NEW PRODUCTS 5 Pch HIGH-SIDE SWITCH FOR USB µPD16855 Takahiro Hattori HOST (PC) Scanner HUB (PC Monitor) USB HUB Printer Modem CCD Camera Keyboard Mouse PC peripheral units can be connected with a standardized cable. USB line Fig. 1 USB System Introduction Personal computers (PCs) have become widespread in recent years, and new proposals have been made concerning the interfaces that connect a PC and peripheral units. These proposals include standards such as the Universal Serial BUS (USB) and IEEE1394 intended to supersede the existing interfaces. Figure 1 shows a USB system. NEC has developed many devices that conform to the USB and IEEE1394 standards, such as the µPD72011 USB hub Published July 1998 controller IC and the µPD789800 keyboard microcontroller with USB capability. When designing a USB-supporting unit, a power management circuit is essential for obtaining power from the USB port and supplying it to the peripheral devices. The µPD16855 Pch high-side switch for USB developed this time is an intelligent power IC that supplies power to peripheral devices, which is a key requirement for USBsupporting units, and overcurrent limiting operation in case of an abnormality. By using this IC with a USB controller IC, system designers can easily design USBsupporting units. Why Is Pch High-Side Switch for USB Necessary? A high-side switch having an overcurrent limiting function for power management is necessary for a USB supporting unit. This is because the USB standard stipulates the following concerning the power supply: • HOST and Self-Powered HUB General-Purpose Device Engineering Department, Semiconductor Solution Engineering Division IN (input) 7 OUT1 8 (output 1) 5 OUT2 (output 2) Reference Voltage Overcurrent Detection Block Reference Voltage Gate Control Gate Control Under Voltage Locked Out Circuit 2 FLG1 (flag output 1) Overcurrent Detection Block 3 FLG2 (flag output 2) Thermal Shutdown Circuit 6 1 4 GND CTL1 (control input 1) CTL2 (control input 2) Fig. 2 Internal Block Diagram of µPD16855 Supply voltage: 4.75 to 5.25 V Supply current: 0.5 A MAX (per port) 5 A MAX (per unit) • Bus-Powered HUB Supply voltage: 4.40 to 5.25 V Supply current: 0.1 A MAX (per port) 0.5 A MAX (per unit) Because of this standard, a USBsupporting unit is required to have a 5-V power supply circuit, current detection circuit, and current limiting circuit. The µPD16855 Pch high-side switch for USB developed this time is an intelligent power IC that implements the above functions with a single chip. This IC helps you design a power circuit conforming to the USB standard. Product Outline The µPD16855 is a Pch high-side switch for USB conforming to the above USB standard. It operates at 5 V and contains two switch circuits. Because it is provided with an overcurrent detection and current limiting circuits corresponding to each switch, it can independently control a USB port. The result of detection can be reported to a USB controller by the signal output from a detection flag pin. In turn, control signals are input from the USB controller to each switch to turn ON/OFF the switch. The µPD16855 also has a thermal shutdown circuit and an under voltage locked out circuit to prevent the system from malfunctioning due to malfunctions of the IC. The features of the µPD16855 are as follows: • Two circuits of Pch power MOS FETs • Overcurrent detection circuit outputting signal from detection flag pin • Overcurrent limiting circuit to prevent voltage drop in system due to overcurrent • Thermal shutdown circuit • Under voltage locked out circuit • Soft start function • Switches turned ON/OFF by control pin • Space-saving 8-pin SOP package Figure 2 shows the block diagram of the µPD16855, and Photograph 1 shows its appearance. Photo 1 Appearance of µPD16855 Features The features of the µPD16855 Pch highside switch for USB are described in detail below. 1. Two circuits of Pch power MOS FETs The µPD16855 has two circuits of internal Pch power MOS FETs. These Pch power MOS FETs have an ON resistance of 100 mΩ TYP and 130 mΩ TYP., respectively. According to the USB standard, the voltage drop due to switching must be 70 mV or less. Therefore, the permissible ON resistance specification for a switch when a maximum current of 0.5 A flows is 140 mΩ (= 0.07 (V)/0.5 (A) x 1,000). The reason for providing a two-switch circuit is that PCs and PC peripheral units supporting USB mainly support two ports. Therefore, only one µPD16855 is used in a unit that supports two USB connector ports. 2. Overcurrent detection circuit outputting signal from detection flag pin The µPD16855 has an overcurrent detection circuit for each switch. The overcurrent detection circuit detects a current value through comparison between a reference voltage created from an input voltage with an output voltage, and judges that an overcurrent has occurred if the detected current value exceeds a certain limit. The accuracy of this overcurrent detection is 0.6 to 1.25 A, which is slightly higher than the 0.5 A defined by the USB standard. This is because, in the case of a USB unit requiring a current exceeding 0.5 A, too high a sensitivity in detecting overcurrents is inconvenient for PC users. By setting the overcurrent detection value to 0.6 A, slightly higher than the USB standard value of 0.5 A, the user can use the IC more easily. The moment a USB unit is connected, a current called inrush current, which is very high (up to 3 A) flows, but only momentarily (up to 10 µs). If this inrush current is detected as an overcurrent, the USB is no longer capable of “plug-and-play connection”, which is its biggest selling point. The µPD16855, therefore, has a delay of 10 µs TYP. for detection time so as not to detect the inrush current as an overcurrent. The overcurrent detection result is output to the Flg pin by an open-drain Nch MOS FET. Usually, this pin is kept “H” by an external pull-up resistor, and goes “L” upon detection of an overcurrent. The detection result is fed to the USB controller IC to inform the USB system of the occurrence of an overcurrent. Figure 3 shows the overcurrent detection operation. 3. Overcurrent limiting circuit to prevent voltage drop in system due to overcurrent The switches of the µPD16855 remains ON even after the IC has detected an overcurrent and reported the detection result, unless control by a control pin to be described later is performed, or a protection circuit is activated. In this case, the µPD16855 limits the current by switching the ON resistance of a switch. By switching the ON resistance, the output voltage drops but the current can be limited. This function prevents the supply voltage of the system from dropping (= system down), which is caused by a high current flowing through the system. 4. Thermal shutdown circuit The power consumption of the µPD16855 due to its heat generation can be calculated as indicated below, using the following values : • Maximum power consumption of IC: Approx. 70 mW (internal circuitry: approx. 5 mW + two switch circuits: 65 mW MAX.) • Thermal resistance of 8-pin SOP: Approx. 240°C/W (measured value for reference only) Therefore, the temperature rises approximately 17°C (= 0.07 (W) x 240 (°C/W)). The maximum ambient temperature is rated at +85°C, which is much less than the rated chip channel temperature (150°C). However, there is a possibility that the operating temperature will rise beyond the rated channel temperature due to internal heat generation in case the ambient temperature exceeds the rated value by accident. To provide for this situation, the µPD16855 has an internal thermal shutdown circuit. This circuit shuts off the internal circuitry to protect the IC from damage if the internal channel temperature exceeds 150°C. The thermal shutdown circuit monitors the internal channel temperature (Tch). If Tch exceeds 150°C, the output pins are turned off and the detection plug pin goes low, regardless of the input of the control pins. Figure 4 shows the thermal shutdown circuit operation. 5. Under voltage locked out circuit The µPD16855 is assumed to operate at 5 V. Its recommended operating voltage range is 4 to 5.5 V. Usually, the output status of an IC 5V IN (input) OUT (output) Flg (output) CTL (input) GND 5V GND 5V GND 5V GND lout Overcurrent Detection Threshold Overcurrent Detected Fig. 3 Overcurrent Detection Operation IN (input) OUT (output) Flg (output) 5V Standby Status GND 5V Non-Standby Status GND 5V GND 5V CTL (input) GND Tch Operating Temperature (rising) of Thermal Shutdown Circuit Fig. 4 Thermal Shutdown Circuit Operation 5V IN (input) OUT (output) Flg (output) CTL (input) lout GND 5V GND 5V GND 5V GND Overcurrent Detection Threshold Fig. 5 Operation at Power ON/OFF Operating Temperature (falling) of Thermal Shutdown Circuit becomes unstable and the IC malfunctions if the supply voltage drops. Therefore, the µPD16855 has an under voltage locked out circuit that stops the IC if the supply voltage drops below a certain level (3.5 V TYP.). This circuit prevents the µPD16855 from malfunctioning even if an abnormality such as instantaneous power failure occurs. While the under voltage locked out circuit operates, the output is turned off and the detection flag pin goes “L”. 6. Soft start function In a USB system, a capacitor of about 150 µF is connected to the power output pin of the USB connector. On power application, a high current (of up to 3 A) flows to charge this capacitor, and this current may be detected as an overcurrent by mistake. Therefore, the µPD16855 performs soft start control of the output voltage immediately after power application (output rise time: 2.5 ms MIN.), so that a high current does not flow through the capacitor connected to the output pin and an overcurrent is not detected by mistake. In the condition where the output of this product is turned ON, the output voltage is raised slowly (2.5 ms MIN.). In the condition where the output is turned OFF, the voltage of the output abruptly drops. Figure 5 shows the operation when the power is turned ON/OFF. 7. Switches turned ON/OFF by control pin As described above, the µPD16855 does not turn off the switch by itself even if it detects an overcurrent. This is because, if the power is arbitrarily turned off, power cannot be effectively managed from the viewpoint of the system. Therefore, the µPD16855 has a control pin that is used to turn off the switch. Power supply to the IC can be controlled by inputting a signal to this pin from a controller. When both switch circuits of the µPD16855 are turned off by the control signal, the IC enters the standby status. At this time, the current consumption drops to 5 µA MAX. When the switches are turned on again by the control signal, the soft start function described above is activated. 8. Space-saving 8-pin SOP package The µPD16855 is housed in an 18-pin SOP (225 mils) package that is useful for saving the space in an application set. Application Examples Having the above features, the µPD16855 can be used for applications using a USB system. Figure 6 to 8 show examples of applications and circuits of various USB systems. Figure 6 shows an application example of the µPD16855 used in a PC (especially, notebook type PC). Figure 7 shows an example used in a PC monitor. Figure 8 shows an application example in a USB hub. Future Development USB-supporting systems are expected to rapidly become widespread after Windows 98 is released. The µPD16855 is an intelligent power IC that is ideal for configuring a USB power system conforming to the USB standard. NEC has developed this USB power IC to satisfy market demands. In the future, NEC will promote the development of new power ICs for USB, such as 1-circuit output products and 4-circuit products supporting multiple outputs. NEC is also studying the feasibility of developing a power IC for the IEEE1394 high-speed serial interface. In this way, NEC plans to continuously develop new products that conform to new products and new standards in the PC market. Pull-Up Resistor Example: PC USB Port1 5V 5-V Power Supply Control1 D+ Output1 Flag1 100µF Input 1µF Output2 Power IC for USB USB Controller GND GND Control2 Flag2 D- µPD16855 USB Port2 100µF 5V D+ DGND ASIC Signal Line (pair) Fig. 6 Example of USB System (PC) Example: PC Monitor Pull-Up Resistor 5-V Power Supply (power supply in set) 100µF Control1 Flag1 Control2 Flag2 Output1 Input 100µF 1µF GND Output2 Regulato Power IC for USB µPD16855 x 2 pcs 100µF 3.3V Control1 Output1 USB Controller Flag1 Input Control2 GND µPD72011 Flag2 1µF 100µF Output2 Supports 4 to 5 USB ports. USB Port1 5V D+ DGND USB Port2 5V D+ DGND USB Port3 5V D+ DGND USB Port4 5V D+ DGND Signal Lines (pair) Fig. 7 Example of USB System (PC Monitor) Example: USB HUB Pull-Up Resistor 100µF VBUS (5V) Control1 Flag1 Control2 Flag2 Output1 Input 100µF 1µF GND Output2 Regulator Power IC for USB µPD16855 (x 2 pcs) 100µF 3.3V USB Controller µPD72011 Supports 4 to 5 USB ports. Fig. 8 Example of USB System (USB HUB) Control1 Flag1 Output1 Input Control2 GND Flag2 Output2 1µF 100µF USB Port1 5V D+ DGND USB Port2 5V D+ DGND USB Port3 5V D+ DGND USB Port4 5V D+ DGND Signal Lines (pair)