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MAX682 Datasheet(PDF) 6 Page - Maxim Integrated Products |
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MAX682 Datasheet(HTML) 6 Page - Maxim Integrated Products |
6 / 12 page 3.3V-Input to Regulated 5V-Output Charge Pumps 6 _______________________________________________________________________________________ Detailed Description The MAX682/MAX683/MAX684 charge pumps provide a regulated 5V output from a 2.7V to 5.5V input. They deliver a maximum of 250mA, 100mA, or 50mA load current, respectively. Designed specifically for com- pact applications, a complete regulator circuit requires only three small external capacitors and one resistor. An externally adjustable switching frequency and inno- vative control scheme allow the circuit to be optimized for efficiency, size, or output noise. The devices also contain a shutdown feature. The MAX682/MAX683/MAX684 consist of an error amplifier, a 1.23V bandgap reference, an internal resis- tive feedback network, an oscillator, high-current MOS- FET switches, and shutdown and control logic (Figure 1). Figure 2 shows an idealized unregulated charge- pump voltage doubler. The oscillator runs at a 50% duty cycle. During one half of the period, the transfer capacitor (CX) charges to the input voltage. During the other half, the doubler stacks the voltage across CX and the input voltage, and transfers the sum of the two voltages to the output filter capacitor (COUT). Rather than simply doubling the input voltage, the MAX682/MAX683/MAX684 provide a regulated fixed output voltage (5V) using either skip mode or constant- frequency mode. Skip mode and constant-frequency mode are externally selected via the SKIP input pin. Skip Mode In skip mode (SKIP = low), the error amplifier disables switching when it detects an output higher than 5V. The device then skips switching cycles until the output volt- age drops. Then the error amplifier reactivates the oscillator. Figure 3 illustrates the regulation scheme. This regulation method minimizes operating current because the device does not switch continuously. SKIP is a logic input and should not remain floating. Constant-Frequency Mode When SKIP is high, the charge pump runs continuously at the selected frequency. Figure 4 shows a block dia- gram of the device in constant-frequency mode. The error amplifier controls the charge on CX by driving the gate of the N-channel FET. When the output voltage falls, the gate drive increases, resulting in a larger volt- age across CX. This regulation scheme minimizes out- put ripple. Since the device switches continuously, the CXP OUT 1.23V EN SHDN PGND SWITCHES CONTROL LOGIC CXN SHDN SKIP IN OSC IN S1 S2 CIN COUT OUT CX OSC Figure 2. Unregulated Voltage Doubler IN S1 S2 CIN CX OUT OSCILLATOR EN Figure 3. Skip-Mode Regulation Figure 1. Functional Block Diagram |
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