Application Note 32 Micrel Application Note 32 Dual Current-Limiting Switch for USB Applications by William Mai Thresholds Overview The need for hot insertion capability has dramatically increased 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. Vhigh threshold = VREF Vlow threshold = VREF R1 + R2 + R3 R3 R1 + R2 + R3 R2 + R3 VREF = 1.24V Set R1+R2+R3 = 1MΩ Vhigh threshold = 4V = 1.24 1MΩ R3 R3 = 310k Vlow threshold = 2V = 1.24 Circuit Description 1MΩ R2 + 310k R2 = 310k 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 programmable 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. R1 = 1MΩ − 310k − 310k = 380k Current-limit Current limit = 230 RSET where: Current factor = 230 100mA = RSET1 = 230 RSET 230 ≈ 2.2k 100mA 500mA = 230 RSET1 || RSET2 (continued) C1 R1 4.7µF 380k R2 310k MIC2778 VDD LTH HTH /RST MIC2545A-2 IN OUT IN OUT EN ILIM GND FLG R4 100k GND R3 310k Load RSET1 600Ω CL RSET2 500µF 2.2k RL 10Ω Q1 VN1206 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.micrel.com February 2000 1 Application Note 32 VIN 5V 4V 3V 2V 1V 0V V/RST 5V 4V 3V 2V 1V 0V Micrel tRST IIN V/RST VIN VOUT (200mA/div) (5V/div) (10V/div) (10V/div) Application Note 32 500mA ILIMIT 100mA Time Figure 2. Timing Diagram RSET1 || RSET2 = 230 = 460Ω 500mA RSET2 ≈ 0.6k Figure 3. Dual Current Limiting C1 4.7µF MIC2545A-2 IN OUT IN OUT EN ILIM GND FLG IIN (200mA/div) V/RST VIN VOUT (5V/div) (10V/div) (10V/div) When VIN is hot-plugged to a 5V power supply, the voltage will ramp up from 0V to 5V (Figure 2). When VIN 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 VIN 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 600Ω to set the current limit for the MIC2545A-2 at 500mA. When VIN is unplugged, its voltage will ramp down. Once the VIN 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. +5V RL = 10Ω CL = 0µF RL = 10Ω CL = 500µF Figure 4. Inrush-Current Limiting Load RSET1 2.2k RL 10Ω CL 500µF RSET2 600Ω Q1 VN1206 Logic Control Figure 5. Logic-Controlled Current-Limit Setpoint MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA 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 any 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 patent right of Micrel Inc. © 2000 Micrel Incorporated Application Note 32 2 February 2000