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ZHCSOE3E August 2021 – September 2023 LMR38020

PRODUCTION DATA

8.3.8 Overcurrent and Short Circuit Protection

The LMR38020 is protected from overcurrent conditions by cycle-by-cycle current limit on both the peak and valley of the inductor current. Hiccup mode is activated if a fault condition persists to prevent overheating.

High-side MOSFET overcurrent protection is implemented by the nature of the peak current mode control. The high-side switch current is sensed when the high-side is turned on after a set blanking time. The high-side switch current is compared to the output of the error amplifier (EA) minus slope compensation every switching cycle. The peak current of high-side switch is limited by a clamped maximum peak current threshold, I_HS-LIMIT, which is constant.

The current going through the low-side MOSFET is also sensed and monitored. When the low-side switch turns on, the inductor current begins to ramp down. The low-side switch is turned OFF at the end of a switching cycle if its current is above the low-side current limit, I_{LS_LIMIT}. The low-side switch is kept on so that inductor current keeps ramping down until the inductor current ramps below the I_{LS_LIMIT}. Then the low-side switch is turned OFF and the high-side switch is turned on after a dead time. This is somewhat different to the more typical peak current limit and results in Equation 7 for the maximum load current.

If the feedback voltage is lower than 40% of V_REF, the current of the low-side switch triggers I_{LS_LIMIT} for 256 consecutive cycles and hiccup current protection mode is activated. In hiccup mode, the converter shuts down and stays off for a period of hiccup, T_HICCUP (76-ms typical), before the LMR38020 tries to start again. If overcurrent or short-circuit fault condition still exist, hiccup repeats until the fault condition is removed. Hiccup mode reduces power dissipation under severe overcurrent conditions and prevents overheating and potential damage to the device.

Equation 7.

I_{{O U T}_{M A X}} = I_{L S} + (\frac{V_{I N -} V_{O U T}}{L \times 2 \times f_{S W}}) \times (\frac{V_{O U T}}{V_{I N}})

For FPWM version, the inductor current is allowed to go negative. When this current exceeds the low-side negative current limit, I_{LS_NEG}, the low-side switch is turned off and high-side switch is turned on immediately. This is used to protect the low-side switch from excessive negative current.