ISL78225 Datasheet, PDF (17/21 Pages) Intersil Corporation – 4-Phase Interleaved Boost PWM Controller with Light Load Efficiency Enhancement

ISL78225
Inductor DCR Sensing
An inductor’s winding is characteristic of a distributed resistance
as measured by the DCR (Direct Current Resistance) parameter.
IL
VIN
DCR
L
VOUT
C
R
ISEN
RSET
INDUCTOR DCR
CURRENT SENSING
ISEN
CSA
ISL78225 INTERNAL CIRCUITS
ISEN(n)P
ISEN(n)N
FIGURE 21. INDUCTOR DCR CURRENT SENSING
Consider the inductor DCR as a separate lumped quantity, as
shown in Figure 21. The channel current I_L, flowing through the
inductor, will also pass through the DCR. Equation 6 shows the
s-domain equivalent voltage across the inductor V_L.
V_L = I_L ⋅ (s ⋅ L + DCR)
(EQ. 6)
A simple R-C network across the inductor extracts the DCR
voltage, as shown in Figure 21.
The voltage on the capacitor V_C, can be shown to be proportional
to the channel current I_L, see Equation 7.
⎝⎛^s
⋅
-----^L------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
V_C
=
----------------------------------------------------------------
(s ⋅ RC + 1)
(EQ. 7)
If the R-C network components are selected such that the RC
time constant (= R*C) matches the inductor time constant
(= L/DCR), the voltage across the capacitor V_C is equal to the
voltage drop across the DCR, i.e., proportional to the channel
current.
With the internal low-offset differential current sense amplifier,
the capacitor voltage V_C is replicated across the sense resistor
R_SET. Therefore, the current flow into the ISENxP pin is
proportional to the inductor current. Equation 8 shows that the
ratio of the channel current to the sensed current I_SEN is driven
by the value of the sense resistor and the DCR of the inductor.
I_SEN
=
I_L
⋅
-^D----^C----^R---
R_SET
(EQ. 8)
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 ISL78225 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 previously 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 ISL78225 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 ISL78225 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 4-phase operation and
the MODE pin is set to 1.2V, the converter will monitor the V_IOUT
and compared to 1.2V; if less than 900mV (75% of 1.2V), it will
drop to 3-phase; if less than 600mV (50% of 1.2V), it will drop to
2-phase. The detailed threshold setting is shown in the “Electrical
Specifications” 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 ISL78225 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 ISL78225 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 previously mentioned phase-adding method can take care of
the condition that the load current increases slowly. However, if
the load is increasing quickly, the IC will use a different phase
adding scheme. The ISL78225 monitors the individual channel
current for all active phases. During phase adding, the system
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FN7909.0
December 15, 2011