ADP1611-EVAL 查看數據表（PDF） - Analog Devices

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ADP1611-EVAL

20 V,1.2 MHz Step-Up DC-to-DC Switching Converter

Analog Devices 

ADP1611

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 × ⎜⎛ VOUT − 1.23 ⎟⎞

(2)

⎝ 1.23 ⎠

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 = VOUT − VIN

(3)

VOUT

Rev. 0 | Page 10 of 20

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