Electronic Components Datasheet Search |
|
LM2753SD Datasheet(PDF) 6 Page - National Semiconductor (TI) |
|
|
LM2753SD Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Application Information (Continued) PFM REGULATION The LM2753 achieves its tightly regulated output voltage with pulse-frequency modulated (PFM) regulation. PFM sim- ply means the part only pumps when charge needs to be delivered to the output in order to keep the output voltage in regulation. When the output voltage is above the target regulation voltage the part idles, consuming minimal supply current with C 1 is connected between VIN and GND and VIN is disconnected from V OUT. In this state, the load current is supplied solely by the charge stored on the output capacitor. As this capacitor discharges and the output voltage falls below the target regulation voltage, the charge pump acti- vates, and charge is delivered to the output. This charge supplies the load current and boosts the voltage on the output capacitor. The primary benefit of PFM regulation is when output cur- rents are light and the part is predominantly in the low- supply-current idle state. Net supply current is minimal be- cause the part only occasionally needs to recharge the output capacitor by activating the charge pump. With PFM regulation, input and output ripple frequencies vary signifi- cantly, and are dependent on output current, input voltage, and to a lesser degree, other factors such as temperature, internal switch characteristics, and capacitor characteristics (voltage tolerance, temperature variation). OUTPUT VOLTAGE RIPPLE The voltage ripple on the output of the LM2753 is highly dependent on the application conditions. The output capaci- tance, input voltage, and output current each play a signifi- cant part in determining the output voltage ripple. Due to the complexity of the LM2753 operation, providing equations or models to approximate the magnitude of the ripple cannot be easily accomplished. However, the following general state- ments can be made. The output capacitor will have a significant effect on output voltage ripple magnitude. Ripple magnitude will typically be linearly proportional to the output capacitance present. The ESR of the output capacitor also contributes to the output voltage ripple, as there is effectively an AC voltage drop across the ESR due to current switching in and out of the capacitor. To keep the voltage ripple small, a low-ESR ce- ramic capacitor is recommended on the output. Placing mul- tiple capacitors in parallel can reduce ripple significantly, by both increasing capacitance and reducing ESR. When ca- pacitors are in parallel the ESR of the capacitors are in parallel as well, resulting in a net ESR according to the properties of parallel resistance. Two identical capacitors in parallel have twice the capacitance and half the ESR as compared to a single capacitor if the same type. On a similar note, if a large-value, high-ESR capacitor (tantalum, for ex- ample) is to be used as the primary output capacitor, the net ESR can be significantly reduced by placing a low-ESR ceramic capacitor in parallel with this primary output capaci- tor. I OUT PIN An internal FET is connected between the V OUT pin and the I OUT pin of the LM2753. When a logic high signal is placed on the Flash input pin, the internal FET turns on and con- nects I OUT to VOUT in less than 10ns (typ). If the IOUT pin is not going to be used, the Flash input pin can be tied to GND and the I OUT pin can be left unconnected. In the typical application circuit there is one resistor between V OUT and IOUT and another resistor between IOUT and the Flash LED. When a LOW logic signal is placed on the Flash input pin, the internal FET opens and current flows from V OUT through both resistors and through the Flash LED. When a logic HIGH signal is applied to the Flash input pin the internal FET closes, shorting out the resistor between V OUT and IOUT, and current flows through the second resis- tor and the Flash LED. Follow the steps below to set the desired current levels for the Flash LED: Setting Flash Current 1. Determine the LED’s forward voltage at the desired Flash current. 2. Find the voltage difference between I OUT and the LED forward voltage. 3. Divide the voltage difference by the desired Flash cur- rent to obtain the needed Flash LED ballast resistance Setting Torch Current 1. First determine required Flash Ballast 2. Determine the LED’s forward voltage at the desired continuous Torch current 3. Find the voltage difference between V OUT and the LED forward voltage. 4. Divide the voltage difference by the desired Torch cur- rent to obtain the total resistance needed. 5. Subtract the Flash Ballast resistance from this total re- sistance to find the required Torch resistance between V OUT and IOUT PWM BRIGHTNESS CONTROL PROCEDURES The brightness of a Flash LED connected to V OUT can be linearly varied from zero up to the maximum programmed current level by applying a Pulse-Width-Modulated signal to the EN pin of the LM2753. The following procedures illus- trate how to program the LED drive current and adjust the output current level using a PWM signal. 1. To select the maximum desired current level, refer to the "I OUT Pin" section and follow the steps detailed in the "Setting Flash Current" and "Setting Torch Current" sub- headings. 2. Brightness control for "Torch" mode can be implemented by pulsing a signal at the EN pin, while Flash is con- nected to a logic LOW signal. Also, brightness control can also be implemented for Flash mode by pulsing a signal on the Flash pin while the part is already enabled (EN = logic HIGH). LED brightness is proportional to the duty cycle (D) of the PWM signal. For linear brightness control over the full duty cycle adjustment range, the PWM frequency (f) should be limited during Torch mode to accommodate the turn-on time (T ON = 640µs) of the device. Also, the PWM frequency should be limited dur- ing "Flash" mode to accommodate the turn-on time (T FLASH = 10ns) of the IOUT output FET. D x (1/f) > T ON,FLASH f MAX =DMIN ÷TON,FLASH If the PWM frequency is much less than 100Hz, flicker may be seen in the LEDs. For the LM2753, zero duty cycle will turn off the LED and a 50% duty cycle will result in an average I OUT being half of the programmed LED current. For example, if the output is programmed for a maximum of 100mA through the Flash LED, a 50% duty cycle will result in an average I LED of 50mA. www.national.com 6 |
Similar Part No. - LM2753SD |
|
Similar Description - LM2753SD |
|
|
Link URL |
Privacy Policy |
ALLDATASHEET.NET |
Does ALLDATASHEET help your business so far? [ DONATE ] |
About Alldatasheet | Advertisement | Contact us | Privacy Policy | Link Exchange | Manufacturer List All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |