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ZHCSOF7B september 2022 – june 2023 TPS25990

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RQP|26
- MPQF569A

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8.3.4.1 Slew rate (dVdt) and Inrush Current Control

During hot-plug events or while trying to charge a large output capacitance, there can be a large inrush current. If the inrush current is not managed properly, it can put excessive stress on the system power supply causing it to droop and even damage the input connectors. This action can lead to unexpected restarts elsewhere in the system. The inrush current during turn-on is directly proportional to the load capacitance and rising slew rate. Equation 2 can be used to find the slew rate (SR) required to limit the inrush current (I_INRUSH) for a given load capacitance (C_LOAD):

Equation 2.

S R (\frac{V}{m s}) = \frac{I_{I N R U S H} (m A)}{C_{O U T} (µ F)}

A capacitor can be added to the DVDT pin to control the rising slew rate and lower the inrush current during turn-on. This is also a function of the dVdt rate scaling factor which can be digitally programmed through PMBus® writes to the DEVICE_CONFIG register. The required C_dVdt capacitance to produce a given slew rate can be calculated using Equation 3.

Equation 3.

C_{d V d t} (p F) = \frac{42000 \times k}{S R (\frac{V}{m s})}

where k = 1, if DEVICE_CONFIG[10:9] = 10 (Default)

k = 0.5, if DEVICE_CONFIG[10:9] = 00

k = 0.75, if DEVICE_CONFIG[10:9] = 01

k = 1.5, if DEVICE_CONFIG[10:9] = 11

The fastest output slew rate is achieved by leaving the dVdt pin open and setting DEVICE_CONFIG[10:9] = 11.

Note: High turn-on slew rates in combination with high input power path inductance can result in oscillations during start-up. This can be mitigated using one or more of the following steps:

Reduce the input inductance.
Increase the capacitance on VIN pin.
Increase the DVDT pin capacitor value or change the DVDT scaling factor using DEVICE_CONFIG[10:9] register bits to reduce the slew rate or increase the start-up time. TI recommends using a minimum start-up time of 5 ms.