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MAX16928DGUPV Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX16928DGUPV Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 20 page ���������������������������������������������������������������� Maxim Integrated Products 11 MAX16928 Automotive TFT-LCD Power Supply with Boost Converter and Gate Voltage Regulators Positive-Gate Voltage Regulator (GH) The positive-gate voltage regulator includes a p-channel FET output stage to generate a regulated output between 5V and (VCP - 2V). The regulator maintains accuracy over wide line and load conditions. It is capable of at least 20mA of output current and includes current-limit protec- tion. VGH is typically used to provide the TFT-LCD gate drivers’ gate-on voltage. The regulator derives its positive supply voltage from a noninverting charge pump, a single-stage example of which is shown in the Typical Operating Circuit. A higher voltage using a multistage charge pump is possible, as described in the Charge Pumps section. Negative-Gate Voltage Regulator (GL) The negative-gate voltage regulator is an analog gain block with an open-drain p-channel output. It drives an external npn pass transistor with a 6.8kI base-to-emitter resistor (see the Pass Transistor Selection section). Its guaranteed base drive source current is at least 2mA. VGL is typically used to provide the TFT-LCD gate driv- ers’ gate-off voltage. The output of the negative-gate voltage regulator (i.e., the collector of the external npn pass transistor) has load- dependent bypassing requirements. Connect a ceramic capacitor between the collector and ground with the value shown in Table 3. The regulator derives its negative supply voltage from an inverting charge pump, a single-stage example of which is shown in the Typical Operating Circuit. A more negative voltage using a multistage charge pump is possible, as described in the Charge Pumps section. The external npn transistor is not short-circuit protected. To maintain proper pulldown capability of the external npn transistor and optimal regulation, a minimum load of at least 500FA is recommended on the output of the GL regulator. Enable (ENP) Use the enable input (ENP) to enable and disable the boost section of the device. Connect ENP to INA for normal operation and to GND to place the device in shut- down. In shutdown, the INA supply current is reduced to 0.5FA. Soft-Start and Supply Sequencing (SEQ) When enabled, the boost output ramps up from VINA to its set voltage. Once the boost output reaches 85% of the set voltage and the soft-start timer expires, the gate volt- age regulators turn on in the order shown in Table 1. The 1.8V/3.3V regulator controller is enabled at the beginning of the boost converter’s soft-start. Both gate voltage regulators have a 7.45ms soft-start time. The second one turns on as soon as the output of the first reaches 85% of its set voltage. Thermal Shutdown Internal thermal shutdown circuitry shuts down the device immediately when the die temperature exceeds +165NC. A 15NC thermal shutdown hysteresis prevents the device from resuming normal operation until the die temperature falls below +150NC. Design Procedure Boost Converter Inductor Selection Three key inductor parameters must be specified for operation with the device: inductance value (L), inductor saturation current (ISAT), and DC resistance (RDC). To determine the inductance value, select the ratio of inductor peak-to-peak ripple current to average output current (LIR) first. For LIR values that are too high, the RMS currents are high, and therefore, I2R losses are high. Use high-valued inductors to achieve low LIR values. Typically, inductance is proportional to resistance for a given package type, Table1.SupplySequencing CONTROLINPUTS SUPPLYSEQUENCING ENP SEQ FIRST SECOND THIRD 0 X Device is in shutdown 1 0 VSH VGH VGL 1 1 VSH VGL VGH |
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