Datasheet ADP1611 (Analog Devices) - 10

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ADP1611 THEORY OF OPERATION
The ADP1611 current-mode step-up switching converter
ON/OFF CONTROL
converts a 2.5 V to 5.5 V input voltage up to an output voltage The SD input turns the ADP1611 regulator on or off. Drive SD as high as 20 V. The 1.2 A internal switch allows a high output low to turn off the regulator and reduce the input current to current, and the high 1.2 MHz switching frequency allows tiny 10 nA. Drive SD high to turn on the regulator. external components. The switch current is monitored on a pulse-by-pulse basis to limit it to 2 A. 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
CURRENT-MODE PWM OPERATION
inductor and output rectifier. This causes the output voltage to The ADP1611 uses current-mode architecture to regulate the remain slightly below the input voltage by the forward voltage output voltage. The output voltage is monitored at FB through a of the rectifier, preventing the output voltage from dropping to resistive voltage divider. The voltage at FB is compared to the 0 when the regulator is shut down. Figure 28 shows the applica- internal 1.23 V reference by the internal transconductance error tion circuit to disconnect the output voltage from the input amplifier to create an error current at COMP. A series resistor- voltage at shutdown. capacitor at COMP converts the error current to a voltage. The switch current is internally measured and added to the
SETTING THE OUTPUT VOLTAGE
stabilizing ramp, and the resulting sum is compared to the error The ADP1611 features an adjustable output voltage range of VIN voltage at COMP to control the PWM modulator. This current- to 20 V. The output voltage is set by the resistive voltage divider mode regulation system allows fast transient response, while (R1 and R2 in Figure 2) from the output voltage (VOUT) to the maintaining a stable output voltage. By selecting the proper 1.230 V feedback input at FB. Use the following formula to resistor-capacitor network from COMP to GND, the regulator determine the output voltage: response is optimized for a wide range of input voltages, output voltages, and load conditions. VOUT = 1.23 × (1 + R1/R2) (1)
FREQUENCY SELECTION
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 The ADP1611 frequency is user-selectable and operates at on the following formula: either 700 kHz to optimize the regulator for high efficiency or at 1.2 MHz for small external components. Connect RT to ⎛ V − 1.23 ⎞ IN for 1.2 MHz operation, or connect RT to GND for 700 kHz R1 = R2 × ⎜ OUT ⎟ (2) ⎝ . 1 23 ⎠ operation. To achieve the maximum duty cycle, which might be required for converting a low input voltage to a high output
INDUCTOR SELECTION
voltage, use the lower 700 kHz switching frequency. The inductor is an essential part of the step-up switching
SOFT START
converter. It stores energy during the on time, and transfers that To prevent input inrush current at startup, connect a capacitor 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, from SS to GND to set the soft-start period. When the device is in shutdown ( lower inductance values have higher saturation current and SD is at GND) or the input voltage is below the lower series resistance for a given physical size. However, lower 2.4 V undervoltage lockout voltage, SS is internally shorted to inductance results in higher peak current that can lead to GND to discharge the soft start capacitor. Once the ADP1611 is reduced efficiency and greater input and/or output ripple and turned on, SS sources 3 µA to the soft-start capacitor at startup. noise. Peak-to-peak inductor ripple current at close to 30% of As the soft-start capacitor charges, it limits the voltage at the maximum dc input current typically yields an optimal COMP. Because of the current-mode regulator, the voltage at compromise. COMP is proportional to the switch peak current, and, therefore, the input current. By slowly charging the soft-start For determining the inductor ripple current, the input (VIN) and capacitor, the input current ramps slowly to prevent it from output (VOUT) voltages determine the switch duty cycle (D) by overshooting excessively at startup. the following equation: V − V D = OUT IN (3) VOUT Rev. 0 | Page 10 of 20 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION CURRENT-MODE PWM OPERATION FREQUENCY SELECTION SOFT START ON/OFF CONTROL SETTING THE OUTPUT VOLTAGE INDUCTOR SELECTION CHOOSING THE INPUT AND OUTPUT CAPACITORS DIODE SELECTION LOOP COMPENSATION SOFT-START CAPACITOR APPLICATION CIRCUITS STEP-UP DC-TO-DC CONVERTER WITH TRUE SHUTDOWN TFT LCD BIAS SUPPLY SEPIC POWER SUPPLY LAYOUT PROCEDURE OUTLINE DIMENSIONS ORDERING GUIDE