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ADP1611 데이터시트(PDF) 10 Page - Analog Devices |
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ADP1611 데이터시트(HTML) 10 Page - Analog Devices |
10 / 20 page ADP1611 Rev. 0 | Page 10 of 20 THEORY OF OPERATION The ADP1611 current-mode step-up switching converter converts a 2.5 V to 5.5 V input voltage up to an output voltage as high as 20 V. The 1.2 A internal switch allows a high output current, and the high 1.2 MHz switching frequency allows tiny external components. The switch current is monitored on a pulse-by-pulse basis to limit it to 2 A. CURRENT-MODE PWM OPERATION The ADP1611 uses current-mode architecture to regulate the output voltage. The output voltage is monitored at FB through a resistive voltage divider. The voltage at FB is compared to the internal 1.23 V reference by the internal transconductance error amplifier to create an error current at COMP. A series resistor- capacitor at COMP converts the error current to a voltage. The switch current is internally measured and added to the stabilizing ramp, and the resulting sum is compared to the error voltage at COMP to control the PWM modulator. This current- mode regulation system allows fast transient response, while maintaining a stable output voltage. By selecting the proper resistor-capacitor network from COMP to GND, the regulator response is optimized for a wide range of input voltages, output voltages, and load conditions. FREQUENCY SELECTION The ADP1611 frequency is user-selectable and operates at either 700 kHz to optimize the regulator for high efficiency or at 1.2 MHz for small external components. Connect RT to IN for 1.2 MHz operation, or connect RT to GND for 700 kHz operation. To achieve the maximum duty cycle, which might be required for converting a low input voltage to a high output voltage, use the lower 700 kHz switching frequency. SOFT START To prevent input inrush current at startup, connect a capacitor from SS to GND to set the soft-start period. When the device is in shutdown (SD is at GND) or the input voltage is below the 2.4 V undervoltage lockout voltage, SS is internally shorted to GND to discharge the soft start capacitor. Once the ADP1611 is turned on, SS sources 3 µA to the soft-start capacitor at startup. As the soft-start capacitor charges, it limits the voltage at COMP. Because of the current-mode regulator, the voltage at COMP is proportional to the switch peak current, and, therefore, the input current. By slowly charging the soft-start capacitor, the input current ramps slowly to prevent it from overshooting excessively at startup. ON/OFF CONTROL The SD input turns the ADP1611 regulator on or off. Drive SD low to turn off the regulator and reduce the input current to 10 nA. Drive SD high to turn on the regulator. When the step-up dc-to-dc switching converter is turned off, there is a dc path from the input to the output through the inductor and output rectifier. This causes the output voltage to remain slightly below the input voltage by the forward voltage of the rectifier, preventing the output voltage from dropping to 0 when the regulator is shut down. Figure 28 shows the applica- tion circuit to disconnect the output voltage from the input voltage at shutdown. SETTING THE OUTPUT VOLTAGE The ADP1611 features an adjustable output voltage range of VIN to 20 V. The output voltage is set by the resistive voltage divider (R1 and R2 in Figure 2) from the output voltage (VOUT) to the 1.230 V feedback input at FB. Use the following formula to determine the output voltage: VOUT = 1.23 × (1 + R1/R2) (1) Use an R2 resistance of 10 kΩ or less to prevent output voltage errors due to the 10 nA FB input bias current. Choose R1 based on the following formula: R1 = R2 × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − 23 . 1 23 . 1 OUT V (2) INDUCTOR SELECTION The inductor is an essential part of the step-up switching converter. It stores energy during the on time, and transfers that energy to the output through the output rectifier during the off time. Use inductance in the range of 1 µH to 22 µH. In general, lower inductance values have higher saturation current and lower series resistance for a given physical size. However, lower inductance results in higher peak current that can lead to reduced efficiency and greater input and/or output ripple and noise. Peak-to-peak inductor ripple current at close to 30% of the maximum dc input current typically yields an optimal compromise. For determining the inductor ripple current, the input (VIN) and output (VOUT) voltages determine the switch duty cycle (D) by the following equation: D = OUT IN OUT V V V − (3) |
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