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ADP2126/ADP2127

Data Sheet

Rev. B | Page 12 of 20

SPREAD SPECTRUM OSCILLATOR

The ADP2126/ADP2127 incorporate spread spectrum

functionality to modulate electromagnetic interference (EMI)

for EMI sensitive applications. A typical switching converter

with a regulated switching frequency has a narrow frequency

spectrum centered at the target switching frequency. This

results in a high spectral density around the target frequency

with peak emission levels that can exceed the regulatory levels

for EMI in many portable, cellular, and wireless applications.

To maintain acceptable levels of EMI, the ADP2126/ADP2127

employs spread spectrum via a controlled variance of the switching

frequency over a wider band of frequencies. Figure 25 shows the

variance of the frequency over time. This distribution of the

frequency content spreads the spectral density over a wider

bandwidth, resulting in lower peak emission levels.

MODE SELECTION

The ADP2126/ADP2127 have two modes of operation (PWM

mode and auto mode), determined by the state of the MODE pin.

Pull the MODE pin high to force the converter to operate in

PWM mode, regardless of the output current. Otherwise, set

MODE low to put the converter into auto mode and allow the

converter to automatically transition from PWM mode to the

power-saving PFM mode at light load currents. Do not leave

this pin floating.

Pulse-Width Modulation (PWM) Mode

The PWM mode forces the part to maintain a fixed frequency

of 6 MHz (maximum) under all load conditions. The ADP2126/

ADP2127 use a proprietary, hybrid voltage-mode control scheme

to control the duty cycle under all load current and line voltage

variations. This control scheme provides excellent stability,

transient response, and output regulation. PWM mode results

in lower efficiencies at light load currents.

Auto Mode (PFM and PWM Switching)

Auto mode is a power-saving feature that enables the converter

to switch between PWM and PFM in response to the output

load. Auto mode is enabled when the MODE pin is pulled low.

In auto mode, the ADP2126/ADP2127 operate in PFM mode for

light load currents and switch to PWM mode for medium and

heavy load currents.

Pulse Frequency Modulation (PFM) Mode

When the converter is operating under light load conditions,

the effective switching frequency and supply current are decreased

and varied using PFM to regulate the output voltage. This results in

improved efficiencies and lower quiescent currents. In PFM mode,

the converter only switches when necessary to keep the output

voltage within the PFM limits set by an internal comparator.

Switching stops when the upper limit is reached and resumes

when the lower limit is reached.

When the upper level is reached, the output stage and most

control circuitry turn off to reduce the quiescent current. During

this stage, the output capacitor supplies the current to the load.

As the output capacitor discharges and the output voltage reaches

the lower PFM comparator threshold, switching resumes and the

process repeats.

Mode Transition

When the MODE pin is low, the converter switches between

PFM and PWM modes automatically to maintain optimal

transient response and efficiency. The mode transition point

depends on the input voltage. Hysteresis exists in the transition

point to prevent instability and decreased efficiencies that could

result if the converter were able to oscillate between PFM and

PWM for a fixed input voltage and load current. See Figure 9 for

the typical PFM and PWM mode boundaries of the

ADP2126/ADP2127.

A switch from PFM to PWM occurs when the output voltage dips

below the nominal value of the output voltage option. Switching

to PWM allows the converter to maintain efficiency and supply

a larger current to the load. The output voltage in PFM mode is

slightly higher to keep the ADP2126/ADP2127 from oscillating

between modes, ensuring stable operation.

The switch from PWM to PFM occurs when the output current

is below the PFM threshold for multiple consecutive switching

cycles. Switching to PFM allows the converter to save power by

supplying the lighter load current with fewer switching cycles.

INTERNAL CONTROL FEATURES

Synchronous Rectification

In addition to the P-channel MOSFET switch, the ADP2126/

ADP2127 include an N-channel MOSFET switch to build the

synchronous rectifier. The synchronous rectifier improves

efficiency, especially for small load currents, and reduces cost

and board space by eliminating the need for an external rectifier.

Soft Start

To prevent excessive input inrush current at startup, the ADP2126/

ADP2127 operate with an internal soft start. When EXTCLK

begins to oscillate, or when the part recovers from a fault (UVLO,

TSD, or SCP), a soft start timer begins. During this time, the

peak current limit is gradually increased to its maximum. The

output voltage increases in stages to ensure that the converter is

able to start up effectively and in proper sequence. After the soft

start period expires, the peak PMOS switch current limit remains

at 1 A (typical), and the part begins normal operation.

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