ADP1621ARMZ-R7 datasheet(18/32 Pages) AD

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ADP1621

Rev. A | Page 18 of 32

RDSON, of the external power MOSFET. Otherwise, RCS

represents the external current-sense resistor.

Solving for RS gives the slope-compensation criterion:

(

)

(

)

MIN

OFF

OUT

−

(34)

Keep in mind that the above inequality is a function of both

ADP1621 parameters and off-chip components, the values of

which vary from part to part and with temperature. Select RS to

ensure current-loop stability for all possible variations.

After accounting for parameter variations, use values of RS that

are as close to the calculated limit as possible because excessive

slope compensation reduces the benefits of current-mode control

and increases the “softness” of the current limit, as discussed in the

Current Limit section. Given a typical peak slope-compensation

current of 70 μA, RS should not exceed 1.6 kΩ because the voltage

at the CS pin is typically clamped at 116 mV. It is also recom-

mended that RS be greater than 20 Ω. If the calculated RS is greater

than 1.6 kΩ, the parameters in Equation 34, such as RCS, fSW, and L,

can be adjusted such that RS is less than 1.6 kΩ.

In conclusion, the value of RS should be 20 Ω ≤ RS ≤ 1.6 kΩ.

CURRENT LIMIT

The current limit in the ADP1621 limits the peak inductor

current and is achieved by the COMP voltage clamp. The peak

inductor current, IL,PK, is given by

MIN

OFF

ZCT

COMP

CLAMP

COMP

−

(35)

where VCOMP,CLAMP is the COMP clamp voltage (typically 2.0 V),

VCOMP,ZCT is the COMP zero-current threshold (typically 1.0 V),

n is the current-sense amplifier gain (typically 9.5), ISC,PK is the

peak slope-compensation current (typically 70 μA), RS is the

slope-compensation resistor, D is the duty cycle, fSW is the

switching frequency, tOFF,MIN is the minimum off time (typically

190 ns), and RCS is the current-sense resistor. In the case of

lossless current sensing, RCS is equal to the on resistance, RDSON,

of the external power MOSFET. Otherwise, RCS represents the

external current-sense resistor.

The current limit in the ADP1621 is a “soft” current limit.

When the inductor current reaches the IL,PK limit given in

Equation 35, the duty cycle decreases, and the output voltage

drops below the desired voltage. The IL,PK limit in Equation 35

then increases in response to the smaller duty cycle, D. The

larger the slope-compensation resistor, RS, the larger the effect

on IL,PK for an incremental decrease in D. This behavior results

in a “soft” current limit for the ADP1621. Use values of RS that are

as close as possible to the calculated limit derived from

Equation 34. If high-precision current limiting is required,

consider inserting a fuse in series with the inductor.

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 part to part 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.

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