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LM3520SD Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM3520SD Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Circuit Description The LM3520 is designed for White LED & OLED backlighting in mobile phone applications. It has a main display loop which can drive up to 5 white LEDS in series and a sub display loop which is designed to drive OLED up to 20V/50 mA. The main display loop employs a fixed frequency cur- rent mode scheme to regulate the LED current. The sub display loop employs a fixed frequency gated oscillator scheme to regulate the output voltage. The device has two independent control pins to enable the Main or Sub displays. Note that both displays can not be ON at the same time. PWM Operation The LM3520 utilizes a synchronous Current Mode PWM control scheme to regulate the feedback voltage over all load and line conditions for the main display. The LM3520 is internally compensated preventing the need for external compensation components yielding a compact solution. The operation can best be understood referring to the functional block diagram. The LM3520 operates as follows: During the first cycle, the oscillator sets the driver logic and turns on the NMOS power device conducting current through the inductor and reverse biases the external diode isolating the output from the V SW node. The LED current is supplied by the output capacitor when the NMOS power device is active. During this cycle, the output voltage of the EAMP controls the current through the inductor. This voltage will increase for larger loads and de- crease for smaller loads limiting the peak current in the inductor. The sum of the EAMP voltage and voltage ramp is compared with the sensed switch voltage. Once these volt- ages are equal, the PWM COMP will then reset the logic turning off the NMOS power device and forward biasing the external diode to the white LED load and flows through the diode to the white LED load and output capacitor. The induc- tor current recharges the output capacitor and supplies the current for the white LED branches. The oscillator then sets the driver logic again repeating the process. PFM Operation The LM3520 utilizes a gated oscillator control scheme for the sub-display. There is a hysteresis window to regulate the output voltage. The oscillator frequency is the same as the frequency in PWM control. The Duty cycle of the oscillator signal is always set to maximum. During the first part of each switching cycle, the internal NMOS switch is turned on until the PFM current limit is reached. When the NMOS is off, the voltage of the inductor reverses and forces current through the diode to the output capacitor. This process continues until the upper comparator hysteresis is reached at which point the NMOS is disabled until the lower comparator threshold is reached and the process repeats again. Current Limit Protection The LM3520 has current limiting protection to prevent exces- sive stress on itself and external components during over- load conditions. The internal current limit comparator will disable the NMOS power device at a typical switch peak current limit of 700 mA. Output Over-Voltage Protection The LM3520 contains dedicated circuitry for monitoring the output voltage. In the event that the primary LED network is disconnected the output will increase and be limited to 23.2V (typ.). There is a ~1V hysteresis associated with this cir- cuitry, which will turn the NMOS off when the output voltage is at 24.2V(max.) until the output voltage reach 22.5V(typ.) or lower. The 23.5V limit allows the use of 25V 1 µF ceramic output capacitors creating an overall small solution for white LED applications. Under Voltage Protection The LM3520 has an UVP comparator to turn the NMOS power device off in case the input voltage or battery voltage is too low preventing an on state of the power device con- ducting large amounts of current. Reliability and Thermal Shutdown The LM3520 has an internal thermal shutdown function to protect the die from excessive temperatures. The thermal shutdown trip point is typically 160˚C, Normal operation re- sumes when the temperature drops below 140˚C. Startup The LM3520 does not include a power on reset circuit and relies on external signal to monitor enable signal. In the event of under voltage condition, the device enable pin must be brought low until the input voltage is above the minimum guarantee voltage (2.7V). Application Information SETTING LED CURRENT The White LED current is set using the following equation: For main display: (1) PWM CONTROL The LED current can be controlled using a PWM signal on the enable pin with frequencies in the range of 100 Hz to 1 kHz. LM3520 LED current can be controlled with PWM signal frequencies above 1 kHz but LED current is not linearly porportional to the duty cycle. The maximum LED current would be achieved using the equation above with 100% duty cycle, ie. The enable pin is always high. SETTING SUB VOLTAGE Sub-display voltage is be set by choosing R SUB1 and RSUB2 as illustrated in Figure 4.V SUB is calculated as follow: (2) www.national.com 11 |
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