ADP1882ARMZ-1.0-R7 datasheet(22/40 Pages) AD

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ADP1882/ADP1883

Rev. 0 | Page 22 of 40

SYNCHRONOUS RECTIFIER

The ADP1882/ADP1883 employ an internal lower-side MOSFET

driver to drive the external upper-side and lower-side MOSFETs.

The synchronous rectifier not only improves overall conduction

efficiency but also ensures proper charging to the bootstrap

capacitor located at the upper-side driver input. This is beneficial

during startup to provide a sufficient drive signal to the external

upper-side MOSFET and attain a fast turn on response, which is

essential for minimizing switching losses. The integrated upper-

side and lower-side MOSFET drivers operate in complementary

fashion with built-in anticross conduction circuitry to prevent

unwanted shoot-through current that may potentially damage the

MOSFETs or reduce efficiency as a result of excessive power loss.

POWER SAVING MODE (PSM) VERSION (ADP1883)

The ADP1883 is the power saving mode version of the ADP1882.

The ADP1883 operates in the discontinuous conduction mode

(DCM) and pulse skips at light load to midload currents. It outputs

pulses, as necessary, to maintain output regulation. Unlike the

continuous conduction mode (CCM), DCM operation prevents

negative current, thus allowing improved system efficiency at

light loads. Current in the reverse direction through this pathway,

however, results in power dissipation and, therefore, a decrease

in efficiency.

HS AND LS ARE OFF

OR IN IDLE MODE

ILOAD

AS THE INDUCTOR

CURRENT APPROACHES

ZERO CURRENT, THE STATE

tON

tOFF

MACHINE TURNS OFF THE

LOWER-SIDE MOSFET.

Figure 74. Discontinuous Mode of Operation (DCM)

To minimize the chance of negative inductor current buildup,

an on-board, zero-cross comparator turns off all upper-side and

lower-side switching activities when the inductor current

approaches the zero current line, causing the system to enter

idle mode, where the upper-side and lower-side MOSFETs are

turned off. To ensure idle mode entry, a 10 mV offset, connected

in series at the SW node, is implemented (see Figure 75).

10mV

ZERO-CROSS

COMPARATOR

IQ2

Figure 75. Zero-Cross Comparator with 10 mV of Offset

As soon as the forward current through the lower-side

MOSFET decreases to a level where

10 mV = IQ2 × RON(Q2)

the zero-cross comparator (or IREV comparator) emits a signal to

turn off the lower-side MOSFET. From this point, the slope of the

inductor current ramping down becomes steeper (see Figure 76)

as the body diode of the lower-side MOSFET begins to conduct

current and continues conducting current until the remaining

energy stored in the inductor has been depleted.

ANOTHER

tON EDGE IS

TRIGGERED WHEN VOUT

FALLS BELOW REGULATION

HS AND LS

IN IDLE MODE

10mV = RON × ILOAD

ZERO-CROSS COMPARATOR

DETECTS 10mV OFFSET AND

TURNS OFF LS

ILOAD

tON

Figure 76. 10 mV Offset to Ensure Prevention of Negative Inductor Current

The system remains in idle mode until the output voltage drops

from within regulation. A PWM pulse is then produced, turning

on the upper-side MOSFET to maintain system regulation. The

ADP1883 does not have an internal clock; therefore, it switches

purely as a hysteretic controller as described in this section.

TIMER OPERATION

The ADP1882/ADP1883 employ a constant on-time architecture

that provides a variety of benefits, including improved load and

line transient responses when compared with a constant (fixed)

frequency current-mode control loop of a comparable loop design.

The constant on-time timer, or tON timer, senses the high input

voltage (VIN) and the output voltage (VOUT) using SW waveform

information to produce an adjustable one-shot PWM pulse that

varies the on time of the upper-side MOSFET in response to

dynamic changes in input voltage, output voltage, and load current

conditions to maintain regulation. It then generates an on-time

(tON) pulse that is inversely proportional to VIN.

tON = K ×

OUT

where

K is a constant that is trimmed using an RC timer product

for the 300 kHz, 600 kHz, and 1.0 MHz frequency options.

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