Electronic Components Datasheet Search |
|
CS5127GDWR16 Datasheet(PDF) 10 Page - Cherry Semiconductor Corporation |
|
CS5127GDWR16 Datasheet(HTML) 10 Page - Cherry Semiconductor Corporation |
10 / 24 page discharge time is typically less than 10% of the charge time. External components CT and RT allow the switching frequency to be set by the user in the range between 10kHz and 500kHz. CT can be chosen first based on size and cost constraints. For proper operation over temperature, the value of RT should be chosen within the range from 20k½ to 40k½. Any type of one-eighth watt resistor will be ade- quate. Larger values of RT will decrease the maximum duty cycle slightly. This occurs because the sink current on the CT lead has an exponential relationship to the charge current. Higher charge currents will discharge the CT lead capacitor more quickly than lower currents, and a shorter discharge time will result in a higher maximum duty cycle. Once the oscillator frequency and a value of CT have been selected, the necessary value of RT can be calculated as fol- lows: RT = where fOSC is the oscillator frequency in hertz, CT is given in farads, and the value of RT is given in ohms. ESR effects are negligible since the charge and discharge currents are fairly small, and any type of capacitor is adequate for CT. As previously noted, the error amplifier does not con- tribute greatly to transient response, but it does influence noise immunity. The fast feedback loop input is compared against the COMP pin voltage. The DC bias to the VFFB pin may be provided directly from the output voltage, or through a resistor divider if output voltage is greater than 2.9V. The desired percentage value of DC accuracy trans- lates directly to the VFFB pin, and the minimum COMP pin capacitor value can be calculated: CCOMP = If fOSC = 200kHz, VFFB DC bias voltage is 2.8V and toler- ance is 0.1%, CCOMP = 28.6µF. This is the minimum value of COMP pin capacitance that should be used. It is a good practice to guard band the tolerance used in the calcula- tion. Larger values of capacitance will improve noise immunity, and a 100µF capacitor will work well in most applications. The type of capacitor is not critical, since the amplifier output sink current of 16mA into a fairly large value or wide range of ESR will typically result in a very small DC output voltage error. The COMP pin capacitor also deter- mines the length of the soft start interval. The input bypass capacitors minimize the ripple current in the input supply, help to minimize EMI, and provide a charge reservoir to improve transient response. The capac- itor ripple current rating places the biggest constraint on component selection. The input bypass capacitor network should conduct all the ripple current. RMS ripple current can be as large as half the load current, and can be calcu- lated as: IRIPPLE(RMS) =IOUT Peak current requirement, load transients, ambient operat- ing temperature and product reliability requirements all play a role in choosing this component. Capacitor ESR and the maximum load current step will determine the maxi- mum transient variation of the supply voltage during normal operation. The drop in the supply voltage due to load transient response is given as: ÆV = IRIPPLE(RMS) ´ ESR The type of capacitor is also an important consideration. Aluminum electrolytic capacitors are inexpensive, but they typically have low ripple current ratings. Choosing larger values of capacitance will increase the ripple current rating, but physical size will increase as well. Size con- straints may eliminate aluminum electrolytics fro consideration. Aluminum electrolytics typically have shorter operating life because the electrolyte evaporates during operation. Tantalum electrolytic capacitors have been associated with failure from inrush current, and man- ufacturers of these components recommended derating the capacitor voltage by a ratio 2:1 in surge applications. Some manufacturers have product lines specifically tested to withstand high inrush current. AVX TPS capacitors are one such product. Ceramic capacitors perform well, but they are also large and fairly expensive. At startup, output switching does not occur until two undervoltage lockouts release. The first lockout monitors the VIN lead voltage. No internal IC activity occurs until VIN lead voltage exceeds the VIN turn-on threshold. This threshold is typically 8.4V. Once this condition is met, the on-chip reference turns on. As the reference voltage begins to rise, a second undervoltage lockout disables switching until VREF lead voltage is about 3.5V. The GATE leads are held in a low state until both lockouts are released. As switching begins, the VFB lead voltage is lower than the output voltage. This causes the error amplifier to source current to the COMP lead capacitor. The COMP lead volt- age will begin to rise. As the COMP lead voltage begins to rise, it sets the threshold level at which the rising VFFB lead voltage will trip the PWM comparator and terminate switch conduction. This process results in a soft start inter- val. The DC bias voltage on VFFB will determine the final COMP voltage after startup, and the soft start time can be approximately calculated as: TSOFT START = VFFB ´ CCOMP ICOMP(SOURCE) Startup VOUT(VIN - VOUT) VIN2 Selecting the Input Bypass Capacitor (16mA)(TOSC) (VFFBDC Bias Voltage)(tolerance) Selecting the Compensation Capacitor 1.88 (fOSC)(CT) 10 Applications Information: continued |
Similar Part No. - CS5127GDWR16 |
|
Similar Description - CS5127GDWR16 |
|
|
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 |