Electronic Components Datasheet Search |
|
MAX1928EUB25 Datasheet(PDF) 8 Page - Maxim Integrated Products |
|
MAX1928EUB25 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Detailed Description The MAX1927/MAX1928 PWM step-down DC-DC con- verters accept inputs as low as 2.6V, while delivering 800mA to output voltages as low as 0.75V. These devices operate in one of two modes to optimize noise and quiescent current. Under heavy loads, MAX1927/ MAX1928 operate in pulse-width modulation (PWM) mode and switch at a fixed 1MHz frequency. Under light loads, they operate in PFM mode to reduce power consumption. In addition, both devices provide selec- table forced PWM operation for minimum noise at all load currents. PFM Operation and PWM Control Scheme The PFM mode improves efficiency and reduces quies- cent current to 140µA at light loads. The MAX1927/ MAX1928 initiate pulse-skipping PFM operation when the peak inductor current drops below 130mA. During PFM operation, the MAX1927/MAX1928 switch only as necessary to service the load, reducing the switching frequency and associated losses in the internal switch, synchronous rectifier, and inductor. During PFM mode, a switching cycle initiates when the error amplifier senses that the output voltage has dropped below the regulation point. If the output volt- age is low, the P-channel MOSFET switch turns on and conducts current to the output filter capacitor and load. The PMOS switch turns off when the PWM comparator is satisfied. The MAX1927/MAX1928 then wait until the error amplifier senses a low output voltage to start again. Some jitter is normal during the transition from PFM to PWM with loads around 100mA. This has no adverse impact on regulation. At loads greater than 130mA, the MAX1927/MAX1928 use a fixed-frequency, current-mode, PWM controller capable of achieving 100% duty cycle. Current-mode feedback provides cycle-by-cycle current limiting, superior load and line response, as well as overcurrent protection for the internal MOSFET and synchronous rectifier. A comparator at the P-channel MOSFET switch detects overcurrent conditions exceeding 1.1A. During PWM operation, the MAX1927/MAX1928 regu- late output voltage by switching at a constant frequency and then modulating the power transferred to the load using the PWM comparator (Figure 1). The error-amp output, the main switch current-sense signal, and the slope compensation ramp are all summed at the PWM comparator. The comparator modulates the output power by adjusting the peak inductor current during the first half of each cycle based on the output-error volt- age. The MAX1927/MAX1928 have relatively low AC- loop gain coupled with a high-gain integrator to enable the use of a small, low-valued, output filter capacitor. The resulting load regulation is 0.3% (typ) from 0 to 800mA. Forced PWM Operation To force PWM-only operation, connect PWM to BATT. Forced PWM operation is desirable in sensitive RF and data-acquisition applications to ensure that switching noise does not interfere with sensitive IF and data sam- pling frequencies. A minimum load is not required dur- ing forced PWM operation because the synchronous rectifier passes reverse inductor current as needed to allow constant frequency operation with no load. Forced PWM operation has higher quiescent current than PFM (2mA typ compared to 140µA) due to contin- uous switching. 100% Duty-Cycle Operation The maximum on-time can exceed one internal oscilla- tor cycle, which permits operation at 100% duty cycle. As the input voltage drops, the duty cycle increases until the internal P-channel MOSFET stays on continu- ously. Dropout voltage at 100% duty cycle is the output current multiplied by the sum of the internal PMOS on- resistance (typically 0.25 Ω) and the inductor resis- tance. Near dropout, switching cycles can be skipped, reducing switching frequency. However, voltage ripple remains small because the current ripple is still low. Synchronous Rectification An N-channel synchronous rectifier eliminates the need for an external Schottky diode and improves efficiency. The synchronous rectifier turns on during the second half of each cycle (off-time). During this time, the volt- age across the inductor is reversed, and the inductor current falls. In normal mode, the synchronous rectifier is turned off when either the output falls out of regula- tion (and another on-time begins) or when the inductor current approaches zero. In forced PWM mode, the synchronous rectifier remains active until the beginning of a new cycle. Shutdown Mode Driving SHDN to GND places the MAX1927/MAX1928 in shutdown mode. In shutdown, the reference, control circuitry, internal switching MOSFET, and synchronous rectifier turn off and the output becomes high imped- ance. Drive SHDN high for normal operation. Input cur- rent falls to 0.1µA (typ) during shutdown mode. POK Output POK is an open-drain output that goes high impedance 20ms after the soft-start ramp has concluded and VFB is within 90% of the threshold. POK is low impedance when in shutdown. Low-Output-Voltage, 800mA, PWM Step-Down DC-DC Converters 8 _______________________________________________________________________________________ |
Similar Part No. - MAX1928EUB25 |
|
Similar Description - MAX1928EUB25 |
|
|
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 |