february 2000 1 application note 32 application note 32 micrel application note 32 dual current-limiting switch for usb applications by william mai overview the need for hot insertion capability has dramatically in- creased due to the demand of reducing system down time and portability. the effect of hot insertion is an inrush current or surge current which is produced when any uncharged capacitors are connected to a power supply. the larger the capacitors, the more energy is needed to charge them. inrush current can cause the voltage of the power supply to droop to a level where system logic can be confused. this application note describes one of the techniques to minimize the inrush current to allow the user to utilize bulk capacitors of any size for hot insertion applications. in addition, with this solution the user does not have to wait for a long period of time between multiple insertions. circuit description instead of charging the capacitor in a short period of time, the idea is to charge the capacitor in a long period of time by limiting the charge current. the circuit in figure 1 uses the mic2545a-2 and the mic2778. the mic2545a-2 is a pro- grammable current limit high-side switch that is capable of having current-limit up to 2.5a. the current-limit is set by the external resistor. in this circuit, there are two current-limits, 100ma and 500ma. usb specifications limit the device current to 100ma prior to enumeration. after enumeration, a high-powered device can request no more than 500ma. this circuit will guarantee these limits are not exceeded. the mic2778 is a voltage monitor with adjustable hysteresis and that operate down to 1.5v. the high and low thresholds are set at 4v and 2v respectively. the equation below shows how to calculate the resistors value for the two thresholds and the two current-limits. vdd c1 4.7 f r4 100k r2 310k r1 380k r3 310k lt h hth gnd /rst r set1 600 ? q1 vn1206 load mic2778 in en gnd flg out in out ilim mic2545a-2 r set2 2.2k r l 10 ? c l 500 f figure 1. dual current-limiting switch micrel, inc. ?1849 fortune drive ?san jose, ca 95131 ?usa ?tel + 1 (408) 944-0800 ?fax + 1 (408) 944-0970 ?http://www.mic rel.com thresholds vv r1 r2 r3 r3 high threshold ref = ++ vv r1 r2 r3 r2 r3 low threshold ref = ++ + v 1.24v ref = set r1+r2+r3 = 1m ? v 4v 1.24 1m r3 high threshold == ? r3 310k = v 2v 1.24 1m r2 310k low threshold == ? + r2 310k = r1 1m 310k 310k 380k =?? ? = current-limit current limit = 230 r set where: current factor = 230 100ma 230 r set = r 230 100ma 2.2k set1 = 500ma 230 r||r set1 set2 = (continued)
application note 32 micrel application note 32 2 february 2000 rr 230 500ma 460 set1 set2 || ==? r 0.6k set2 when v in is hot-plugged to a 5v power supply, the voltage will ramp up from 0v to 5v (figure 2). when v in crosses 4v, the delay generator of the mic2778 turns on allowing the v /rst to remains low for a minimum of 140ms. the delay feature is important in this application because it guarantees v in will be at 5v when the delay is off. it also allows the mic2545a-2 to charge the capacitor with less than 100ma (figure 4). once the delay is off, the v /rst is pulled up to 5v. q1 turns on to parallel the 2.2k and 600 ? to set the current limit for the mic2545a-2 at 500ma. when v in is unplugged, its voltage will ramp down. once the v in crosses 2v, the mic2545a-2 s current limit is reset back to 100ma. in figure 5, a logic control can be used to switch the current limit from 100ma to 500ma. the difference between the two circuits is that the mic2778 allows the circuit to switch automatically. c1 4.7 f +5v r set2 600 ? q1 vn1206 load in en gnd flg out in out ilim mic2545a-2 logic control r set1 2.2k r l 10 ? c l 500 f figure 5. logic-controlled current-limit setpoint v in (10v/div) r l = 10 ? c l = 0 f v /rst (10v/div) v out (5v/div) i in (200ma/div) figure 3. dual current limiting v in (10v/div) r l = 10 ? c l = 500 f v /rst (10v/div) v out (5v/div) i in (200ma/div) figure 4. inrush-current limiting 5v 100ma i limit time 500ma 4v v in v /rst 3v 2v 1v 0v 5v 4v 3v 2v 1v 0v t rst figure 2. timing diagram micrel inc. 1849 fortune drive san jose, ca 95131 usa tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 web http://www.micrel.com this information is believed to be accurate and reliable, however no responsibility is assumed by micrel for its use nor for an y infringement of patents or other rights of third parties resulting from its use. no license is granted by implication or otherwise under any patent or pat ent right of micrel inc. ? 2000 micrel incorporated
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