ADP1879 Analog Devices, ADP1879 Datasheet - Page 22

• Manufacturer Part Number: ADP1879
• Description: Synchronous Buck Controller with Constant On-Time and Valley Current Mode with Power Saving Mode
• Manufacturer: Analog Devices

ADP1878/ADP1879
The system remains in idle mode until the output voltage drops
below regulation. Next, a PWM pulse is produced, turning on the
high-side MOSFET to maintain system regulation. The
does not have an internal clock; it switches purely as a hysteretic
controller, as described in this section.
TIMER OPERATION
The
which provides a variety of benefits, including improved load
and line transient response when compared with a constant
(fixed) frequency current-mode control loop of comparable
loop design. The constant on-time timer, or t
the high-side input voltage (V
using SW waveform information to produce an adjustable one
shot PWM pulse. The pulse varies the on-time of the high-side
MOSFET in response to dynamic changes in input voltage, output
voltage, and load current conditions to maintain output regula-
tion. The timer generates an on-time (t
proportional to V
where K is a constant that is trimmed using an RC timer product
for the 300 kHz, 600 kHz, and 1.0 MHz frequency options.
The constant on-time (t
varies with V
a way as to keep the switching frequency virtually independent
of V
Figure 76. 10 mV Offset to Ensure Prevention of Negative Inductor Current
ADP1878/ADP1879
IN
I
and V
LOAD
SW
LS
0A
OUT
IN
t
ON
and V
.
IN
Figure 77. Constant On-Time Time
.
ANOTHER
TRIGGERED WHEN V
FALLS BELOW REGULATION
OUT
ON
employ a constant on-time architecture,
. However, this variation occurs in such
INFORMATION
ZERO-CROSS COMPARATOR
DETECTS 10mV OFFSET AND
TURNS OFF LS
) is not strictly constant because it
10mV = R
SW
t
ON
IN
) and the output voltage (V
EDGE IS
C
ON
VREG
I
× I
OUT
ON
LOAD
R
IN IDLE MODE
(TRIMMED)
) pulse that is inversely
HS AND LS
ON
timer, senses
t
V
ON
IN
ADP1879
OUT
Rev. 0 | Page 22 of 40
)
The t
to the constant on-time control loop, makes it a pseudo fixed
frequency to a first-order approximation.
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 if the feedforward technique is not used.
The feedforward technique establishes the following relationship:
where f
600 kHz, and 1.0 MHz).
The t
variation as previously explained. This provides pseudo fixed
frequency as explained in the Pseudo Fixed Frequency section.
To allow headroom for V
following equations:
For typical applications where V
not relevant; however, for lower V
PSEUDO FIXED FREQUENCY
The
scheme. During steady state operation, the switching frequency
stays relatively constant, or pseudo fixed. This is due to the one
shot t
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 quicker 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 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
with the output, V
output (COMP). In addition, the current sense amplifier senses
new inductor current information during this positive load
transient event. The output voltage reaction of the error amplifier 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
ADP1878/ADP1879
V
V
ON
ON
ON
REG
REG
SW
timer uses a feedforward technique that, when applied
timer senses V
timer that produces a high-side PWM pulse with a
≥ V
≥ V
is the controller switching frequency (300 kHz,
1
IN
OUT
/8 + 1.5
/4
OUT
) produces new voltage information at its
IN
and V
employ a constant on-time control
IN
and V
OUT
REG
REG
OUT
to minimize frequency
inputs, care may be required.
is 5 V, these equations are
sensing, adhere to the
Data Sheet