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MAX16826 Datasheet(PDF) 15 Page - Maxim Integrated Products |
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MAX16826 Datasheet(HTML) 15 Page - Maxim Integrated Products |
15 / 26 page rising edge of the clock. When using external synchro- nization, the clock frequency set by RTCT must be 10% lower than the synchronization signal frequency. Overvoltage Protection (OVP) OVP limits the maximum voltage of the switching regu- lator output for protection against overvoltage due to circuit faults, for example a disconnected FB. Connect OVP to the center of a resistor-divider connected between the switching regulator output and GND to set the output-voltage OVP limit. Typically, the OVP output voltage limit is set higher than the load dump voltage. Calculate the value of R15 and R16 as follows: R15 = (VOVP/1.25 - 1) x R16 Or to calculate VOVP: VOVP = 1.25 x (1 + R15/R16) where R15 and R16 are shown in the Typical Application Circuit. The internal OVP comparator compares the volt- age at OVP with the internal reference (1.25V typ) to decide if an overvoltage error occurs. If an overvoltage error is detected, switching stops, the switching regula- tor gate-drive output is latched off, and the soft-start capacitor is discharged. The latch can only be reset by toggling SYNC/EN, activating the I2C standby mode, or cycling power. The internal ADC also uses OVP to sense the switching regulator output voltage. Output voltage measurement information can be read back from the I2C interface. Voltage is digitized to 7-bit resolution. Undervoltage Lockout (UVLO) When the voltage at VCC is below the VCC undervolt- age threshold (VVCC_UVLO, typically 4.3V falling), the MAX16826 enters undervoltage lockout. VCC UVLO forces the linear regulators and the switching regulator into shutdown mode until the VCC voltage is high enough to allow the device to operate normally. In VCC UVLO, the VCC regulator remains active. Thermal Shutdown The MAX16826 contains an internal temperature sensor that turns off all outputs when the die temperature exceeds +160°C. The outputs are enabled again when the die temperature drops below +140°C. In thermal shutdown, all internal circuitry is shut down with the exception of the shunt regulator. Linear Current Sources (CS1–CS4, DL1–DL4) The MAX16826 uses transconductance amplifiers to con- trol each LED current sink. The amplifier outputs (DL1–DL4) drive the gates of the external current sink FETs (Q2 to Q5 in the Typical Application Circuit). The source of each MOSFET is connected to GND through a current- sense resistor. CS1–CS4 are connected to the respective inverting input of the amplifiers and also to the source of the external current sink FETs where the LED string cur- rent-sense resistors are connected. The noninverting input of each amplifier is connected to the output of an internal DAC. The DAC output is programmable using the I2C inter- face to output between 97mV and 316mV. The regulated string currents are set by the value of the current-sense resistors (R28 to R31 in the Typical Application Circuit) and the corresponding DAC output voltages. LED PWM Dimming (DIM1–DIM4) The MAX16826 features a versatile dimming scheme for controlling the brightness of the four LED strings. Independent LED string dimming is accomplished by dri- ving the appropriate DIM1–DIM4 inputs with a PWM sig- nal with a frequency up to 100kHz. Although the brightness of the corresponding LED string is proportional to the duty cycle of its respective PWM dimming signal, finite LED current rise and fall times limit this linearity when the dim pulse width approaches 2µs. Each LED string can be independently controlled. Simultaneous control of the PWM dimming and the LED string currents in an ana- log way over a 3:1 range provides great flexibility allowing independent two-dimensional brightness control that can be used for color point setup and brightness control. Analog-to-Digital Converter (ADC) The MAX16826 has an internal ADC that measures the drain voltage of the external current sink driver FETs (Q2 to Q5 in the Typical Application Circuit) using DR1 - DR4 and the switching regulator output voltage using OVP. Fault monitoring and switching stage out- put-voltage optimization is possible by using an exter- nal microcontroller to read out these digitized voltages through the I2C interface. The ADC is a 7-bit SAR (suc- cessive-approximation register) topology. It sequential- ly samples and converts the drain voltage of each channel and VOVP. An internal 5-channel analog MUX is used to select the channel the ADC is sampling. Conversions are driven by an internally generated 1MHz clock and gated by the external dimming sig- nals. After a conversion, each measurement is stored into its respective register and can be accessed through the I2C interface. The digital circuitry that con- trols the analog MUX includes a 190ms timer. If the ADC does not complete a conversion within this 190ms measurement window then the analog MUX will sequence to the next channel. For the ADC to complete one full conversion, the cumulative PWM dimming on- time must be greater than 10µs within the 190ms mea- surement window. The minimum PWM dimming on-time Programmable, Four-String HB LED Driver with Output-Voltage Optimization and Fault Detection ______________________________________________________________________________________ 15 |
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