ADP1875 Analog Devices, ADP1875 Datasheet - Page 23

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

As soon as the forward current through the lower-side MOSFET
decreases to a level where
the zero-cross comparator (or I
turn off the lower-side MOSFET. From this point, the slope of the
inductor current ramping down becomes steeper (see Figure 76)
as the body diode of the lower-side MOSFET begins to conduct
current and continues conducting current until the remaining
energy stored in the inductor has been depleted.
The system remains in idle mode until the output voltage drops
below regulation. A PWM pulse is then produced, turning on the
upper-side MOSFET to maintain system regulation. The ADP1875
does not have an internal clock, so it switches purely as a hysteretic
controller as described in this section.
TIMER OPERATION
The ADP1874/ADP1875 employ a constant on-time architecture,
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 upper-side
MOSFET in response to dynamic changes in input voltage,
output voltage, and load current conditions to maintain output
regulation. The timer generates an on-time (t
inversely 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.
Figure 76. 10 mV Offset to Ensure Prevention of Negative Inductor Current
10 mV = I
t
I
ON
LOAD
SW
LS
0A
=
K
×
Q2
V
V
OUT
× R
IN
ANOTHER
TRIGGERED WHEN V
FALLS BELOW REGULATION
ON(Q2)
ZERO-CROSS COMPARATOR
DETECTS 10mV OFFSET AND
TURNS OFF LS
10mV = R
IN
t
.
ON
IN
REV
) and the output voltage (V
EDGE IS
ON
comparator) emits a signal to
× I
OUT
LOAD
IN IDLE MODE
HS AND LS
ON
ON
) pulse that is
timer, senses
t
ON
OUT
Rev. 0 | Page 23 of 44
)
The constant on-time (t
varies with V
a way as to keep the switching frequency virtually independent
of V
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
as previously explained. This provides pseudo-fixed frequency
as explained in the Pseudo-Fixed Frequency section. To allow
headroom for V
equations:
For typical applications where VREG is 5 V, these equations are not
relevant; however, care may be required for lower VREG/VIN
inputs.
IN
VREG ≥ V
VREG ≥ V
ON
ON
f
and V
SW
SW
timer senses V
timer uses a feedforward technique, which when applied
=
is the controller switching frequency (300 kHz,
K
1
OUT
t
IN
ON
IN
OUT
and V
.
IN
/8 + 1.5
Figure 77. Constant On-Time Time
and V
/4
OUT
IN
and V
ON
INFORMATION
OUT
. However, this variation occurs in such
) is not strictly constant because it
sensing, adhere to the following
SW
OUT
to minimize frequency variation
C
ADP1874/ADP1875
VREG
I
R
(TRIMMED)
V
IN