參數(shù)資料
型號： ADP1871ACPZ-0.3-R7
廠商： ANALOG DEVICES INC
元件分類： 穩(wěn)壓器
英文描述： SWITCHING CONTROLLER, PDSO10
封裝： 3 X 3 MM, ROHS COMPLIANT, LFCSP-10
文件頁數(shù)： 23/44頁
文件大小： 1958K
代理商： ADP1871ACPZ-0.3-R7
ADP1870/ADP1871
Rev. A | Page 23 of 44
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.
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
with 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
OUT
to transient down, which causes COMP to
transient up and therefore shortens the off-time. This resulting
increase in frequency during a positive load transient helps to
quickly bring V
OUT
back up in value and within the regulation
window.
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.
Because the ADP1870/ADP1871 has the ability to 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
significantly better load transient performance than using a
fixed frequency.
SW
INFORMATION
0
Figure 77. Constant On-Time Time
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 a 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 utilized.
The feedforward technique establishes the following relationship:
1
=
K
f
SW
where
f
SW
is the controller switching frequency (300 kHz,
600 kHz, and 1.0 MHz).
The t
ON
timer senses V
IN
and V
OUT
to minimize frequency
variation as previously explained. This provides a pseudo-fixed
frequency as explained in the Pseudo-Fixed Frequency section.
To allow headroom for V
IN
and V
OUT
sensing, adhere to the
following equations:
V
REG
≥
V
IN
/8 + 1.5
V
REG
≥ V
OUT
/4
For typical applications where V
REG
is 5 V, these equations are
not relevant; however, for lower V
REG
inputs, care may be
required.
PSEUDO-FIXED FREQUENCY
The ADP1870/ADP1871 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 that 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
frequency value to a first order.
VALLEY
LODEMAND
EOUTPUT
PWM OUTPUT
f
SW
>
f
SW
CS AMP
0
Figure 78. Load Transient Response Operation
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參數(shù)描述
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