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MC34845 데이터시트(PDF) 11 Page - Freescale Semiconductor, Inc |
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MC34845 데이터시트(HTML) 11 Page - Freescale Semiconductor, Inc |
11 / 21 page Analog Integrated Circuit Device Data Freescale Semiconductor 11 34845 FUNCTIONAL DEVICE OPERATION INTRODUCTION Figure 5. Power up sequence case 2, VIN applied after the ENABLE and PWM signals. VIN ramp up time between 0V and 5V should be not higher than 2ms. BOOST CONVERTER The boost converter uses a Dynamic Headroom Control (DHC) loop to automatically set the output voltage needed to drive the LED strings. The DHC is designed to operate under specific pulse width conditions in the LED drivers. It operates for pulse widths higher than 400 ns. If the pulse widths are shorter than specified, the DHC circuit will not operate and the voltage across the LED drivers will increase to a value given by the OVP, minus the total LED voltage in the LED string. It is therefore imperative to select the proper OVP level to avoid exceeding the max off state voltage of the LED drivers (45 V). The boost operates in current mode and is compensated externally through a type 2 network on the COMP pin. A modification of the compensation network is suggested to minimize the amplitude of the ripple at VOUT. The details of the suggested compensation network are shown in Figures 10 and 11. An integrated 2.0 A minimum FET supplies the required output current. An Over-current Protection circuit limits the output current cycle-by-cycle to IOCP. If the condition exists longer than 10 ms, then the device will shut down. The frequency of the boost converter is internally set to 300 kHz, 600 kHz or 1.2 MHz, depending on the device’s version. The boost also includes a soft start circuit. Each time the IC comes out of shutdown mode, the soft start period lasts for tSS. Over-voltage Protection is also included. The device has an internally fixed OVP value of 60 V (typical) which serves as a secondary fault protection mechanism, in the event the externally programmed OVP fails (i.e. resistor divider opens up). While the internal 60 V OVP detector can be used exclusively without the external OVP network, this is only recommended for applications where the LED string voltage approaches 55 V or more. The OVP level can be set by using an external resistor divider connected between the output voltage and ground with its output connected to the OVP pin. The OVP can be set up to 60 V by varying the resistor divider to match the OVP internal reference of 6.9 V (typical). LED DRIVER The 6 channel LED driver provides current matching for 6 LED strings to within ±2% maximum. The current in the strings is set using a resistor tied to GND from the ISET pin. The LED current level is given by the equation: RSET = 153/ ILED. The accuracy of the RSET resistor should be 0.1% for best performance. LED ERROR DETECT If an LED is open, the output voltage ramps to the OVP level. If there is still no current in the LED string, the LED channel is turned off and the output voltage ramps back down to normal operating level. If LEDs are shorted and the voltage in any of the channels is greater than the SFDV threshold (7.0 V typical), then the device will turn off that channel. However if the on-time of the channels is less than 10 μs, the SFDV circuit will not disable any of the channels, regardless of the voltage across them. All the LED errors can be cleared by recycling the EN pin or applying a complete power-on-reset (POR). WAKE OPERATION The WAKE pin provides the means to set the device for low power consumption (shutdown mode) without the need of an extra logic signal for enable. This is achieved by connecting the WAKE and PWM pins together, and tying the EN pin to ground. In this configuration, the PWM signal is used to control the LED channels, while allowing low power consumption by setting the device into its shutdown mode every time the PWM signal is kept low for longer time than the WAKE time out of 27 ms. OVER-TEMPERATURE SHUTDOWN AND TEMPERATURE CONTROL CIRCUITS The 34845 includes over-temperature protection. If the internal temperature exceeds the over-temp threshold OTTSHUTDOWN, then the device shuts down all functions. Once the temperature falls below the low level threshold, the device is re-enabled. FAIL PIN The FAIL pin is at its low-impedance state when no error is detected. However, if an error such as an LED channel open or boost over-current is detected, the FAIL pin goes into high-impedance. Once a failure is detected, the FAIL pin can be cleared by recycling the EN pin or applying a complete power-on-reset (POR). If the detected failure is an Over- current time-out, the EN pin or a POR must be cycled/ executed to restart the part. VIN EN PWM VOUT Boost Soft Start 5V 2ms VIN ramp UVLO Rising |
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