Datasheet

ADP1870/ADP1871 Data Sheet
Rev. B | Page 22 of 44
SYNCHRONOUS RECTIFIER
The ADP1870/ADP1871 employ an internal lower-side MOSFET
driver to drive the external upper- and lower-side MOSFETs.
The synchronous rectifier not only improves overall conduction
efficiency, but also ensures proper charging to the bootstrap
capacitor located at the upper-side driver input. This is beneficial
during startup to provide sufficient drive signal to the external
upper-side MOSFET and to attain fast turn-on response, which is
essential for minimizing switching losses. The integrated upper-
and lower-side MOSFET drivers operate in complementary
fashion with built-in anticross conduction circuitry to prevent
unwanted shoot-through current that may potentially damage the
MOSFETs or reduce efficiency as a result of excessive power loss.
POWER SAVING MODE (PSM) VERSION (ADP1871)
The power saving mode version of the ADP1870 is the ADP1871.
The ADP1871 operates in the discontinuous conduction mode
(DCM) and pulse skips at light load to mid load currents. It
outputs pulses as necessary to maintain output regulation. Unlike
the continuous conduction mode (CCM), DCM operation
prevents negative current, thus allowing improved system
efficiency at light loads. Current in the reverse direction through
this pathway, however, results in power dissipation and therefore
a decrease in efficiency.
HS
HS AND LS ARE OFF
OR IN IDLE MODE
LS
0A
I
LOAD
AS THE INDUCTOR
CURRENT APPROACHES
ZERO CURRENT, THE STATE
MACHINE TURNS OFF THE
LOWER-SIDE MOSFET.
t
ON
t
OFF
08730-072
Figure 74. Discontinuous Mode of Operation (DCM)
To minimize the chance of negative inductor current buildup,
an on-board zero-cross comparator turns off all upper- and
lower-side switching activities when the inductor current
approaches the zero current line, causing the system to enter
idle mode, where the upper- and lower-side MOSFETs are
turned off. To ensure idle mode entry, a 10 mV offset, connected
in series at the SW node, is implemented (see Figure 75).
10mV
ZERO-CROSS
COMPARATOR
Q2
LS
SW
I
Q2
08730-073
Figure 75. Zero-Cross Comparator with 10 mV of Offset
As soon as the forward current through the lower-side
MOSFET decreases to a level where
10 mV = I
Q2
× R
ON(Q2)
the zero-cross comparator (or I
REV
comparator) emits a signal to
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.
HS AND LS
IN IDLE MODE
10mV = R
ON
× I
LOAD
ZERO-CROSS COMPARATOR
DETECTS 10mV OFFSET AND
TURNS OFF LS
SW
LS
0A
I
LOAD
t
ON
ANOTHER t
ON
EDGE IS
TRIGGERED WHEN V
OUT
FALLS BELOW REGULATION
08730-074
Figure 76. 10 mV Offset to Ensure Prevention of Negative Inductor Current
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
ADP1871 does not have an internal clock, so it switches purely
as a hysteretic controller as described in this section.
TIMER OPERATION
The ADP1870/ADP1871 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
ON
timer, senses
the high input voltage (V
IN
) and the output voltage (V
OUT
) using
SW waveform information to produce an adjustable one-shot
PWM pulse that 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 regulation. It then generates
an on-time (t
ON
) pulse that is inversely proportional to V
IN.
IN
OUT
ON
V
V
Kt 
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.