Three Approaches to Creating Negative Low-Voltage Hot-Swap Circuits
IntroductionIn addition to the usual array of positive-voltage supplies, many systems that need to be hot-plugged into a live backplane utilize at least a single -5V or
This application note presents two circuits for implementing hot-swap control on dual +5V/
The two-chip solution shown in Figure 1 allows independent circuit-breaker action on both negative- and positive-voltage channels. The two single-chip circuits shown in Figures 2 and 3 provide startup inrush-current control, but neither provides current-limiting or circuit-breaker functions for the negative-voltage channel.
Two-Chip SolutionThe Figure 1 circuit provides full hot-swap control, including current-limit and circuit-breaker functions for both a +5V channel and a
Each channel is set for overload fault shutdown at approximately 1A with autorestart action after a fault condition. The MAX5900 utilizes the M2 MOSFET's RDS(ON) as the current-sense resistor, and the VSENSE trip point is 200mV. Selecting M2 for RDS(ON) = 0.2Ω thus allows an approximate 1A overload trip point. Please refer to the individual data sheets for delay and timing information.
Figure 1. A two-chip approach to providing hot-swap control on positive and negative low-voltage supplies provides overload fault detection on both channels.
One-Chip SolutionThe Figure 2 circuit provides current-limited, hot-swap startup control on both channels, but includes current-limit and circuit-breaker functions for only the +5V channel. This one-chip solution employs a single two-channel MAX5904 controller to control both the +5V and the
With the following configuration, the MAX5904 treats the +5V channel as a +10.2V circuit with full circuit-breaker function, and it treats the
- Connect the MAX5904's GND pin to -5.2V
- Connect the IN1 and SENSE1 pins to ground
- Connect the IN2 pin to +5V
A fault on the negative channel will not be recognized by the MAX5904, but a fault on the +5V channel will shut down both channels. Both channels will restart after an autoretry delay time. The ON pin may be brought out for logic control of the on/off function, but a simple level shifter is required because the MAX5904 is referenced to
Figure 2. A single-chip solution provides inrush-current-limited turn-on functionality to positive and negative supplies, but without a circuit-breaker function on the -5.2V channel.
Single-Channel SolutionThe -5.2V, single-channel hot-swap circuit of Figure 3 provides full startup control using a MAX4272 positive low-voltage hot-swap controller. The MAX4272's GND pin is connected to the
Figure 3. A single-chip design provides hot-swap control without circuit-breaker function on a single negative supply, while also allowing for adjustment of the turn-on voltage.
The MAX4272's ON pin may be connected directly to circuit ground, and the circuit will start 150ms after the
There is no internal current limiting at startup, so CG has been included to slow the gate voltage rate of rise at turn on. As the internal gate drive is set to approximately 100µA, the load capacitor will charge at a rate described by the following equation:
where CGATE = CG + CGS (of M1).
As no current-limiting shutdown function is supplied, it may be desirable to include a fuse in series with the