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LM36922H Datasheet(PDF) 22 Page - Texas Instruments |
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LM36922H Datasheet(HTML) 22 Page - Texas Instruments |
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22 / 46 page  22 LM36922H SNVSAF2A – FEBRUARY 2016 – REVISED FEBRUARY 2016 www.ti.com Product Folder Links: LM36922H Submit Documentation Feedback Copyright © 2016, Texas Instruments Incorporated 7.4.5 Fault Protection/Detection 7.4.5.1 Overvoltage Protection (OVP) The LM36922H provides four OVP thresholds (17 V, 24 V, 32 V, and 38 V). The OVP circuitry monitors the boost output voltage (VOUT) and protects OUT and SW from exceeding safe operating voltages in case of open load conditions or in the event the LED string voltage requires more voltage than the programmed OVP setting. The OVP thresholds are programmed in register 13 bits[3:2]. The operation of OVP differentiates between two overvoltage conditions (see Case 1 OVP Fault Only (OVP Threshold Hit and All Enabled Current Sink Inputs > 40 mV) , Case 1 OVP Fault Only (OVP Threshold Hit and All Enabled Current Sink Inputs > 40 mV) , and Case 2b OVP Fault and Open LED String Fault (OVP Threshold Duration and Any Enabled Current Sink Input ≤ 40 mV) ). 7.4.5.1.1 Case 1 OVP Fault Only (OVP Threshold Hit and All Enabled Current Sink Inputs > 40 mV) In steady-state operation with VOUT near the OVP threshold a rapid change in VIN or brightness code can result in a momentary transient excursion of VOUT above the OVP threshold. In this case the boost circuitry is disabled until VOUT drops below OVP – hysteresis (1 V). Once this happens the boost is re-enabled and steady state regulation continues. If VOUT remains above the OVP threshold for > 1 ms the OVP Flag is set (register 0x1F bit[0]). 7.4.5.1.2 Case 2a OVP Fault and Open LED String Fault (OVP Threshold Occurrence and Any Enabled Current Sink Input ≤ 40 mV) When any of the enabled LED strings is open the boost converter tries to drive VOUT above OVP and at the same time the open string(s) current sink headroom voltage(s) (LED1, LED2) drop to 0. When the LM36922H detects three occurrences of VOUT > OVP and any enabled current sink input (VLED1 or VLED2) ≤ 40 mV, the OVP Fault flag is set (register 0x1F bit[0]), and the LED Open Fault flag is set (register 0x1F bit[4]). 7.4.5.1.3 Case 2b OVP Fault and Open LED String Fault (OVP Threshold Duration and Any Enabled Current Sink Input ≤ 40 mV) When any of the enabled LED strings is open the boost converter tries to drive VOUT above OVP and at the same time the open string(s) current sink headroom voltage(s) (LED1, LED2) drop to 0. When the LM36922H detects VOUT > OVP for > 1 msec and any enabled current sink input (VLED1 or VLED2) ≤ 40 mV, the OVP Fault flag is set (register 0x1F bit[0]), and the LED Open Fault flag is set (register 0x1F bit[4]). 7.4.5.1.4 OVP/LED Open Fault Shutdown The LM36922H has the option of shutting down the device when the OVP flag is set. This option can be enabled or disabled via register 0x1E bit[0]. When the shutdown option is disabled the fault flag is a report only. When the device is shut down due to an OVP/LED String Open fault, the fault flags register must be read back before the LM36922H can be re-enabled. 7.4.5.1.5 Testing for LED String Open The procedure for detecting an open in a LED string is: • Apply power the the LM36922H . • Enable all LED strings (Register 0x10 = 0x07). • Set maximum brightness (Register 0x18 = 0x07 and Register 0x19 = 0xFF). • Set the brightness control (Register 0x11 = 0x00). • Open LED1 string. • Wait 4 msec. • Read LED open fault (Register 0x1F). • If bit[4] = 1, then a LED open fault condition has been detected. • Connect LED1 string. • Repeat the procedure for the other LED strings. |
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