ADP1882ARMZ-0.3-R7 datasheet(23/40 Pages) AD

23 / 40 page

ADP1882/ADP1883

Rev. 0 | Page 23 of 40

R (TRIMMED)

VREG

tON

VIN

To illustrate this feature more clearly, this section describes

one such load transient event—a positive load step—in detail.

During load transient events, the high-side driver output pulse

width stays relatively consistent from cycle to cycle; however,

the off time (DRVL on time) dynamically adjusts according to

the instantaneous changes in the external conditions mentioned.

INFORMATION

When a positive load step occurs, the error amplifier (out of

phase of the output, VOUT) produces new voltage information

at its output (COMP). In addition, the current-sense amplifier

senses new inductor current information during this positive

load transient event. The error amplifier’s output voltage

reaction is compared to the new inductor current information

that sets the start of the next switching cycle. Because current

information is produced from valley current sensing, it is

sensed at the down ramp of the inductor current, whereas the

voltage loop information is sensed through the counter action

upswing of the error amplifier’s output (COMP).

Figure 77. Constant On-Time Timer

The constant on time (tON) is not strictly constant because it

varies with VIN and VOUT. However, this variation occurs in such

a way as to keep the switching frequency virtually independent

of VIN and VOUT.

The tON timer uses a feedforward technique, applied to the constant

on-time control loop, making it pseudo-fixed frequency to a first

order. Second-order effects, such as dc losses in the external power

MOSFETs (see the Efficiency Consideration section), cause some

variation in frequency vs. load current and line voltage. These

effects are shown in Figure 23 to Figure 34. The variations in

frequency are much reduced, compared with the variations

generated when the feedforward technique is not used.

The result is a convergence of these two signals (see Figure 78),

which allows an instantaneous increase in switching frequency

during the positive load transient event. In summary, a positive

load step causes VOUT to transient down, which causes COMP to

transient up and, therefore, shortens the off time. This resultant

increase in frequency during a positive load transient helps to

quickly bring VOUT back up in value and within the regulation

window.

The feedforward technique establishes the following relationship:

fSW = 1

where fSW is the controller switching frequency (300 kHz,

600 kHz, and 1.0 MHz).

Similarly, a negative load step causes the off time to lengthen in

response to VOUT rising. This effectively increases the inductor

demagnetizing phase, helping to bring VOUT within regulation.

In this case, the switching frequency decreases, or experiences

a foldback, to help facilitate output voltage recovery.

The tON timer senses VIN and VOUT to minimize frequency

variation with VIN and VOUT as previously explained. This

provides a pseudo fixed frequency that is explained in the

Pseudo-Fixed Frequency section. To allow headroom for VIN

and VOUT sensing, adhere to the following two equations:

Because the ADP1882/ADP1883 can respond rapidly to sudden

changes in load demand, the recovery period in which the output

voltage settles back to its original steady state operating point is

much quicker than it would be for a fixed-frequency equivalent

Therefore, using a pseudo-fixed frequency results in signifi-

cantly better load transient performance than using a fixed

frequency.

VDD ≥ VIN/8 + 1.5

VDD ≥ VOUT/4

For typical applications where VDD = 5 V, these equations are

not relevant; however, for lower VDD inputs, care may be

required.

PSEUDO-FIXED FREQUENCY

The ADP1882/ADP1883 employ a constant on-time control

scheme. During steady state operation, the switching frequency

stays relatively constant, or pseudo-fixed. This is due to the one-

shot tON timer, which produces a high-side PWM pulse with

a fixed duration, given that external conditions such as input

voltage, output voltage, and load current are also at steady state.

During load transients, the frequency momentarily changes for

the duration of the transient event so that the output comes back

within regulation more quickly than if the frequency were fixed

or if it were to remain unchanged. After the transient event is

complete, the frequency returns to a pseudo-fixed value to

a first-order.

VALLEY

TRIP POINTS

LOAD CURRENT

DEMAND

ERROR AMP

OUTPUT

PWM OUTPUT

fSW

CS AMP

OUTPUT

Figure 78. Load Transient Response Operation

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