Moteur de recherche de fiches techniques de composants électroniques |
|
ADP1147AR-5 Fiches technique(PDF) 7 Page - Analog Devices |
|
ADP1147AR-5 Fiches technique(HTML) 7 Page - Analog Devices |
7 / 12 page ADP1147-3.3/ADP1147-5 –7– REV. 0 APPLICATIONS The ADP1147 family of regulators incorporate a current mode, constant off-time architecture to switch an external P-channel MOSFET. The external MOSFET can be switched at frequen- cies up to 250 kHz. The switching frequency of the device is determined by the value selected for capacitor CT. A regulated output voltage is maintained by the feedback volt- age at the SENSE(–) pin. The SENSE(–) pin is connected to an internal voltage divider. The voltage from this internal divider is fed to comparator V, and gain block G. It is then compared to an internal 1.25 volt reference. The ADP1147 is capable of maintaining high levels of efficiency by automatically switching between the power saving and con- tinuous modes. The internal R-S flip-flop #2 controls the device in the power saving mode, and gain block G assumes control when the device is in the continuous mode of operation. During the P-MOSFET on time, the voltage developed across RSENSE is monitored by the SENSE(–) and SENSE(+) pins of the device. When this voltage reaches the threshold level of comparator C the output trips, switching the P drive to VIN, and turns the external P-MOSFET off. At this point capacitor CT begins to discharge at a rate that is determined by the off-time controller. The CT discharge current is proportional to the voltage measured at the SENSE(–) pin. When the voltage on cap CT decays to the threshold voltage (VTH1), comparator T switches and sets R-S flip-flop #1. This forces the P-drive out- put low, and turns on the P-MOSFET. The sequence is then repeated. As the load current is increased, the output voltage starts to drop. This causes the gain circuit to raise the threshold of the current comparator, and the load current is now tracked. When load currents are low, comparator B sets the R-S flip-flop #2 and asserts the power savings mode of operation. Compara- tor B monitors the voltage developed across RSENSE. As the load current decreases to 50% of the designed inductor ripple cur- rent, the voltage reverses polarity. This reversal causes compara- tor B to trip, setting the Q-bar output of R-S flip-flop #2 to a logic zero, and interrupts the cycle by cycle operation of the output. The output storage capacitors are then slowly discharged by the load. When the output cap voltage decays to the VOS level of comparator V, it resets flip-flop #2, and the normal cycle by cycle mode of operation resumes. If load currents are extremely small, the time it takes for flip-flop #2 to reset increases. During the extended wait for reset period, capacitor CT will discharge below the value of VTH2 causing comparator S to trip. This forces the internal sleep bar low and the device enters the sleep mode. A significant amount of the IC is disabled during the sleep mode, reducing the ground current from 1.6 mA to 160 µA, typical. In sleep mode the P-MOSFET is turned off until additional inductor current is required. The sleep mode is terminated when flip-flop #2 is reset. Due to the constant off-time architecture, the input voltage has an effect on the device switching frequency. To limit the effects of this variation in frequency the discharge current is increased as the device approaches the dropout voltage of VIN +1.5 V. In the dropout mode the P-MOSFET is constantly turned on. Determining the Output Current and the Value for RSENSE The value selected for RSENSE is determined by the required output current. The current comparator C has a threshold volt- age range of 10 mV/RSENSE to 150 mV/RSENSE maximum. This threshold sets the peak current in the external inductor and yields a maximum output current of: I MAX = I PEAK – I RIPPLE p − p 2 The resistance values for RSENSE can range from 20 m Ω to 200 m Ω. A graph for selecting R SENSE vs. the maximum out- put current is shown in Figure 3. The value of RSENSE can be determined by using the following equation: RSENSE (in mΩ) = 100/IMAX This equation allows for a design margin due to component variations. The following equations are used to approximate the trip point for the power savings mode and the peak short circuit current. IPOWER SAVINGS ~ 5 mV/RSENSE + VO tOFF/2L ISC(PK) = 150 mV/RSENSE The ADP1147 automatically increases the tOFF time when a short circuit condition is encountered. This allows sufficient time for the inductor to decay between switching cycles. Due to the resulting inductor ripple current the average short circuit current ISC(AVG) is reduced to approximately IMAX. Determining the Operating Frequency and Selecting Values for CT and L The ADP1147 incorporates a constant off-time architecture to switch an external P-MOSFET. The off-time (tOFF) is deter- mined by the value of the external timing cap CT. When the P-MOSFET is turned on the voltage across CT is charged to approximately 3.3 volts. During the switch off-time the voltage on CT is discharged by a current that is proportional to the voltage level of VOUT. The voltage across CT is representative of the current in the inductor, which decays at a rate that is pro- portional to VOUT. Due to this relationship the value of the inductor must track the value selected for CT. The following equation is used to determine the desired con- tinuous mode operating frequency: CT = 1 − VOUT +V D V IN +V D 1.3 ×104 × f VD = the voltage drop across the Schottky diode. The graph in Figure 4 can be used to help determine the capaci- tance value of CT vs. the operating frequency and input voltage. The P-MOSFET gate charge losses increase with the operating frequency and results in lower efficiency (see the Efficiency section). |
Numéro de pièce similaire - ADP1147AR-5 |
|
Description similaire - ADP1147AR-5 |
|
|
Lien URL |
Politique de confidentialité |
ALLDATASHEET.FR |
ALLDATASHEET vous a-t-il été utile ? [ DONATE ] |
À propos de Alldatasheet | Publicité | Contactez-nous | Politique de confidentialité | Echange de liens | Fabricants 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 |