ADP2108ACBZ-3.0-R7(RevA) 데이터 시트보기 (PDF) - Analog Devices

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ADP2108ACBZ-3.0-R7
(Rev.:RevA)

Compact, 600 mA, 3 MHz, Step-Down DC-to-DC Converter

Analog Devices 

ADP2108

THEORY OF OPERATION

GM ERROR

AMP

PWM

COMP

VIN

SOFT START

ILIMIT

FB

PSM

COMP

PWM/

PSM

LOW

CONTROL CURRENT

SW

OSCILLATOR

UNDERVOLTAGE

LOCKOUT

DRIVER

AND

ANTISHOOT-

THROUGH

ADP2108

THERMAL

SHUTDOWN

GND

EN

Figure 28. Functional Block Diagram

The ADP2108 is a step-down dc-to-dc converter that uses a

fixed frequency and high speed current mode architecture. The

high switching frequency and tiny 5-ball WLCSP package allow

for a small step-down dc-to-dc converter solution.

The ADP2108 operates with an input voltage of 2.3 V to 5.5 V

and regulates an output voltage down to 1.0 V.

CONTROL SCHEME

The ADP2108 operates with a fixed frequency, current mode

PWM control architecture at medium to high loads for high

efficiency, but shifts to a power save mode control scheme at

light loads to lower the regulation power losses. When operating

in fixed frequency PWM mode, the duty cycle of the integrated

switches is adjusted and regulates the output voltage. When

operating in power save mode at light loads, the output voltage

is controlled in a hysteretic manner, with higher VOUT ripple.

During part of this time, the converter is able to stop switching

and enters an idle mode, which improves conversion efficiency.

PWM MODE

In PWM mode, the ADP2108 operates at a fixed frequency of

3 MHz, set by an internal oscillator. At the start of each oscillator

cycle, the PFET switch is turned on, sending a positive voltage

across the inductor. Current in the inductor increases until the

current sense signal crosses the peak inductor current threshold

that turns off the PFET switch and turns on the NFET synchronous

rectifier. This sends a negative voltage across the inductor, causing

the inductor current to decrease. The synchronous rectifier stays

on for the rest of the cycle. The ADP2108 regulates the output

voltage by adjusting the peak inductor current threshold.

POWER SAVE MODE

The ADP2108 smoothly transitions to the power save mode of

operation when the load current decreases below the power

save mode current threshold. When the ADP2108 enters power

save mode, an offset is induced in the PWM regulation level,

which makes the output voltage rise. When the output voltage

reaches a level approximately 1.5% above the PWM regulation

level, PWM operation is turned off. At this point, both power

switches are off, and the ADP2108 enters an idle mode. COUT

discharges until VOUT falls to the PWM regulation voltage, at

which point the device drives the inductor to make VOUT rise

again to the upper threshold. This process is repeated while the

load current is below the power save mode current threshold.

Power Save Mode Current Threshold

The power save mode current threshold is set to 80 mA. The

ADP2108 employs a scheme that enables this current to remain

accurately controlled, independent of VIN and VOUT levels. This

scheme also ensures that there is very little hysteresis between

the power save mode current threshold for entry to and exit from

the power save mode. The power save mode current threshold

is optimized for excellent efficiency over all load currents.

ENABLE/SHUTDOWN

The ADP2108 starts operation with soft start when the EN pin

is toggled from logic low to logic high. Pulling the EN pin low

forces the device into shutdown mode, reducing the shutdown

current below 1 μA.

Rev. A | Page 11 of 16

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