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MAX16936SAUEA Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX16936SAUEA Datasheet(HTML) 11 Page - Maxim Integrated Products |
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11 / 17 page  MAX16936 36V, 220kHz to 2.2MHz Step-Down Converter with 28µA Quiescent Current 11 Maxim Integrated Spread-Spectrum Option The MAX16936 has an internal spread-spectrum option to optimize EMI performance. This is factory set and the S-version of the IC should be ordered. For spread- spectrum-enabled ICs, the operating frequency is varied ±6% centered on FOSC. The modulation signal is a trian- gular wave with a period of 110µs at 2.2MHz. Therefore, FOSC will ramp down 6% and back to 2.2MHz in 110µs and also ramp up 6% and back to 2.2MHz in 110µs. The cycle repeats. For operations at FOSC values other than 2.2MHz, the modulation signal scales proportionally, e.g., at 400kHz, the 110µs modulation period increases to 110µs x 2.2MHz/400MHz = 550µs. The internal spread spectrum is disabled if the IC is synced to an external clock. However, the IC does not fil- ter the input clock and passes any modulation (including spread-spectrum) present on the driving external clock to the SYNCOUT pin. Automatic Slew-Rate Control on LX The MAX16936 has automatic slew-rate adjustment that optimizes the rise times on the internal HSFET gate drive to minimize EMI. The IC detects the internal clock frequency and adjusts the slew rate accordingly. When the user selects the external frequency setting resistor RFOSC such that the frequency is > 1.1MHz, the HSFET is turned on in 4ns (typ). When the frequency is < 1.1MHz the HSFET is turned on in 8ns (typ). This slew-rate control optimizes the rise time on LX node externally to minimize EMI while maintaining good efficiency. Internal Oscillator (FOSC) The switching frequency, fSW, is set by a resistor (RFOSC) connected from FOSC to AGND. See Figure 3 to select the correct RFOSC value for the desired switching frequency. For example, a 400kHz switching frequency is set with RFOSC = 732kI. Higher frequencies allow designs with lower inductor values and less output capacitance. Consequently, peak currents and I2R losses are lower at higher switching frequencies, but core losses, gate charge currents, and switching losses increase. Synchronizing Output (SYNCOUT) SYNCOUT is an open-drain output that outputs a 180N out-of-phase signal relative to the internal oscillator. 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 internal bias regulator and the step-down con- troller, allowing the device to cool. The thermal sensor turns on the device again after the junction temperature cools by 15NC. Applications Information Setting the Output Voltage Connect FB to BIAS for a fixed +5V/+3.3 output voltage. To set the output to other voltages between 1V and 10V, connect a resistive divider from output (OUT) to FB to AGND (Figure 2). Use the following formula to determine the RFB2 of the resistive divider network: RFB2 = RTOTAL x VFB/VOUT where VFB = 1V, RTOTAL = selected total resistance of RFB1, RFB2 in ω, and VOUT is the desired output in volts. Calculate RFB1 (OUT to FB resistor) with the following equation: OUT FB1 FB2 FB V RR 1 V = − where VFB = 1V (see the Electrical Characteristics table). FPWM/Skip Modes The MAX16936 offers a pin selectable skip mode or fixed-frequency PWM mode option. The IC has an internal LS MOSFET that turns on when the FSYNC pin is connected to VBIAS or if there is a clock present on the FSYNC pin. This enables the fixed-frequency-forced PWM mode operation over the entire load range. This option allows the user to maintain fixed frequency over the entire load range in applications that require tight control on EMI. Even though the MAX16936 has an inter- nal LS MOSFET for fixed-frequency operation, an exter- nal Schottky diode is still required to support the entire load range. If the FSYNC pin is connected to GND, the skip mode is enabled on the MAX16936. Figure 2. Adjustable Output Voltage Setting RFB2 RFB1 FB VOUT MAX16936 |
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