ADP1621ARMZ-R7 datasheet(19/32 Pages) AD

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Data Sheet

ADP1621

Rev. D | Page 19 of 32

Also, keep in mind that the current limit is a function of both

ADP1621 parameters and off-chip components, the values of

which vary from device to device and with temperature. If

lossless current sensing is used, consider that the on resistance

of a MOSFET typically increases with increasing junction

temperature.

The peak inductor current limit also limits the maximum load

current at a given output voltage. The maximum load current,

assuming CCM operation, is given by

(

)×

−

MAX

LOAD

















−

MIN

OFF

ZCT

COMP

CLAMP

COMP

(36)

If the load current exceeds ILOAD,MAX, the output voltage drops

below the desired voltage.

LIGHT LOAD OPERATION

Discontinuous Conduction Mode

With light loads, the average inductor current is small, and,

depending on the converter design, the instantaneous inductor

current may reach 0 during the time when the MOSFET is off.

This mode of operation is termed discontinuous conduction

mode. The condition for entering discontinuous conduction

mode in a boost converter is

(

)

LOAD

−

(37)

When the instantaneous inductor current reaches 0 during the

cycle, the inductor ceases to be a current source, and ringing

can be observed in the waveforms of the MOSFET drain voltage

and the inductor current. The frequency of the ringing is the

resonant frequency of the inductor and the total capacitance

from the SW node to GND, which includes the capacitances of

the MOSFET and diode, and any parasitic capacitances from

the PCB. While adding a resistive element, such as a snubber, to

the system further dampens the resonance, it also decreases the

efficiency of the regulator.

Pulse-Skipping Modulation

The ADP1621 features circuitry that improves the converter

efficiency and minimizes power consumption with no load or

very light loads. When the COMP voltage drops below VCOMP,ZCT

(typically 1.0 V), which can occur at sufficiently light loads, the

MOSFET is powered off until the FB voltage drops below 1.215 V.

Then, the error amplifier drives the COMP voltage higher, and

the converter resumes switching when the COMP voltage rises

above the VCOMP,ZCT voltage. While the MOSFET is powered off,

the output capacitor supplies current to the load.

With light loads, the COMP voltage hovers around 1.0 V, and

short periods of switching are followed by long periods of the

MOSFET being powered off. This pulse-skipping modulation

operation improves converter efficiency by reducing the number of

switching cycles and therefore reducing the gate drive current and

the switching transition power loss.

Given the minimum on time of the ADP1621, pulse-skipping

modulation is also a requirement to maintain output voltage

regulation with light loads. During the short switching periods

of pulse-skipping modulation, the MOSFET is turned on for the

minimum on time each cycle, storing just enough energy in the

inductor to charge the output capacitor. During the long period

when the MOSFET is off, no current flows through the inductor,

and the light load current is supplied by the output capacitor.

RECOMMENDED COMPONENT MANUFACTURERS

Table 5. Recommended Component Manufacturers

Vendor

Components

AVX Corporation

Capacitors

Central Semiconductor Corp.

Diodes

Coilcraft, Inc.

Inductors

Diodes, Inc.

Diodes

International Rectifier

Diodes, MOSFETs

Murata Manufacturing Co., Ltd.

Capacitors, inductors

ON Semiconductor

Diodes, MOSFETs

Rubycon Corporation

Capacitors

Sanyo

Capacitors

Sumida

Inductors

Taiyo Yuden, Inc.

Capacitors, inductors

Toko America, Inc.

Inductors

United Chemi-Con, Inc.

Capacitors

Vishay Siliconix

Diodes, MOSFETs, resistors, capacitors

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