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TPS61162 Datasheet(PDF) 9 Page - Texas Instruments |
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TPS61162 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 31 page 9 TPS61162D www.ti.com SLVSC13A – JULY 2013 – REVISED MARCH 2016 Product Folder Links: TPS61162D Submit Documentation Feedback Copyright © 2013–2016, Texas Instruments Incorporated 8.3 Feature Description 8.3.1 Normal Operation In order to provide high brightness backlighting for large-sized or high-resolution smart-phone panels, a greater number of white LED diodes are used. Having all LED diodes in a string improves overall current matching; however, the output voltage of a boost converter is limited when input voltage is low, and normally the efficiency drops when the output voltage is very high. For these reasons, the TPS61162D is designed to configure the LED diodes in two parallel strings. 8.3.2 Boost Converter The boost converter of the TPS61162D integrates a 40-V, 1.5-A low-side switch MOSFET and has a fixed switching frequency of 1.2 MHz. The control architecture is based on traditional current-mode PWM control. (For operation see Functional Block Diagram.) Two current sinks regulate the dual-channel current, and the boost output is automatically set by the voltage of the regulating IFBx pin. The output of the error amplifier and the sensed current of the switch MOSFET are applied to a control comparator to generate the boost switching duty cycle; slope compensation is added to the current signal to allow stable operation for duty cycles larger than 50%. In order to ensure that both current sinks remain in regulation whenever there is a mismatch in string voltages while the power dissipation of the current sink regulators is minimized, the minimum headroom voltage between IFB1 and IFB2 becomes the regulation point for the boost converter. For example, if the LEDs connected to IFB1 require 20 V, and the LEDs connected to IFB2 require 20.5 V at the programmed current, then the voltage at IFB2 is about 90 mV, and the voltage at IFB1 is about 0.59 V. In other words, the boost makes the cathode of the highest voltage LED string the regulation point. 8.3.3 IFBx Pin Unused If only one channel is needed, a user can easily disable the unused channel by connecting its IFBx pin to ground. If both IFBx pins are connected to ground, the device does not start up. 8.3.4 Enable and Start-Up In order to enable the device from shutdown mode, three conditions have to be met: 1. Power On Reset (POR), that is, VIN voltage is higher than UVLO threshold 2. Logic high on EN pin 3. PWM signal (logic high or PWM pulses) on PWM pin When all these conditions are met, an internal LDO linear regulator is enabled to provide supply to internal circuits, and the device can start up. The TPS61162D supports two dimming interfaces: one-wire digital interface (EasyScale interface) and PWM interface. The device begins an EasyScale detection window after start-up to detect which interface is selected. If the EasyScale interface is needed, signals of a specific pattern must be input into the EN pin during the EasyScale detection window; otherwise, PWM dimming interface is enabled (see details in One-Wire Digital Interface (Easyscale Interface) ). After the EasyScale detection window, the TPS61162D checks the status of IFBx pins. If one IFBx pin is detected to connect to ground, the corresponding channel is disabled and removed from the control loop. The soft start then begins, and the boost converter starts switching. If both IFBx pins are shorted to ground, the TPS61162D does not start up. Either pulling EN pin low for more than 2.5 ms, or pulling the PWM pin low for more than 20 ms, can disable the device, and the TPS61162D enters into shutdown mode. 8.3.5 Soft Start Soft start is implemented internally to prevent voltage overshoot and inrush current. After the IFBx pin status detection, the COMP pin voltage starts ramp up, and the boost starts switching. During the beginning 5 ms (tHalf_LIM) of the switching, the peak current of the switch MOSFET is limited at ILIM_Start (0.7 A typical) to prevent excess inrush input current. After 5 ms the current limit is changed to ILIM (1.5 A typical) to allow the normal operation of the boost converter. |
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