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ML4790 Scheda tecnica(PDF) 4 Page - Micro Linear Corporation |
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ML4790 Scheda tecnica(HTML) 4 Page - Micro Linear Corporation |
4 / 10 page 4 ML4790 FUNCTIONAL DESCRIPTION The ML4790 combines Pulse Frequency Modulation (PFM) and synchronous rectification to create a boost converter that is followed by a low dropout linear regulator (LDO). This combination creates a low output ripple boost converter that is both highly efficient and simple to use. The PFM regulator charges a single inductor for a fixed period of time and then completely discharges before another cycle begins, simplifying the design by eliminating the need for conventional current limiting circuitry. Synchronous rectification is accomplished by replacing an external Schottky diode with an on-chip PMOS device, reducing switching losses and external component count. The integrated LDO reduces the output ripple voltage to less than 5mV peak-to-peak. Integrating the LDO along with the PFM regulator allows the circuit to be optimized for very high efficiency using a patented feedback technique. It also allows the LDO to provide the maximum ripple rejection over the operating frequency range of the regulator. A block diagram of the ML4790 is shown in Figure 2. The PFM stage is comprised of Q1, Q2, A1, A2, the one shot, the flip-flop, and externals L1 and C2. The LDO stage is comprised of Q3, A3, the offset voltage control, and external components R1, R2 and COUT. Since the LDO actually controls the operation of the PFM regulator, the operation of the LDO stage will be covered first. LDO OPERATION The LDO stage operates as a linear regulator. A3 is the error amplifier, which compares the output voltage through the divider R1 and R2 to the reference, and Q3 is the pass device. When the output voltage is lower than desired, the output of A3 increases the gate drive of Q3, which reduces the voltage drop across it and brings the output back into regulation. Similarly, if the output voltage is higher than desired, A3 adjusts the gate drive of Q3 for more drop and the output is brought back into regulation. 450 400 350 300 250 200 150 100 0 IOUT (mA) 10 20 30 40 50 60 70 80 90 100 Figure 3. LDO VOS versus output current. Also included in the LDO stage is an offset voltage control. This circuit monitors the output current and adjusts the offset voltage according the general characteristic shown in Figure 3. The offset control ensures that the PFM stage provides just enough “overhead” voltage for the LDO stage to operate properly. + – 6 – 5 µs ONE SHOT R S L1 Q3 A2 A1 Q1 R1 COUT + – + 5 C2 VOS = f (ILOAD) + – A3 VREF R2 4 ILOAD Q2 3 CFB Figure 2. PFM Regulator and LDO Block Diagram |
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