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USB Fuse(MAX1562)

2017-08-11 20:47  
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This article describes the USB Fuse (MAX1562). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: MAX1562.

Life in the 21st century would be almost unbearable without some of the computer peripherals that PC users now look on as essentials – take for example theUSBpowered teacup warmer; this device is obviously an invaluable productivity tool for all users but it could prove a little tire some if the extra current it draws from theUSBport is sufficient to produce a localised meltdown on the motherboard. In a slightly more serious vein a similar situation could result from a carelessly wired connector in the design lab during prototyping and development of aUSBported peripheral. What’s needed here is some form of current limiting or fuse to prevent damage to the motherboard.

Figure:1 USB Fuse Circuit

Figure 1 USBFuse Circuit

The MAX1562 shown in Figure 1 is a purpose-builtUSBcurrent limiter from the chip manufacturers Maxim. The device operates with a supply voltage from 4.0 to 5.5 V with an operating current of typically 40 μA or 3 μA in standby mode. The circuit introduces a very low resistance in the power line (typically 26 m but guaranteed less than 50 m) from an internalMOSFET. TheFETgate bias voltage is generated on-chip from a charge pump circuit.

Figure:1 USB Fuse Circuit Diagram

Figure 2 USBFuse Circuit Diagram

The chip can distinguish between an overload and a short circuit condition in the supply line by measuring the voltage drop across its internal resistance; if the voltage is less than 1 V a short circuit is assumed and the chip pulses a (limited) output current every 20 ms in an effort to raise the output voltage. This approach will eventually be successful if the short circuit was caused by a large value capacitor across theUSBsupply pins or an external hard drive which have a high in-rush at start up. If the supply rail is not pulled up within the first 20 ms theFAULToutput (pin 2) is driven low. The output current limit is set by a single resistor on pin 4 (ISET):LIM= 17120 /RSET.

Figure:1 PCB and Parts layout usb fuse circuit project

Figure 3 PCBand Parts layout usb fuse circuit project

The circuit diagram shows a fixed 5.6k resistor in series with a 10k preset giving an adjustable current limit between 1.097 and 3.057 A. This range should be sufficient for the majority of applications. Increasing the preset resistance reduces the current limit level. Any intermittent connection in the preset (caused by a dirty track etc.) will switch the chip into shut down. The MAX1562 also contains a thermal cut out which turns off the output when the chip temperature exceeds 160 degrees C.

Figure 2 shows a diagram of the manufacturer’s application circuit. TheFAULToutput drives anLEDvia a series limiting resistor which reduces theLEDcurrent to 2 to 3 mA. The MAX1562 is available in aHESAvariant (with an active high ON signal) orESAversion (with an active low ON signal). The chip is packaged in an 8-pinSMDoutline. Figure 3 shows a smallPCBlayout for the circuit using mostlySMDcomponents.

COMPONENTSLIST
Resistors
R1 = 5k6 (SMD1206)
R2 = 1k5 (SMD1206)
P1 = 10k preset
Capacitors
C1 = 1μF (SMD1206)
C2 = 4μF7 10 V, tantalum
C3 = 220nF (SMD1206)
Semiconductors
D1 =LED, low current
IC1 = MAX1562ESA

 

 


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