ADP1883-1.0-EVALZ Analog Devices Inc, ADP1883-1.0-EVALZ Datasheet - Page 23

• Manufacturer Part Number: ADP1883-1.0-EVALZ
• Description: 1 MHz Synchronous Current-Mode Buck Controller Eval. Board
• Manufacturer: Analog Devices Inc

Specifications of ADP1883-1.0-EVALZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: PWM Buck Controller
• Kit Application Type: Power Management - Voltage Regulator
• Silicon Core Number: ADP1883
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The constant on time (t
varies with V
a way as to keep the switching frequency virtually independent
of V
The t
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 feedforward technique establishes the following relationship:
where f
600 kHz, and 1.0 MHz).
The t
variation with V
provides a pseudo fixed frequency that is explained in the
Pseudo-Fixed Frequency section. To allow headroom for V
and V
For typical applications where V
not relevant; however, for lower V
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
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.
IN
f
V
V
ON
ON
SW
ON
OUT
and V
DD
DD
SW
timer uses a feedforward technique, applied to the constant
=
timer senses V
timer, which produces a high-side PWM pulse with
≥ V
≥ V
sensing, adhere to the following two equations:
is the controller switching frequency (300 kHz,
K
1
OUT
IN
OUT
t
IN
ON
/8 + 1.5
and V
.
/4
IN
Figure 77. Constant On-Time Timer
and V
OUT
IN
ON
INFORMATION
. However, this variation occurs in such
OUT
and V
) is not strictly constant because it
as previously explained. This
SW
OUT
DD
C
DD
to minimize frequency
= 5 V, these equations are
VREG
I
inputs, care may be
R
(TRIMMED)
V
IN
IN
Rev. 0 | Page 23 of 40
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.
When a positive load step occurs, the error amplifier (out of
phase of the output, V
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).
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
transient up and, therefore, shortens the off time. This resultant
increase in frequency during a positive load transient helps to
quickly bring V
window.
Similarly, a negative load step causes the off time to lengthen in
response to V
demagnetizing phase, helping to bring V
In this case, the switching frequency decreases, or experiences
a foldback, to help facilitate output voltage recovery.
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.
PWM OUTPUT
ERROR AMP
OUTPUT
CS AMP
Figure 78. Load Transient Response Operation
OUTPUT
OUT
LOAD CURRENT
OUT
DEMAND
rising. This effectively increases the inductor
OUT
back up in value and within the regulation
f
to transient down, which causes COMP to
SW
OUT
) produces new voltage information
ADP1882/ADP1883
>
f
SW
VALLEY
TRIP POINTS
OUT
within regulation.