ISL78226 Datasheet, PDF (39/94 Pages) Intersil Corporation – Cycle-by-cycle peak current limiting

ISL78226
Light-Load Efficiency Enhancement
For switching mode power supplies, the total loss is related to
conduction loss and switching loss. At heavy load, the conduction
loss dominates, while the switching loss dominates at light load
condition. Therefore, if a multiphase converter is running at a
fixed phase number for the entire load range, the efficiency
starts to drop significantly below a certain load current. The
ISL78226 has selectable automatic phase dropping, Also, the
cycle-by-cycle diode emulation and the pulse skipping features
serve to enhance the light-load efficiency. By observing the total
output current in Buck mode and input current in Boost mode
on-the-fly, and dropping the active phases, the system can
achieve optimized efficiency over the entire load range.
DIODE EMULATION AT LIGHT-LOAD CONDITION
MODE pin enables the selection of Diode Emulation (DE) mode or
Forced PWM (FPWM or CCM) mode. When MODE pin is
connected to GND directly or a 68kΩ (typical) resistor is
connected from MODE pin to GND, the DE mode will be activated.
If the MODE pin is connected to VCC directly or a 33kΩ(typical)
resistor is connected to MODE pin to GND, the Forced PWM mode
will be activated.
The MODE pin consolidates the selection of switching mode (DE
mode or FPWM mode) and fault response (hiccup or latch-off).
For details on MODE pin setting, refer to the “Switching Mode and
Fault Response Selection” section on page 47.
When the Diode Emulation mode is enabled, the ISL78226 has
cycle-by-cycle diode emulation operation at light load and
achieves Discontinuous Conduction Mode (DCM) operation. With
DE mode operation, negative current is prevented and the
conduction loss is reduced, therefore, high efficiency can be
achieved at light load conditions.
PULSE SKIPPING AT DEEP LIGHT-LOAD CONDITION
If the converter enters DE mode and the load is still reducing,
eventually pulse skipping will occur to increase the deep light-
load efficiency. Either one enabled phase or a combination of
phases will be pulse skipping at these deep light-load conditions.
AUTOMATIC PHASE DROPPING/ADDING
To enhance the light-load efficiency, the ISL78226 has an
Adjustable Automatic Phase Dropping/Adding option. The phase
dropping/adding decision will be done by the master device only.
The slave devices will follow the phase dropping/adding decision
made by the master device.
Phase dropping is available only when the tri-state driver is used.
So, if PWM_TRI is low, phase dropping related signals will be
ignored and all phases will be active.
The PD_CTRL pin controls enabling or disabling the phase
dropping feature, and phase dropping/adding threshold. The
phase dropping function can be enabled by connecting the
appropriate resistor from PD_CTRL to GND. By connecting
PD_CTRL pin to VCC or leaving it floating, the phase dropping
function is disabled.
When the phase drop function is enabled, the ISL78226
automatically drops or adds phases by comparing the voltage at
IMON pin (VIMON) to the phase dropping/adding thresholds.
These thresholds are determined by the resistor value connected
from PD_CTRL pin to GND, where V_IMON is proportional to the
average output current in Buck mode or average input current in
Boost mode.
Phase Dropping/Adding at Master Device
If the PD_CTRL pin of the master device is connected to the VCC,
the Adjustable Automatic Phase Dropping/Adding function is
disabled and the device operates with its maximum phase count.
If a resistor (R_PDCTRL) is connected between PD_CTRL of the
master device and GND, and the voltage at the PD_CTRL pin
(V_PDCTRL) is lower than its disable threshold 4V, the Adjustable
Automatic Phase Dropping/Adding function is enabled. When
the ISL78226 controller works in this mode, it will automatically
adjust the active phase number by comparing the voltage at
IMON pin (VIMON), which is proportional to the total output
current, to the threshold voltage defined by the PD_CTRL voltage.
PD_CTRL pin sources a 40µA constant current to the resistor
(R_PDCTRL), which is connected to this pin and generates the
reference voltage (V_PDCTRL) to determine the threshold of phase
dropping and adding. 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 IMON pin (V_IMON) voltage at the PD_CRTL
pin (V_PDCTRL) voltage.
The phase dropping/adding threshold is defined as follows:
V_PDCTRL = 40µA*R_PDCTRL:
If maximum phase count is configured as 6-phase:
• 6 to 4 threshold = 88.2% of V_PDCTRL (typical)
• 4 to 3 threshold = 84.2% of V_PDCTRL (typical)
• 3 to 2 threshold = 80.3% of V_PDCTRL (typical)
If maximum phase count is configured as 4-phase:
• 4 to 3 threshold = 88.2% of VPDCTRL (typical)
• 3 to 2 threshold = 83.6% of VPDCTRL (typical)
If maximum phase count is configured as 3-phase:
• 3 to 2 threshold = 86.8% of VPDCTRL (typical)
If PWM_TRI is tied to VCC, the PWM output of dropped phases
will be 2.5V. The external driver has to identify this tri-state signal
and turn off both the low-side and high-side switches accordingly.
For better transient response during phase dropping, the
ISL78226 will gradually reduce the duty cycle of the dropping
phase from steady state to zero, typically within 15 switching
cycles. This gradual dropping scheme will help with a smooth
change of the PWM signal and, in turn, will help to stabilize the
system when phase dropping happens.
The ISL78226 also has an automatic phase adding feature
similar to phase dropping. When adding a phase, the required
adding phases will be added instantly to take care of the
increased load condition. The phase adding scheme is controlled
by three factors.
1. The maximum configured phase number.
Submit Document Feedback ₃₉
FN8887.0
November 7, 2016