Datasheet

ADP1882/ADP1883
Rev. 0 | Page 23 of 40
C
R (TRIMMED)
VREG
t
ON
V
IN
I
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.
SW
INFORMATION
When a positive load step occurs, the error amplifier (out of
phase of the output, V
OUT
) 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).
08901-076
Figure 77. Constant On-Time Timer
The constant on time (t
ON
) is not strictly constant because it
varies with V
IN
and V
OUT
. However, this variation occurs in such
a way as to keep the switching frequency virtually independent
of V
IN
and V
OUT
.
The t
ON
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 V
OUT
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 V
OUT
back up in value and within the regulation
window.
The feedforward technique establishes the following relationship:
f
SW
=
1
K
where f
SW
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 V
OUT
rising. This effectively increases the inductor
demagnetizing phase, helping to bring V
OUT
within regulation.
In this case, the switching frequency decreases, or experiences
a foldback, to help facilitate output voltage recovery.
The t
ON
timer senses V
IN
and V
OUT
to minimize frequency
variation with V
IN
and V
OUT
as previously explained. This
provides a pseudo fixed frequency that is explained in the
Pseudo-Fixed Frequency section. To allow headroom for V
IN
and V
OUT
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.
V
DD
≥ V
IN
/8 + 1.5
V
DD
≥ V
OUT
/4
For typical applications where V
DD
= 5 V, these equations are
not relevant; however, for lower V
DD
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 t
ON
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
f
SW
> f
SW
CS AMP
OUTPUT
08901-077
Figure 78. Load Transient Response Operation