Keywords: hot swap, pcb layout, circuit breaker
The MAX5977A is a versatile, high-performance hot-swap controller with electronic fuse, and high-side current-sense output. Proper component placement and routing are critical in achieving the MAX5977’s full performance. Here, we discuss the benefits and inadequacies of specific layout/placement techniques.
Review the critical nodes in the application diagram to better understand the operation and layout requirements of the MAX5977. See Figure 1. Standard layout techniques can be used for the traces not shown.
Figure 1. Applications diagram showing the critical components and traces for a MAX5977 circuit.
The circuit-breaker function compares the voltage across RSENSE to the voltages across RSCOMP and RFCOMP. See Figure 2. When the voltage across RSENSE exceeds the voltage across either RSCOMP or RFCOMP, the circuit breaker will trip. The voltages being compared will typically be in the 25mV to 100mV range. Improper component placement and layout can result in nuisance trips or failure to trip under some fault conditions.
Figure 2. Simplified applications diagram of the MAX5977 fast and slow circuit-breaker functions.
The MAX5977’s current reporting senses the voltage across RSENSE through the IN and SENSE pins. See Figure 3. Internally, a transimpedance amplifier with a gain of 2500µS outputs a current on the CSOUT pin. An external resistor, RCSOUT, converts this current into a voltage and, along with RSENSE, sets the overall gain of the circuit. Similar to the circuit-breaker function, the voltage measured across RSENSE is quite small, typically ranging from a couple of millivolts to 50mV. Improper placement or routing of the components shown in Figure 3 results in reduced accuracy across RCSOUT.
Figure 3. Simplified applications diagram for the MAX5977 current-reporting function.
The CALSENSE pin allows for a single-point calibration of the current reporting. See Figure 4. An external current source generates a known voltage across RCALSENSE. This calibration voltage is multiplexed into an external ADC through the CAL pin. The calibration voltage will typically be in the 25mV to 50mV range. Improper placement and routing will cause errors during calibration and adversely affect all future current measurements.
Figure 4. Simplified applications diagram showing the circuitry for single-point calibration of the current reporting.
This section includes rough guidelines for proper placement and routing. These guidelines are not hard-and-fast rules, but provide a framework to help maximize the performance of the MAX5977. While proper placement and routing are always good ideas, circuits with higher RSENSE values are less susceptible to errors. As such, circuits with high RSENSE values can take some liberties without adversely affecting performance. Conversely, lower RSENSE values need to adhere more strictly to the suggested guidelines.
Figure 5. Connect the high-current traces to the ends of RSENSE and not the sides. Keep the trace widths roughly equal to the pad widths at the points of connection.
Figure 6. Diagram showing a kelvin connection on a two-terminal RSENSE.
RCALSENSE, RSCOMP, and RFCOMP
Figure 7. Connect RCALSENSE, RSCOMP, and RFCOMP to the differentially routed voltage-sense leads.
Figure 8. Ground RCSOUT directly at the ADC or signal-conditioning circuitry. This is especially important if the ADC or signal-conditioning circuitry is on a different board than the MAX5977.
Figure 9. Block Diagram of the MAX5977. AGND (pin 3 and the exposed pad) is used for the precision or low-noise circuitry. GND (pin 13) is used for the digital circuitry and the gate pulldown.
Figure 10. Connect pin 3 (AGND) directly to the exposed pad. Connect the exposed pad to the ground plane with a via or vias. Connect the GND pin (pin 13) to the ground plane with a via.
GATE and SOURCE
Figure 11. Connect the GATE and SOURCE pins to the external FET with short and wide traces. This keeps the turn-off time short by minimizing trace inductance.
Putting it All Together
Figure 12 shows one possible layout with all of the above suggestions along with other guidelines.
Figure 12. Summary of placement and routing guidelines for the MAX5977.
Assume in Figure 13 that RSENSE is a 2010-sized 2.5mΩ resistor with 0.12in wide pad. Let’s also assume that the sensing points are incorrectly connected 0.12in away from the pads of the sense resistor.
Figure 13. Improperly connecting to the sense resistor can lead to significant errors.
Each 0.12in × 0.12in trace between the sensing point and RSENSE will add roughly 250µΩ of resistance and 0.14nH of inductance (Note 1). Figure 14 shows the equivalent circuit.
Figure 14. Equivalent circuit of the improperly connected RSENSE shown in Figure 13.
The current reporting and slow-trip circuit breaker have filtering and should not be affected by the added inductance. However, the added resistance of 500µΩ will create a +20% error with respect to the 2.5mΩ sense resistor. For reference, the data-sheet current reporting error is ±4.1% worst case (Note 2). This small routing mistake has increased the overall error from roughly ±4% to +16%/+24%. A full 4x to 6x increase in error!
Both the added resistance and inductance impact the fast-trip comparator since it lacks filtering. A 10A/µs current transient through RSENSE will create an additional 2.8mV error on top of the 20% resistance error. Faster slew rates will create proportionally larger errors.
The errors introduced into the slow and fast circuit breaker thresholds can result in the MAX5977 turning off the downstream circuitry unnecessarily. Depending on the severity of these nuisance trips, the design could be rendered unusable.
Figure 15 shows an example of bad placement and routing.
Figure 15. Example of incorrect placement and routing on a MAX5977 circuit.