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MAX1703ESE Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX1703ESE Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 16 page Synchronized PWM Operation The MAX1703 can be synchronized in PWM mode to a 200kHz to 400kHz frequency by applying an external clock to CLK/SEL. This allows the user to set the har- monics to avoid IF bands in wireless applications. The synchronous rectifier is also active during synchronized PWM operation. Low-Power PFM Operation Pulling CLK/SEL low places the MAX1703 in a low- power mode. During low-power mode, PFM operation regulates the output voltage by transferring a fixed amount of energy during each cycle, and then modulat- ing the pulse frequency to control the power delivered to the output. The devices switch only as needed to service the load, resulting in the highest possible effi- ciency at light loads. Output current capability in PFM mode is 150mA (max). The output voltage is typically 1% higher than in PWM mode. During PFM operation, the error comparator detects the output voltage falling out of regulation and sets a flip- flop, which turns on the N-channel MOSFET switch (Figure 4). When the inductor current ramps to the PFM mode current limit (800mA typical) and stores a fixed amount of energy, the current-sense comparator resets a flip-flop. The flip-flop turns off the N-channel switch and turns on the P-channel synchronous rectifier. A second flip-flop, previously reset by the switch’s “on” sig- nal, inhibits the error comparator from initiating another cycle until the energy stored in the inductor is transferred to the output filter capacitor and the synchronous rectifier current has ramped down to 80mA. This forces operation with a discontinuous inductor current. Synchronous Rectifier The MAX1703 features an internal 140m Ω, P-channel synchronous rectifier to enhance efficiency. Synchro- nous rectification provides a 5% efficiency improve- ment over similar nonsynchronous boost regulators. In PWM mode, the synchronous rectifier is turned on dur- ing the second half of each switching cycle. In low- power mode, an internal comparator turns on the synchronous rectifier when the voltage at LX exceeds the boost regulator output, and then turns it off when the inductor current drops below 80mA. Low-Voltage Start-Up Oscillator The MAX1703 uses a CMOS, low-voltage start-up oscil- lator for a 1.1V guaranteed minimum start-up input volt- age at +25°C. On start-up, the low-voltage oscillator switches the N-channel MOSFET until the output volt- age reaches 2.15V. Above this level, the normal boost- converter feedback and control circuitry take over. Once the device is in regulation, it can operate down to a 0.7V input, since internal power for the IC is boot- strapped from the output via the OUT pin. Do not apply full load until the output exceeds 2.3V (max). 1-Cell to 3-Cell, High-Power (1.5A), Low-Noise, Step-Up DC-DC Converter 10 ______________________________________________________________________________________ 2.7A TYP CURRENT LIMIT OSC FB R Q S REF LXP LXN POUT PGND P N Figure 3. Simplified PWM Controller Block Diagram ERROR COMPARATOR FB REF 800mA TYP CURRENT LIMIT LOGIC HIGH POUT P N PGND S S Q R D Q R LXP LXN Figure 4. Controller Block Diagram in Low-Power PFM Mode |
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