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MAX1800 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1800 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 24 page Digital Camera Step-Up Power Supply _______________________________________________________________________________________ 9 Detailed Description The MAX1800 typical application circuit is shown in Figure 1. It features a main step-up DC-DC converter, three auxiliary step-up DC-DC controllers, an uncom- mitted gain block, a power-ready comparator, and con- trol capability for multiple external MAX1801 slave DC-DC controllers. The uncommitted gain block can be used with an external P-Channel MOSFET to make a linear regulator. The linear regulator can be used with the main output for step-up/step-down functionality or to make a separate stand-alone output voltage. Together, these provide a complete high-efficiency power-supply solution for digital still cameras. Figure 2 shows the MAX1800 functional diagram. Master-Slave Configuration The MAX1800 supports MAX1801 “slave” controllers that obtain input power, a voltage reference, and an oscillator signal directly from the MAX1800 “master” DC-DC converter. The master-slave configuration reduces system cost by eliminating redundant circuitry and controlling the harmonic content of noise with syn- chronized converter switching. Main DC-DC Converter The MAX1800 main step-up DC-DC switching convert- er generates a 2.7V to 5.5V output voltage from a +0.7V to +5.5V battery input voltage. An internal switch and synchronous rectifier allow conversion efficiencies as high as 95% while reducing both circuit size and the number of external components. The converter oper- ates in a low-noise, constant-frequency PWM mode to regulate the voltage across the load. Switching har- monics generated by fixed-frequency operation are consistent and easily filtered. The internal N-channel MOSFET switch turns on during the first part of each cycle, allowing current to ramp up in the inductor and store energy in a magnetic field. During the second part of each cycle, the MOSFET turns off and the voltage across the inductor reverses, forcing current through the internal P-channel synchro- nous rectifier to the output filter capacitor and load. As the energy stored in the inductor is depleted, the cur- rent ramps down. The synchronous rectifier turns off when the inductor current approaches zero or at the beginning of a cycle. The current-mode PWM controller uses the voltage at COMPM to program the inductor current and regulate Pin Description (continued) PIN NAME FUNCTION 24 ONM Main Converter Enable Input. High level turns the main converter on. Connect ONM to POUT to automatically start the main converter. When the main converter is off, all other outputs are disabled. 25 AI Analog Gain Block Input. AI is the positive input to the gain block. The negative input is internally connected to the 1.25V reference. 26 AO Analog Gain Block Output. AO is a push-pull output driven between GND and POUT. The voltage gain of the block is approximately 100. 27 ONA Analog Gain Block Enable Input. Connect ONA to POUT to enable the gain block. When ONA is low, the AO output is driven to POUT. 28 DCON3 Maximum Duty-Cycle Control Input for Auxiliary Controller 3. Connect to POUT to set the default maximum duty cycle. Connect a resistive voltage-divider from REF to DCON3 to set the maximum duty cycle between 40% and 90%. Pull DCON3 below 400mV to turn the controller off. 29 COMP3 Compensation for Auxiliary Controller 3. Output of transconductance error amplifier. Connect a series resistor and capacitor to GND to compensate the control loop. See Compensation Design. 30 FB3 Feedback Input for Auxiliary Controller 3. Connect a feedback resistive voltage-divider from the output to FB3 to set the output voltage. Regulation voltage is VREF (1.25V). 31 ON3 Enable Input for Auxiliary Controller 3. Connect ON2 to POUT to automatically start auxiliary controller 3. 32 DL3 External MOSFET Gate Drive Output for Auxiliary Controller 3. DL3 swings between POUT and GND with typical 500mA drive current. Connect DL3 to the gate of the external switching N-channel MOSFET for auxiliary controller 3. |
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