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ADP1871 Datasheet(PDF) 23 Page - Analog Devices

Part # ADP1871
Description  Synchronous Buck Controller with Constant On-Time and Valley Current Mode
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

ADP1871 Datasheet(HTML) 23 Page - Analog Devices

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ADP1870/ADP1871
Rev. 0 | Page 23 of 44
C
R (TRIMMED)
VREG
tON
VIN
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, 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
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).
Figure 77. Constant On-Time Time
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 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 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 resulting
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:
K
f
SW
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 as previously explained. This provides a pseudo-fixed
frequency as explained in the Pseudo-Fixed Frequency section.
To allow headroom for VIN and VOUT sensing, adhere to the
following equations:
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.
VREG ≥ VIN/8 + 1.5
VREG ≥ VOUT/4
For typical applications where VREG is 5 V, these equations are
not relevant; however, for lower VREG inputs, care may be
required.
VALLEY
TRIP POINTS
LOAD CURRENT
DEMAND
ERROR AMP
OUTPUT
PWM OUTPUT
fSW
>
fSW
CS AMP
OUTPUT
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 tON 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.
Figure 78. Load Transient Response Operation


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