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MAX16904 Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX16904 Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 14 page 2.1MHz, High-Voltage, 600mA Mini-Buck Converter 10 ______________________________________________________________________________________ If the input voltage is reduced and the device approaches dropout, it tries to turn on the high-side FET continuously. To maintain gate charge on the high- side FET, the BST capacitor must be periodically recharged. To ensure proper charge on the BST capacitor when in dropout, the high-side FET is turned off every 6.5μs and the low-side FET is turned on for about 150ns. This gives an effective duty cycle of > 97% and a switching frequency of 150kHz when in dropout. Spread-Spectrum Option The device has an optional spread-spectrum version. If this option is selected, then the internal operating fre- quency varies by +6% relative to the internally generat- ed operating frequency of 2.1MHz (typ). Spread spectrum is offered to improve EMI performance of the device. By varying the frequency 6% only in the posi- tive direction, the device still guarantees that the 2.1MHz frequency does not drop into the AM band limit of 1.8MHz. Additionally, with the low minimum on-time of 80ns (typ) no pulse skipping is observed for a 5V output with 18V input maximum battery voltage in steady state. The internal spread spectrum does not interfere with the external clock applied on the SYNC pin. It is active only when the device is running with internally generat- ed switching frequency. Power-Good (PGOOD) The device features an open-drain power-good output. PGOOD is an active-high output that pulls low when the output voltage is below 91% of its nominal value. PGOOD is high impedance when the output voltage is above 93% of its nominal value. Connect a 20k Ω (typ) pullup resistor to an external supply or the on-chip BIAS output. Overcurrent Protection The device limits the peak output current to 1.05A (typ). To protect against short-circuit events, the device shuts off when OUTS is below 1.5V (typ) and one overcurrent event is detected. The device attempts a soft-start restart every 30ms and stays off if the short circuit has not been removed. When the current limit is no longer present, it reaches the output voltage by following the normal soft-start sequence. If the device die reaches the thermal limit of +175°C (typ) during the current-limit event, it immediately shuts off. Thermal-Overload Protection The device features thermal-overload protection. The device turns off when the junction temperature exceeds +175°C (typ). Once the device cools by 15°C (typ), it turns back on with a soft-start sequence. Applications Information Inductor Selection Three key inductor parameters must be specified for operation with the device: inductance value (L), peak inductor current (IPEAK), and inductor saturation current (ISAT). The minimum required inductance is a function of operating frequency, input-to-output voltage differen- tial, and the peak-to-peak inductor current ( ΔIP-P). Higher ΔIP-P allows for a lower inductor value, while a lower ΔIP-P requires a higher inductor value. A lower inductor value minimizes size and cost, improves large-signal and transient response, but reduces effi- ciency due to higher peak currents and higher peak-to- peak output-voltage ripple for the same output capacitor. On the other hand, higher inductance increases efficiency by reducing the ripple current. Resistive losses due to extra wire turns can exceed the benefit gained from lower ripple current levels especial- ly when the inductance is increased without also allow- ing for larger inductor dimensions. A good compromise is to choose ΔIP-P equal to 30% of the full load current. Use the following equation to calculate the inductance: VIN and VOUT are typical values so that efficiency is optimum for typical conditions. The switching frequency is ~2.1MHz. The peak-to-peak inductor current, which reflects the peak-to-peak output ripple, is worse at the maximum input voltage. See the Output Capacitor sec- tion to verify that the worst-case output ripple is accept- able. The inductor saturation current is also important to avoid runaway current during continuous output short circuit. The output current may reach 1.22A since this is the maximum current limit. Choose an inductor with a saturation current of greater than 1.22A to ensure prop- er operation and avoid runaway. Input Capacitor The discontinuous input current of the buck converter causes large input ripple current. The switching frequen- cy, peak inductor current, and the allowable peak-to- peak input-voltage ripple dictate the input capacitance requirement. Increasing the switching frequency or the inductor value lowers the peak-to-average current ratio yielding a lower input capacitance requirement. The input ripple comprises mainly of ΔVQ (caused by the capacitor discharge) and ΔVESR (caused by the L VV V Vf I OUT IN OUT IN SW P P = − ×× − () Δ |
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