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| 型号： | ADP1872-BL2-EVZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 22/40页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR ADP1872 |
| 标准包装： | 1 |
| 主要目的： | DC/DC，步降 |
| 稳压器拓扑结构： | 降压 |
| 板类型： | 裸（未填充） |
| 已供物品： | 裸板 |
| 已用 IC / 零件： | ADP1872 |
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ADP1872/ADP1873
Data Sheet
t ON
C
SW
INFORMATION
I
VDD
R (TRIMMED)
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.
When a positive load step occurs, the error amplifier (out of
phase of the output, V OUT ) produces new voltage information
Figure 76. Constant On-Time Timer
The constant on-time (t ON ) is not strictly constant because it varies
with VIN and V OUT . However, this variation occurs in such a
way as to keep the switching frequency virtually independent
of VIN 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 22 to Figure 33. 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:
f SW = 1/ K
where f SW is the controller switching frequency (300 kHz,
600 kHz, and 1.0 MHz).
The t ON timer senses VIN and V OUT to minimize frequency variation
with VIN and V OUT as previously explained. This provides a
pseudo-fixed frequency, see the Pseudo-Fixed Frequency section
for additional information. To allow headroom for VIN/V OUT
sensing, the following two equations must be adhered to. For
typical applications where V DD is 5 V, these equations are not
relevant; however, for lower V DD , care may be required.
V DD ≥ VIN /8 + 1.5
V DD ≥ V OUT /4
PSEUDO-FIXED FREQUENCY
The ADP1872/ADP1873 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
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).
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 to within regulation.
In this case, the switching frequency decreases, or experiences a
foldback, to help facilitate output voltage recovery.
Because the ADP1872/ADP1873 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.
LOAD CURRENT
DEMAND
CS AMP
OUTPUT
ERROR AMP
within regulation quicker than if the frequency were fixed or if
OUTPUT
VALLEY
TRIP POINTS
it were to remain unchanged. After the transient event is complete,
the frequency returns to a pseudo-fixed value to a first-order.
PWM OUTPUT
f SW
> f SW
Figure 77. Load Transient Response Operation
Rev. B | Page 22 of 40
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