ADP2105ACPZ-3.3-R7 datasheet(12/32 Pages) AD

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ADP2105/ADP2106/ADP2107

Rev. 0 | Page 12 of 32

THEORY OF OPERATION

The ADP2105/ADP2106/ADP2107 are step-down, dc-to-dc

converters that use a fixed frequency, peak current-mode

architecture with an integrated high-side switch and low-side

synchronous rectifier. The high 1.2 MHz switching frequency

and tiny 16-lead, 4 mm × 4 mm LFCSP_VQ package allow for

a small step-down dc-to-dc converter solution. The integrated

high-side switch (P-channel MOSFET) and synchronous rectifier

(N-channel MOSFET) yield high efficiency at medium-to-

heavy loads. Light load efficiency is improved by smoothly

transitioning to variable frequency PFM mode.

The ADP2105/ADP2106/ADP2107-ADJ operate with an input

voltage from 2.7 V to 5.5 V and regulate an output voltage down

to 0.8 V. The ADP2105/ADP2106/ADP2107 are also available with

preset output voltage options of 3.3 V, 1.8 V, 1.5 V, and 1.2 V.

CONTROL SCHEME

The ADP2105/ADP2106/ADP2107 operate with a fixed

frequency, peak current-mode PWM control architecture at

medium-to-high loads for high efficiency, but shift to a variable

frequency PFM control scheme at light loads for lower quies-

cent current. When operating in fixed frequency PWM mode,

the duty cycle of the integrated switches is adjusted to regulate

the output voltage, but when operating in PFM mode at light

loads, the switching frequency is adjusted to regulate the output

voltage.

The ADP2105/ADP2106/ADP2107 operate in the PWM mode

only when the load current is greater than the pulse-skipping

threshold current. At load currents below this value, the converter

smoothly transitions to the PFM mode of operation.

PWM MODE OPERATION

In PWM mode, the ADP2105/ADP2106/ADP2107 operate at

a fixed frequency of 1.2 MHz set by an internal oscillator. At the

start of each oscillator cycle, the P-channel MOSFET switch is

turned on, putting a positive voltage across the inductor. Current

in the inductor increases until the current sense signal crosses

the peak inductor current level that turns off the P-channel

MOSFET switch and turns on the N-channel MOSFET synchro-

nous rectifier. This puts a negative voltage across the inductor,

causing the inductor current to decrease. The synchronous

rectifier stays on for the rest of the cycle, unless the inductor

current reaches zero, which causes the zero-crossing comparator

to turn off the N-channel MOSFET, as well. The peak inductor

current is set by the voltage on the COMP pin. The COMP pin

is the output of a transconductance error amplifier that compares

the feedback voltage with an internal 0.8 V reference.

PFM MODE OPERATION

The ADP2105/ADP2106/ADP2107 smoothly transition to the

variable frequency PFM mode of operation when the load current

decreases below the pulse-skipping threshold current, switching

only as necessary to maintain the output voltage within regulation.

When the output voltage dips below regulation, the ADP2105/

ADP2106/ADP2107 enter PWM mode for a few oscillator cycles

to increase the output voltage back to regulation. During the wait

time between bursts, both power switches are off, and the output

capacitor supplies all the load current. Because the output voltage

dips and recovers occasionally, the output voltage ripple in this

mode is larger than the ripple in the PWM mode of operation.

PULSE-SKIPPING THRESHOLD

The output current at which the ADP2105/ADP2106/ADP2107

transition from variable frequency PFM control to fixed frequency

PWM control is called the pulse-skipping threshold. The pulse-

skipping threshold has been optimized for excellent efficiency

over all load currents. The variation of pulse-skipping threshold

with input voltage and output voltage is shown in Figure 23,

Figure 25, and Figure 26.

100% DUTY CYCLE OPERATION (LDO MODE)

As the input voltage drops, approaching the output voltage,

the ADP2105/ADP2106/ADP2107 smoothly transition to 100%

duty cycle, maintaining the P-channel MOSFET switch on continu-

ously. This allows the ADP2105/ADP2106/ADP2107 to regulate

the output voltage until the drop in input voltage forces the

P-channel MOSFET switch to enter dropout, as shown in the

following equation:

VIN(MIN) = IOUT × (RDS(ON) − P + DCRIND) + VOUT(NOM)

The ADP2105/ADP2106/ADP2107 achieve 100% duty cycle

operation by stretching the P-channel MOSFET switch on-time

if the inductor current does not reach the peak inductor current

level by the end of the clock cycle. Once this happens, the oscil-

lator remains off until the inductor current reaches the peak

inductor current level, at which time the switch is turned off and

the synchronous rectifier is turned on for a fixed off-time. At

the end of the fixed off-time, another cycle is initiated. As the

ADP2105/ADP2106/ADP2107 approach dropout, the switching

frequency decreases gradually to smoothly transition to 100%

duty cycle operation.

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