ISL78220ANEZ Datasheet, PDF (17/22 Pages) Intersil Corporation – 6-Phase Interleaved Boost PWM Controller with Light Load Efficiency Enhancement

ISL78220
VIN
RSEN
L
ISEN
RSET
VOUT
ISEN
CSA
ISL78220 INTERNAL CIRCUITS
ISEN(n)P
ISEN(n)N
SENSE RESISTOR
CURRENT SENSING
FIGURE 20. SENSE RESISTOR CURRENT SENSING
Inductor DCR Sensing
An inductor’s winding is characteristic of a distributed resistance
as measured by the DCR (Direct Current Resistance) parameter.
VIN
DCR
C
IL
L
R
VOUT
ISEN
RSET
INDUCTOR DCR
CURRENT SENSING
ISEN
CSA
ISEN(n)P
ISEN(n)N
ISL78220 INTERNAL CIRCUITS
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.
V_C
=
⎛
⎝
s
⋅
-----^L------
DCR
+
1_⎠⎞
⋅
(DCR
⋅
I_L)
----------------------------------------------------------------
(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 flows 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 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 compare 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 “Electrical
Specifications” table on page 8.
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
17
FN7688.3
December 24, 2013