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MP8726EL Datasheet(PDF) 10 Page - Monolithic Power Systems |
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MP8726EL Datasheet(HTML) 10 Page - Monolithic Power Systems |
10 / 14 page MP8726 – 6A PEAK CURRENT, 21V, SYNCHRONOUS STEP-DOWN CONVERTER MP8726 Rev. 0.92 www.MonolithicPower.com 10 6/15/2010 MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 3) Synchronized by External Sync Clock Signal The chip can be synchronized to external clock range from 300kHz up to 2MHz through this pin 2ms right after output voltage is set, with the internal clock rising edge synchronized to the external clock rising edge. Vin EN/Sync Vcc Vout CLK 5us Foldback 500kHz External CLK 0.625*Vout_set 2ms 1ms VCC_Rising Vout_set Figure 4—Startup Sequence Using External Sync Clock Signal Under-Voltage Lockout (UVLO) Under-voltage lockout (UVLO) is implemented to protect the chip from operating at insufficient supply voltage. The MP8726 UVLO comparator monitors the output voltage of the internal regulator, VCC. The UVLO rising threshold is about 4.0V while its falling threshold is a consistent 3.2V. Internal Soft-Start The soft-start is implemented to prevent the converter output voltage from overshooting during startup. When the chip starts, the internal circuitry generates a soft-start voltage (SS) ramping up from 0V to 1.2V. When it is lower than the internal reference (REF), SS overrides REF so the error amplifier uses SS as the reference. When SS is higher than REF, REF regains control. The SS time is internally fixed to 4ms. Over-Current-Protection and Latch off The MP8726 has cycle-by-cycle over current limit when the inductor current peak value exceeds the set current limit threshold. Meanwhile, output voltage starts to drop until FB is below the Under- Voltage (UV) threshold, typically 30% below the reference. Once a UV is triggered, the MP8726 is latched off until En or IN is recycled. This protection mode is especially useful when the output is dead-short to ground. Thermal Shutdown Thermal shutdown is implemented to prevent the chip from operating at exceedingly high temperatures. When the silicon die temperature is higher than 150 °C, it shuts down the whole chip. When the temperature is lower than its lower threshold, typically 140 °C, the chip is enabled again. Floating Driver and Bootstrap Charging The floating power MOSFET driver is powered by an external bootstrap capacitor. This floating driver has its own UVLO protection. This UVLO’s rising threshold is 2.2V with a hysteresis of 150mV. The bootstrap capacitor voltage is regulated internally by VIN through D1, M3, C4, L1 and C2 (Figure 5). If (VIN-VSW) is more than 5V, U2 will regulate M3 to maintain a 5V BST voltage across C4. SW Figure 5—Internal Bootstrap Charging Circuit Startup and Shutdown If both VIN and EN are higher than their appropriate thresholds, the chip starts. The reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. The regulator provides stable supply for the remaining circuitries. Three events can shut down the chip: EN low, VIN low and thermal shutdown. In the shutdown procedure, the signaling path is first blocked to avoid any fault triggering. The COMP voltage and the internal supply rail are then pulled down. The floating driver is not subject to this shutdown command. |
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