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TPS7510X Datasheet(PDF) 10 Page - Texas Instruments |
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TPS7510X Datasheet(HTML) 10 Page - Texas Instruments |
10 / 16 page www.ti.com USE OF ENABLE PINS FOR PWM DIMMING LOAD REGULATION LINE REGULATION ENA D1A D2A D1B D2B ENB I SET GND TPS7510x V IN DimmingPWM orCPUGPIO R SET (optional) Li-Ion Battery TPS7510x SBVS080E – SEPTEMBER 2006 – REVISED JANUARY 2008 The TPS7510x divides control of the LED outputs into The TPS7510x is designed to provide very tight load two banks of two current sources each. Each bank is regulation. In the case of a fixed current source, the controlled by the use of an independent, active-high output load change is a change in voltage. Tight load enable pin (ENA and ENB). The enable pin can be regulation means that output voltages (LED forward used for standard ON/OFF operation of the current voltages) with large variations can be used without source, driven by standard logic levels from impacting the fixed current being sourced by the processor GPIO pins, for example. Drive EN high to output or the output-to-output current matching. The turn on the bank of LEDs; drive EN low to turn off the permissible variation on the output not only allows for bank of LEDs. large variations in white LED forward voltages, but even permits the use of different color LEDs on Another use of the enable pin is for LED dimming. different outputs with minimal effect on output current. LED brightness is a function of the current level being driven across the diode and the time that current is being driven through the diode. The perceived brightness of an LED can be changed by either The TPS7510x is also designed to provide very tight varying the current level or, more effectively, by line regulation. This architecture allows for voltage changing the time in which that current is present. transient events to occur on the power supply When a PWM signal is input into the enable pin, the (battery) without impacting the fixed output current duty cycle (high or ON time) determines how long the levels or the output to output current matching. A fixed current is driven across the LEDs. Reducing or prime example of such a supply transient event is the increasing that duration has the effect of dimming or occurrence of a transmit pulse on the radio of a brightening the LED, without having to employ the mobile handset. These transient pulses can cause more complex method of varying the current level. variations of 300mV and 600mV on the supply to the This technique is particularly useful for reducing LED TPS7510x. The line regulation limitation is that the brightness in low ambient light conditions, where LED lower supply voltage level of the event does not brightness is not required, thereby decreasing current cause the input to output voltage difference to drop consumption. The enable pins can also be used for below the dropout voltage range. LED blinking, varying blink rates based on system status. Although providing many useful applications, PWM dimming does have a minimum duty cycle required to achieve the required current level. The recommended minimum on time of the TPS7510x is approximately 33 µs. On times less than 33µs result in reductions in the output current by not allowing enough time for the output to reach the desired current level. Also, having both enables switching together, asynchronously, or having one enable on at all times, impacts the minimum recommended on time (see Figure 4 and Figure 5). If one enable is already on, the speed at which the other channel turns on is faster than if both channel were turning on together or if the other channel is off. Therefore, connecting one enable on Figure 13. Typical Application Diagram allows for approximately 10 µs to 12µs shorter minimum on times of the switching enable channel. Unused enable pins can be left unconnected or connected to ground to minimize current consumption. Connecting unused enable pins to ground increases ESD protection. If connected to VIN, a small amount of current drains through the enable input (see the Electrical Characteristics table). 10 Submit Documentation Feedback Copyright © 2006–2008, Texas Instruments Incorporated Product Folder Link(s): TPS7510x |
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