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MAX1709 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1709 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page 4A, Low-Noise, High-Frequency, Step-Up DC-DC Converter _______________________________________________________________________________________ 9 troller functional diagram. The MAX1709 enters syn- chronized current-mode PWM when a clock signal (350kHz < fCLK < 1000kHz) is applied to CLK. For wireless or noise-sensitive applications, this ensures that switching harmonics are predictable and kept out- side the IF frequency band(s). High-frequency opera- tion permits low-magnitude output ripple voltage and minimum inductor and filter capacitor size. Switching losses will increase at the higher frequencies (see Power Dissipation). Setting the Output Voltage The MAX1709 features Dual Mode™ operation. When FB is connected to ground, the MAX1709 generates a fixed output voltage of either +3.3V or +5V, depending on the logic applied to the 3.3/5 input (Figure 1). The output can be configured for other voltages, using two external resistors as shown in Figure 4. To set the out- put voltage externally, choose an R3 value that is large enough to minimize load at the output but small enough to minimize errors due to leakage and the time constant to FB. A value of R4 ≤ 50k Ω is required. where VFB = 1.24V. Soft-Start/Current-Limit Adjustment (SS/LIM) The soft-start pin allows the soft-start time to be adjust- ed by connecting a capacitor from SS/LIM to ground. Select capacitor C3 (connected to SS/LIM pin) as: C3 (in µF) = 3.2 ✕ tSS where tSS is the time (in seconds) it takes the switch current limit to reach full value. To improve efficiency or reduce inductor size at reduced load currents, the current limit can be reduced from its nominal value (see Electrical Characteristics). A resistor (R1 in Figure 1) between SS/LIM and ground reduces the current limit as follows: Rk I I Rk LIM 1 312 5 1 312 5 1 . . =× ≤ () ΩΩ RR V V OUT FB 34 1 =− _______________________________________________________________________________________ ONA ONB MAX1709 00 On 0 1 Off 10 On 11 On Figure 2. Simplified Functional Diagram Table 2. Selecting Switching Frequency Table 1. On/Off Logic Control CLK MODE 0 Not allowed 1 PWM External clock (350kHz −1000kHz) Synchronized PWM N PWM CONTROLLER STARTUP OSCILLATOR SEE FIGURE 3. OSC UNDERVOLTAGE LOCKOUT FB EN D EN Q 600kHz OSCILLATOR EN REFERENCE ON RDY DUAL MODE FB LX PGND 1.260V 2.15V OUT ONA ONB REF CLK FB 3.3/5 GND IC POWER MAX1709 OUT Dual Mode is a trademark of Maxim Integrated Products. |
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