ISL78220 Datasheet, PDF (18/22 Pages) Intersil Corporation – 6-Phase Interleaved Boost PWM Controller with Light Load Efficiency Enhancement

ISL78220
Light Load Efficiency
Enhancement Schemes
For switching mode power supplies, the total loss is related to
both the conduction loss and the switching loss. At heavy load
the conduction loss is dominant while the switching loss will take
charge at light load condition. So, if a multiphase converter is
running at a fixed phase number for the entire load range, we will
observe that below a certain load point the total efficiency starts
to drop heavily. The ISL78220 has automatic phase dropping,
cycle-by-cycle diode emulation and pulse skipping features to
enhance the light load efficiency. By observing the total input
current on-the-fly and dropping the active phase numbers
accordingly, the overall system can achieve optimized efficiency
over the entire load range. All the above mentioned light load
enhancement features can be disabled by simply pulling the
MODE pin to VCC.
Adjustable Automatic Phase
Dropping/Adding at Light Load Condition
If the MODE pin is connected to a resistor to GND, and the voltage
on the MODE pin is lower than its disable threshold 4V, the
adjustable automatic phase dropping/adding mode will be
enabled. When the ISL78220 controller works in this mode, it
will automatically adjust the active phase number by comparing
the V_MODE and V_IOUT, which represents sensed total current
information. The V_MODE sets the overall phase dropping
threshold, and the V_IOUT is proportional to the input current,
which is in turn proportional to the load current. The smaller the
load current, the lower the voltage observed on the IOUT pin, and
the ISL78220 will drop phases in operation. Once the MODE pin
voltage is fixed, the threshold to determine how many phases are
in operation is dependent on two factors:
1. The maximum configured phase number.
2. The voltage on the IOUT pin (V_IOUT).
For example, if the converter is working in 6-phase operation and
the MODE pin is set to 1.2V, in this case the converter will
monitor the V_IOUT and compared to 1.2V, such that when the
V_IOUT is less than 800mV (66.6% of 1.2V), it will drop from
6-phase to 4-phase; if less than 600mV (50% of 1.2V), it will drop
to 3-phase; if less than 400mV (33% of 1.2V), it will drop to
2-phase. The detailed threshold setting is shown in the table on
page 7.
If PWM_TRI is tied to VCC, the dropped phase will provide a 2.5V
tri-level signal at its PWM output. The external driver has to
identify this tri-state signal and turn off both the lower and upper
switches accordingly. For better transient response during phase
dropping, the ISL78220 will gradually reduce the duty cycle of
the phase from steady state to zero, typically within 15 switching
cycles. This gradual dropping scheme will help smooth the
change of the PWM signal and, in turn, will help to stabilize the
system when phase dropping happens.
The ISL78220 also has an automatic phase adding feature
similar to phase dropping, but when doing phase adding there
will not be 15 switching cycles gradually adding. It will add
phases instantly to take care of the increased load condition. The
phase adding scheme is controlled by three factors.
1. The maximum configured phase number
2. The voltage on the IOUT pin (V_IOUT).
3. Individual phase current
Factors 1 and 2 are similar to the phase dropping scheme. If the
V_IOUT is higher than the phase dropping threshold plus the
hysteresis voltage, the dropped phase will be added back one by
one instantly.
The above mentioned phase-adding method can take care of the
condition that the load current increases slowly. However, if the
load is fast increasing the IC will using different phase adding
scheme. The ISL78220 monitors the individual channel current
for all active phases. During phase adding the system will bring
down the pre-set channel current limit to 2/3 of its original value
(160µA). If any of the phase’s sensed current hit the 2/3 of
pre-set channel current limit threshold (i.e: 106.7µA), all the
phases will be added back instantly. After a fixed 1.5ms delay,
the phase dropping circuit will be activated and the system will
react to drop the phase number to the correct value.
During phase adding when either phase hit the pre-set channel
current limit, there will be 200µs blanking time such that per-
channel OCP will not be triggered during this blanking time.
Diode Emulation at Very Light Load Condition
When phase dropping is asserted and the minimum phase
operation is 2 phases, if the load is still reducing and
synchronous boost structure is used, the ISL78220 controller will
enter into forced cycle-by-cycle diode emulation mode. The PWM
output will be tri-stated when inductor current falls to zero, such
that the synchronous MOSFET can be turned off accordingly
cycle-by-cycle for forced diode emulation. This cycle-by-cycle
diode emulation scheme will only be asserted when two
conditions are met:
1. The PWM_TRI pin voltage is logic HIGH.
2. Only two phases are running either by phase dropping or
initial configuration.
By utilizing the cycle-by-cycle diode emulation scheme in this
way, negative current is prevented and the system can still
optimize the efficiency even at very light load condition.
Pulse Skipping at Deep Light Load Condition
If the converter enters diode emulation mode and the load is still
reducing, eventually pulse skipping will occur to increase the
deep light load efficiency.
18
FN7688.0
December 15, 2011