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MAX1729 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX1729 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page inductor current to ramp down and VPS to increase. If, at the end of tOFF, VPS - VOUT is still too low, then another tON is initiated immediately. Otherwise, the boost con- verter remains idle in a low-quiescent-current state until VPS - VOUT drops again and the error comparator initi- ates another cycle. Linear Regulator The PNP low-dropout linear regulator of the MAX1729 regulates the boost-converter output to the desired out- put voltage. The boost converter’s regulation circuitry holds the linear regulator’s input voltage (VPS) approxi- mately 0.6V above the output voltage to keep the regu- lator out of dropout, thereby enhancing ripple rejection. The linear regulator incorporates short-circuit protec- tion, which limits the output current to approximately 6mA. Temperature Sensor Output The MAX1729 generates a temperature sensor voltage (VTC) that varies at 16.5mV/°C (typ) and is nominally equal to the reference voltage at room temperature. TC is capable of sinking or sourcing 50µA. This output is used to compensate for ECB color or LCD contrast variations caused by changes in temperature. It may be read with an ADC and used to modify an external PWM control signal or, in external feedback mode, summed directly into the feedback-resistor network. Control Signal An externally generated PWM control signal on CTLIN controls VOUT in internal feedback mode and influ- ences VOUT in external feedback mode. In either mode, if CTLIN is held low for longer than 1.24ms, the MAX1729 enters shutdown mode, decreasing the sup- ply current below 2µA. Shutdown mode limits the mini- mum duty cycle and frequency that may be used to keep the device active. CTLIN frequencies between 2kHz and 12kHz are recommended. Internal Feedback Mode In internal feedback mode, the signal at CTLIN is inverse- ly buffered, level-shifted, and output at COMP through a resistor. Internal resistance (33k Ω typical) and C6 then fil- ter the signal before it is used by the internal feedback network to set VOUT. If temperature compensation is used, the temperature sensor output voltage is read by an ADC and used to adjust the duty cycle of the PWM control signal. See the Designing for Internal Feedback Mode section for more information. External Feedback Mode In external feedback mode, the output voltage of the MAX1729 is controlled by the duty cycle of the PWM control signal and an external resistor network, as shown in Figure 3. In this mode, the signal at CTLIN is inverted, level-shifted, and presented directly to COMP. R3, R4, and C6 filter the signal, before it is summed into the feedback node. Design Procedure Designing for Internal Feedback Mode For a 3kHz PWM control signal use a 1µF low-leakage ceramic capacitor for C6. For applications requiring a higher-frequency PWM control signal, reduce the value of C6 to between 1µF and 0.22µF for frequencies between 3kHz and 12kHz. Higher C6 values reduce output ripple. In Figure 2, VOUT is governed by the fol- lowing equation: where VOUT(MIN) is 2.45V and Gain is nominally 13.95V/100%, as listed in the Electrical Characteristics. V = V Duty Cycle Gain OUT OUT(MIN) +⋅ ECB and LCD Display Bias Supply with Accurate Output Voltage and Temperature Compensation _______________________________________________________________________________________ 7 122mV VREF GND NOTE: SWITCH STATES SHOWN FOR INTERNAL FEEDBACK MODE. COMP TC LX PS OUT MAX1729 IN REF FB CTLIN BOOST CONVERTER TEMPERATURE SENSOR VOLTAGE REFERENCE SHUTDOWN CONTROL LDO LINEAR REGULATOR FEEDBACK CONTROL GND Figure 1. Internal Block Diagram |
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