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MAX16976 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX16976 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 17 page ���������������������������������������������������������������� Maxim Integrated Products 11 MAX16976 28V, 600mA Automotive Step-Down Converter with Low Operating Current Overvoltage Protection The device includes overvoltage protection circuitry that protects the device when there is an overvoltage condi- tion at the output. If the output voltage increases by more than 12% of its set voltage, the device stops switching. The device resumes regulation once the overvoltage condition is removed. Overload Protection The overload protection circuitry is activated when the device is in current limit and VOUT is below the reset threshold. Under these conditions, the device enters a soft-start mode. When the overcurrent condition is removed before the soft-start mode is over, the device regulates the output voltage to the set value. Otherwise, the soft-start cycle repeats until the overcurrent condition is removed. Skip Mode During light-load operation, IINDUCTOR P 100mA, the device enters skip-mode operation. Skip mode turns off the internal switch and allows the output to drop below regulation voltage before the switch is turned on again. The lower the load current, the longer it takes for the regulator to initiate a new cycle effectively increasing light-load efficiency. During skip mode, the device qui- escent current drops to as low as 45FA. Overtemperature Protection Thermal-overload protection limits the total power dis- sipation in the device. When the junction temperature exceeds +175NC (typ), an internal thermal sensor shuts down the step-down controller, allowing the device to cool. The thermal sensor turns on the device again after the junction temperature cools by +15NC. Applications Information Output Voltage/Reset Threshold Resistive Divider Network Although the device’s output voltage and reset threshold can be set individually, Figure 1 shows a combined resis- tive divider network to set the desired output voltage and the reset threshold using three resistors. Use the follow- ing formula to determine the RFB3 of the resistive divider network: TOTAL REF FB3 OUT R V R V × = where VREF = 1V, RTOTAL = selected total resistance of RFB1, RFB2, and RFB3 in ohms, and VOUT is the desired output voltage in volts. Use the following formula to calculate the value of RFB2 of the resistive divider network: TOTAL REF_RES FB2 FB3 RES R V R R V × = − where VREF_RES is 1.25V (see the Electrical Characteristics table) and VRES is the desired reset threshold in volts. The precision of the reset threshold function is depen- dent on the tolerance of the resistors used for the divider. BST Capacitor Selection for Dropout Operation The device includes an internal boost capacitor refresh algorithm for dropout operation. This is required to ensure proper boost capacitor voltage that delivers power to the gate-drive circuitry. When the HSFET is on consecutively for 3.65 clock cycles, the internal counter detects this and turns off the HSFET for 0.35 clock cycles. This is of particular concern when VIN is falling and approaching VOUT at the minimum switching frequency (220kHz). The worst-case condition for boost capacitor refresh time is with no load on the output. For the boost capacitor to recharge completely, the LX node must be pulled to ground. If there is no current through the inductor then the LX node does not go to ground. To solve this issue, an internal load of about 100mA turns on at the sixth clock cycle, which is determined by a separate counter. In the worst-case condition with no load, the LX node does not go below ground during the first detect of the Figure 1. Output Voltage/Reset Threshold Resistive Divider Network RFB3 RFB2 RFB1 VOUT RESETI FB MAX16976 |
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