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RT8058PQW Datasheet(PDF) 10 Page - Richtek Technology Corporation |
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RT8058PQW Datasheet(HTML) 10 Page - Richtek Technology Corporation |
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10 / 15 page  RT8058 Preliminary 10 DS8058-02 August 2007 www.richtek.com Application Information Function Description The RT8058 is a 1MHz constant frequency, current mode PWM step-down converter. High switching frequency and high efficiency make it suitable for applications where high efficiency and small size are critical. Frequency compensation is done internally. The output voltages are set by external dividers returned to the FB pin. The output voltage can be set from 0.8V to 5V. Main Control Loop During normal operation, the internal top power switch (P-MOSFET) is turned on at the beginning of each clock cycle. Current in the inductor increases until the peak inductor current reach the value defined by the output voltage of the error amplifier. The error amplifier adjusts its output voltage by comparing the feedback signal from a resistor divider on the FB pin with an internal 0.6V reference. When the load current increases, it causes a reduction in the feedback voltage relative to the reference. The error amplifier raises its output voltage until the average inductor current matches the new load current. When the top power MOSFET shuts off, the synchronous power switch (N-MOSFET) turns on until the beginning of the next clock cycle. Soft Start / Enable For convenience of power up sequence control, RT8058 has an enable pin. Logic high at EN pin will enable the converter. When the converter is enabled, the clamped error amplifier output ramps up during 1024-clock period to increase the current provided by converter until the output voltage reach the target voltage. If EN is kept at high during Vin applying, RT8058 will be enabled when VDD surpass Under-Voltage Lockout threshold. Output Voltage Programming The output voltage is set by an external resistive divider according to the following equation : VOUT = VREF x (1+ R1/R2) where VREF equals to 0.6V typical. The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. RT8058 FB GND VOUT R1 R2 Figure 1. Setting the Output Voltage Slope Compensation and Inductor Peak Current Slope compensation prov ides stability in constant frequency architectures by preventing sub harmonic oscillations at duty cycles greater than 50%. It is accomplished internally by adding a compensating ramp to the inductor current signal. Normally, the maximum inductor peak current is reduced when slope compensation is added. In RT8058, however, separated inductor current signal is used to monitor over current condition and this keeps the maximum output current relatively constant regardless of duty cycle. Dropout Operation When input supply voltage decreases toward the output voltage, the duty cycle increases toward the maximum on time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle eventually reaching 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-MOSFET and the inductor. Low Supply Operation The RT8058 is designed to operate down to an input supply voltage of 2.7V. One important consideration at low input supply voltages is that the RDS(ON) of the P-Channel and N-Channel power switches increases. The user should calculate the power dissipation when the RT8058 is used at 100% duty cycle with low input voltages to ensure that thermal limits are not exceeded. |
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