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JMK316BJ226ML Datasheet(PDF) 9 Page - Richtek Technology Corporation |
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JMK316BJ226ML Datasheet(HTML) 9 Page - Richtek Technology Corporation |
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9 / 16 page  RT8015B 9 DS8015B-04 March 2011 www.richtek.com Application Information The basic RT8015B application circuit is shown in Typical Application Circuit. External component selection is determined by the maximum load current and begins with the selection of the inductor value and operating frequency followed by CIN and COUT. Output Voltage Programming The output voltage is set by an external resistive divider according to the following equation : Figure 1. Setting the Output Voltage ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + × = R2 R1 1 V V REF OUT Soft-Start The RT8015B contains an internal soft-start clamp that gradually raises the clamp on the COMP pin. The full current range becomes available on COMP after 2048 switching cycles as shown in Figure 2. Figure 2. Soft-Start Power Good Output The power good output is an open-drain output and requires a pull up resistor. When the output voltage is 12.5% above or 12.5% below its set voltage, PGOOD will be pulled low. It is held low until the output voltage returns to within the allowed tolerances once more. In soft start, PGOOD is actively held low and is allowed to transition high until soft start finished over and the output voltage reaches 87.5% of its set voltage. Operating Frequency Selection of the operating frequency is a tradeoff between efficiency and component size. High frequency operation allows the use of smaller inductor and capacitor values. Operation at lower frequency improves efficiency by reducing internal gate charge and switching losses but requires larger inductance and/or capacitance to maintain low output ripple voltage. The operating frequency of the RT8015B is determined by an external resistor that is connected between the RT pin and ground. The value of the resistor sets the ramp current that is used to charge and discharge an internal timing capacitor within the oscillator. The RT resistor value can be determined by examining the frequency vs. RT curve. Although frequencies as high as 2MHz are possible, the minimum on-time of the RT8015B imposes a minimum limit on the operating duty cycle. The minimum on-time is typically 110ns. Therefore, the minimum duty cycle is equal to 100 x 110ns x f(Hz). Figure 3 0 0.5 1 1.5 2 2.5 0 200 400 600 800 1000 ROSC (k ) ROSC (k Ω) RT = 152k for 2MHz RT = 330k for 1MHz RT8015B FB GND VOUT R1 R2 where VREF equals to 0.8V typical. The resistive divider allows the FB pin to sense a fraction of the output voltage as shown in Figure 1. Time (1ms/Div) ILX (1A/Div) VOUT (500mV/Div) VIN (2V/Div) VIN = 5V, VOUT = 1.05V, IOUT = 2A |
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