ADP1875 Analog Devices, ADP1875 Datasheet - Page 24

• Manufacturer Part Number: ADP1875
• Description: Synchronous Buck Controller with Constant On-Time, Valley Current Mode, and Power Saving Mode
• Manufacturer: Analog Devices

ADP1874/ADP1875
PSEUDO-FIXED FREQUENCY
The ADP1874/ADP1875 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
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 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 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
transient up and, therefore, shortens the off time. This resulting
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 ADP1874/ADP1875 have 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 compared to
using a fixed frequency.
ON
timer that produces a high-side PWM pulse with a fixed
OUT
OUT
rising. This effectively increases the inductor
OUT
OUT
back up in value and within the regulation
) produces new voltage information at its
to transient down, which causes COMP to
OUT
within regulation.
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POWER GOOD MONITORING
The ADP1874/ADP1875 power good circuitry monitors the
output voltage via the FB pin. The PGOOD pin is an open-drain
output that can be pulled up by an external resistor to a voltage
rail that does not necessarily have to be VREG. When the internal
NMOS switch is in high impedance (off state), this means that
the PGOOD pin is logic high, and the output voltage via the FB
pin is within the specified regulation window. When the internal
switch is turned on, PGOOD is internally pulled low when the
output voltage via the FB pin is outside this regulation window.
The power good window is defined with a typical upper
specification of +90 mV and a lower specification of −70 mV
below the FB voltage of 600 mV. When an overvoltage event occurs
at the output, there is a typical propagation delay of 12 μs prior
to the PGOOD pin deassertion (logic low). When the output
voltage re-enters the regulation window, there is a propagation
delay of 12 μs prior to PGOOD reasserting back to a logic high
state. When the output is outside the regulation window, the
PGOOD open drain switch is capable of sinking 1mA of current
and provides 140 mV of drop across this switch. The user is free
to tie the external pull-up resistor (R
20 V. The following equation provides the proper external pull-up
resistor value:
where:
R
V
PGD
EXT
is the PGOOD external resistor.
is a user-chosen voltage rail.
R
PWM OUTPUT
PGD
Figure 79. Power Good, Output Voltage Monitoring Circuit
690mV
530mV
600mV
=
ERROR AMP
OUTPUT
CS AMP
FB
OUTPUT
V
Figure 78. Load Transient Response Operation
LOAD CURRENT
EXT
DEMAND
1
−
mA
140
f
SW
mV
>
f
SW
RES
) to any voltage rail up to
+
140mV
–
VALLEY
TRIP POINTS
PGOOD
1mA
V
EXT
R
PGD