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ISL6552 Datasheet(PDF) 13 Page - Renesas Technology Corp

Part # ISL6552
Description  Microprocessor CORE Voltage Regulator Multi-Phase Buck PWM Controller
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Manufacturer  RENESAS [Renesas Technology Corp]
Direct Link  http://www.renesas.com
Logo RENESAS - Renesas Technology Corp

ISL6552 Datasheet(HTML) 13 Page - Renesas Technology Corp

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ISL6552
FN4918 Rev 2.00
Page 13 of 18
July 2004
under full load. This droop compensation allows larger transient
voltage deviations and thus reduces the size and cost of the
output filter components.
RIN should be selected to give the desired “droop” voltage at the
normal full load current 50
A applied through the RISEN
resistor (or at a different full load current if adjusted as outlined
in the Over-Current, Selecting RISEN section).
RIN = Vdroop/50A
For a Vdroop of 80mV, RIN = 1.6k
The AC feedback components, RFB and Cc, are scaled in
relation to RIN.
Current Balancing
The detected currents are also used to balance the phase
currents.
Each phase’s current is compared to the average of all phase
currents, and the difference is used to create an offset in that
phase’s PWM comparator. The offset is in a direction to reduce
the imbalance.
The balancing circuit can not make up for a difference in rDS(ON)
between synchronous rectifiers. If a FET has a higher rDS(ON),
the current through that phase will be reduced.
Figures 8 and 9 show the inductor current of a two phase
system without and with current balancing.
Inductor Current
The inductor current in each phase of a multi-phase Buck
converter has two components. There is a current equal to the
load current divided by the number of phases (ILT / n), and a
sawtooth current, (iPK-PK) resulting from switching. The
sawtooth component is dependent on the size of the inductors,
the switching frequency of each phase, and the values of the
input and output voltage. Ignoring secondary effects, such as
series resistance, the peak to peak value of the sawtooth
current can be described by:
iPK-PK = (VIN x VCORE - VCORE2)/(L x FSW x VIN)
Where:
VCORE = DC value of the output or VID voltage
VIN = DC value of the input or supply voltage
L = value of the inductor
FSW = switching frequency
Example: For VCORE= 1.6V,
VIN = 12V,
L = 1.3
H,
FSW = 250kHz,
Then iPK-PK = 4.3A
The inductor, or load current, flows alternately from VIN
through Q1 and from ground through Q2. The ISL6552
samples the on-state voltage drop across each Q2 transistor to
indicate the inductor current in that phase. The voltage drop is
sampled 1/3 of a switching period, i/FSW, after Q1 is turned
OFF and Q2 is turned on. Because of the sawtooth current
component, the sampled current is different from the average
current per phase. Neglecting secondary effects, the sampled
current (ISAMPLE) can be related to the load current (ILT) by:
ISAMPLE = ILT / n + (VINVCORE -3VCORE2)/(6L x FSW x VIN)
Where:
ILT = total load current
n = the number of channels
Example: Using the previously given conditions, and
For
ILT = 100A,
n = 4
Then
ISAMPLE = 25.49A
As discussed previously, the voltage drop across each Q2
transistor at the point in time when current is sampled is rDSON
(Q2) x ISAMPLE. The voltage at Q2’s drain, the PHASE node,
is applied through the RISEN resistor to the ISL6552 ISEN pin.
This pin is held at virtual ground, so the current into ISEN is:
ISENSE = ISAMPLE x rDS(ON) (Q2)/RISEN.
RIsen = ISAMPLE x rDS(ON) (Q2)/50A
0
5
10
15
20
25
FIGURE 8. TWO CHANNEL MULTI-PHASE SYSTEM WITH
CURRENT BALANCING DISABLED
0
5
10
15
20
25
FIGURE 9. TWO CHANNEL MULTI-PHASE SYSTEM WITH
CURRENT BALANCING ENABLED


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