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RT8089A Datasheet(PDF) 11 Page - Richtek Technology Corporation |
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RT8089A Datasheet(HTML) 11 Page - Richtek Technology Corporation |
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11 / 15 page  RT8089A 11 DS8089A-01 March 2013 www.richtek.com Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation. © Application Information The RT8089A is a single-phase step-down converter. It provides single feedback loop, current mode control with fast transient response. An internal 0.6V reference allows the output voltage to be precisely regulated for low output voltage applications. A fixed switching frequency (1MHz) oscillator and internal compensation are integrated to minimize external component count. Protection features include over current protection, under voltage protection and over temperature protection. Output Voltage Setting Connect a resistive voltage divider at the FB between VOUT and GND to adjust the output voltage. The output voltage is set according to the following equation : Chip Enable and Disable The EN pin allows for power sequencing between the controller bias voltage and another voltage rail. The RT8089A remains in shutdown if the EN pin is lower than 400mV. When the EN pin rises above the VEN trip point, the RT8089A begins a new initialization and soft-start cycle. Internal Soft-Start The RT8089A provides an internal soft-start function to prevent large inrush current and output voltage overshoot when the converter starts up. The soft-start (SS) automatically begins once the chip is enabled. During soft- start, the internal soft-start capacitor becomes charged and generates a linear ramping up voltage across the capacitor. This voltage clamps the voltage at the FB pin, causing PWM pulse width to increase slowly and in turn reduce the output surge current. The internal 0.6V reference takes over the loop control once the internal ramping-up voltage becomes higher than 0.6V. UVLO Protection The RT8089A has input Under Voltage Lockout protection (UVLO). If the input voltage exceeds the UVLO rising threshold voltage (2.4V typ.), the converter resets and prepares the PWM for operation. If the input voltage falls below the UVLO falling threshold voltage during normal operation, the device will stop switching. The UVLO rising and falling threshold voltage has a hysteresis to prevent noise-caused reset. The power sequence of the VCC and VIN need to be considered if they are powered separately. The driver voltage of high side MOSFET comes from VIN input and internal control circuit is powered by VCC. The VCC has to be powered earlier than the VIN to ensure that the high side MOSFET never turns on before the internal control circuit is ready. At power off, the voltage at the VIN has to be removed before the VCC voltage goes below the threshold of UVLO. Inductor Selection The switching frequency (on-time) and operating point (% ripple or LIR) determine the inductor value as shown below: ( ) OUT IN OUT SW LOAD(MAX) IN VV V L = f LIR I V ×− ×× × where LIR is the ratio of the peak-to-peak ripple current to the average inductor current. Find a low loss inductor having the lowest possible DC resistance that fits in the allotted dimensions. Ferrite cores are often the best choice, although powdered iron is inexpensive and can work well at 200kHz. The core must be large enough not to saturate at the peak inductor current (IPEAK) : PEAK LOAD(MAX) LOAD(MAX) LIR I = I + I 2 ⎛⎞ × ⎜⎟ ⎝⎠ OUT REF R1 V = V 1 R2 ⎛⎞ ×+ ⎜⎟ ⎝⎠ where VREF is the feedback reference voltage 0.6V (typ.). Figure 1. Setting VOUT with a Voltage Divider FB GND VOUT R1 R2 The calculation above serves as a general reference. To further improve transient response, the output inductor can be further reduced. This relation should be considered along with the selection of the output capacitor. |
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