Electronic Design

Dual Current-Limiting Switch For USB Applications

The number of hot-plug applications has increased dramatically due to its benefits of system down-time reduction and portability. The consequence of implementing hot-plug capability is that inrush or surge currents are produced when any uncharged capacitors are connected to the power-supply rails. The larger the capacitance, the more energy is needed to charge them.

Inrush currents can cause the power-supply output voltage to droop to a level where system logic can become scrambled. This circuit shown demonstrates one of the techniques used to minimize these inrush currents, allowing designers to use bulk capacitors in their hot-plug application (Fig. 1). In addition, with this solution, the user doesn't have to wait for a long period of time between multiple insertions.

Instead of charging the the input capacitor in a short period of time, the intent is to charge this capacitor over a longer period of time while limiting the charging current. The circuit uses the MIC2545A-2 and the MIC2778. The MIC2545A-1 is a programmable-current-limit, high-side switch that can handle currents up to 2.5 A. The current limit is set by an external resistor.

In this circuit there are two current-limits, 100 mA and 500 mA (when Q1 is on). USB specifications limit the device current to 100 mA prior to enumeration. After enumeration, a high-powered device can request no more than 500 mA. The circuit will guarantee these limits aren't exceeded. The MIC2778 is a voltage monitor with adjustable hysteresis that can operate down to 1.5 V. The high and low thresholds are set at 4 V and 2 V, respectively.

The equations below show how to calculate the resistor values for the first two thresholds and the two current-limits.

Thresholds:

VHIGH_THRESHOLD= VREF * ((R1+R2+R3)/R3))

VLOW_THRESHOLD= VREF* ((R1+R2+R3)/R2+R3)) VREF = 1.24 V

Set R1 + R2 + R3 = 1M, then:

VHIGH_THRESHOLD= 4 V = 1.24 * ((1M)/R3)) R3 = 310k

VLOW_THRESHOLD= 2 V = 1.24 * ((1M )/R2+310k)) R2 = 310k R1 = 1M − 310k − 310k = 380k

Current limits:

Current limit = 230/RSET

Current factor = 230

100 mA = 230/RSET

RSET1 = 230/100 mA = 2.2k

500 mA = 230/(RSET1 || RSET2 )

RSET1 || RSET2 = 230/500 mA = 460 Ω

RSET2 ≈ 0.6k

When VIN is "hot-plugged" to a 5-V power supply, the voltage will ramp up from 0 V to 5 V (Fig. 2). When VIN crosses 4 V, the delay generator of the MIC2778 turns on, allowing the V—RST signal to remain low for a minimum of 140 ms (Fig. 3). Such a delay feature is important here, since it guarantees that VIN will be at 5 V when the delay is off. It also allows the MTC2545A-2 to charge the capacitor with less than 100 mA (Fig. 4). Once the delay is off, the V—RST signal is pulled up to 5 V. Q1 turns on, placing the 600 Ω in parallel with the 2.2k to set the current-limit to 500 mA. When VIN is unplugged, its voltage will ramp down. Once the VIN crosses 2 V, the MIC2545's current-limit is reset to 100 mA.

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