC2004 has Vin pin, Vout pin, enable pin, and Gnd pin. When the enable signal is high, the Vout = Vin - (Ron*Iout), where Ron is the switch’s on resistance when it is enabled. The switch has 70mΩ typical on resistance. The MIC2004-0.5 has a fixed current limit of at least 500mA. Thus, the USB device can draw a maximum current of 500mA. When the enable signal is low, the switch is disabled and Iout = 0mA.

Generally, one USB switch rated for 500mA is present per USB connector. One USB switch can supply power to multiple devices. To allow connection to multiple devices, the switch with current rating of 1A or higher is used.

Figure 3 shows an application of a current limit switch powering two USB devices. Since each USB device can draw at least 500mA, the MIC2004-1.2 is used. The MIC2004-1.2 will allow at least 1.2A of current to the load. Thus, MIC2004-1.2 rated at 1.2A can be used to power two devices. If only one USB device is connected – for example, an external USB hard drive consuming higher current – then it would be able to draw the full 1.2A of current.

The rate at which the output Vout turns on is known as slew rate (dV/dt). The slew rate is dependent on the size of the capacitor at the gate of the switch. Thus, slew rate control is achieved by setting appropriate capacitor value at the gate of the switch. A built-in slew rate is available for the MIC2000 family of switches due to the internal capacitor connected at the gate. If slew rate needs to be adjusted, slew rate control is available for some of the switches in the MIC2000 family – for example, the MIC2005 switch. The MIC2005 shown in Figure 4 has the Cslew pin, where an external capacitor can be connected to increase or decrease the slew rate.

Figure 5 shows the output voltage and output current during turn on for MIC2005. The Cload =10uF, and no external capacitor is connected. When the enable pin is turned on, the capacitor will begin charging. The internal slew rate control will control the rate at which the output turns on and thus controlling the dV/dt. In the graph, Channel 1 is enabled, Channel 2 is Vout, and Channel 4 is Iout. The initial current spike is I = C*dV/dt and thus is dependent on dV/dt. Theoretically, Iout = 10µF*dV/dt = 10µF*5V/730µs = 68.5mA. From the graph, the output current spike is measured to be 50mA. If current spike needs to be decreased, an external capacitor on the Cslew pin must be added. Figure 6 shows the graph for MIC2005 with Cslew capacitor of 2.7nf.

The rise time is found to be 1.6ms and the current spike is decreased to 25mA. Slew rate dV/dt is decreased due to Cslew capacitor.

When the load capacitance is high, the initial turn on is controlled by slew rate. After the initial turn on time, the current control loop of the current limit switch becomes active and thus controlling the maximum current charging the capacitive load. The current control loop will ensure that the maximum current drawn by the capacitive load does not exceed the current limit setting of the switch. Figure 7 shows the MIC2005 for Cload = 940µF and without external Cslew capacitor. The initial current spike is controlled due to internal Cslew capacitor setting. After the initial current spike, the switch enters current limit, thus limiting the output current to current limit setting.

In Figure 8, the graph of MIC2005 for Cload = 940µF and with external Cslew capacitor of 2.7nF is shown. There is an initial turn on delay of 8ms due to Cslew capacitor and then during initial turn on Cslew capacitor controls the slew rate dV/dt. The duration of current limit time is also decreased to 1.5ms from 2ms.

The effect of Cslew pin with just a resistive load is shown in Figure 9. The additional Cslew capacitor causes both a turn on delay and increased turn on rise time.

The MIC2004 and MIC2005 have fixed current limits. They are offered as 0.5A, 0.8A or 1.2A fixed current limit parts. For an application that requires different current limit settings, an adjustable current limit switch such as the MIC2007 can be used. The MIC2007 is shown in Figure 10. The current limit can be set between 0.2A to 2A. The Ilimit pin of MIC2007 is used to set the current limit by connecting an external resistor. The Ilimit = CLF(current limit factor)/Rset (external Ilimit resistor). For example, the CLF is specified to be 235 for 0.5A of load current. Thus to have a current limit of 0.5A, an external Rset = CLF/Ilimit = 235/0.5A = 470Ω must be connected on the Ilimit pin.

The MIC2005 and MIC2009 feature a fault output. The block diagram of MIC2005 is shown in Figure 4 and the block diagram of MIC2009 is shown in Figure 11. The MIC2009 has an adjustable current limit pin and fault output pin.

The fault output pin indicates a fault when the switch limits the output current to current limit setting. In Figure 12, the graph is for MIC2009 powering a 0.68Ω resistor. Channel 1 is enabled, Channel 2 is fault, Channel 3 is Vout, and Channel 4 is Iout. VIn is 3V and the current limit setting is at 2A. When the switch is enabled and if there is no current limit, then Iout would be 3V/(Rdson + 0.68Ω). The On resistance of the switch is typically 70mΩ, as specified in the datasheet. The Iout would be =3/(0.07+0.68) = 4A. Since the MIC2009 current limit setting is at 2A, the switch does current limiting and the output current is limited to 2A. When current limiting occurs, the fault output becomes active low (Figure 12).

The MIC2000 family of switches is also protected from overheating. When there is too much power dissipation in the IC, the switch turns off when the junction temperature rises above 145ºC. The switch turns on again when the temperature falls bellow 135ºC. This is known as thermal cycling. Figure 12 shows the MIC2009 doing thermal cycling.

In Figure 12, the total power dissipated in the IC can be calculated as follows:

Power = (Vin-Vout)*Iout = (3-Vout)*2A

Vout = 2A*0.68Ω = 1.36V. (Close to 1.6V in Figure 12)

Therefore, power dissipated in the IC = (3-1.6)*2 = 2.8W.

Since the thermal resistance of the SOT 23 packaged part is 230ºC/W, Tj (junction temperature) = 25 (room temp) + 230*2.8 = 669ºC. Since this temperature is much higher then the maximum junction temperature of 150ºC, the switch protects itself by doing thermal cycling.

The MIC2000 family of switches has very low RDS(on) of 70mΩ (typical). They are offered in small SOT 23-6 packages and MLF packages, and can pass up to 2A.



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