ISL8102 Intersil Corporation, ISL8102 Datasheet - Page 19

• Manufacturer Part Number: ISL8102
• Description: Two-Phase Buck PWM Controller
• Manufacturer: Intersil Corporation

Current Balancing Component Selection
The ISL8102 senses the channel load current by sampling
the voltage across the lower MOSFET r
Figure 17. The ISEN pins are denoted ISEN1, and ISEN2.
The resistors connected between these pins and the
respective phase nodes determine the gains in the channel
current balance loop.
Select values for these resistors based on the room
temperature r
operating current, I
Equation 23.
PVCC
R
FIGURE 17. ISL8102 INTERNAL AND EXTERNAL CURRENT-
FIGURE 16. TYPICAL LOWER-GATE DRIVE TURN-ON PATH
ISEN
FIGURE 15. TYPICAL UPPER-GATE DRIVE TURN-ON PATH
PVCC
ISL8102
PHASE
=
R
-----------------------
50 10
R
r
DS ON
BOOT
LO2
HI2
R
R
⋅
LO1
HI1
SENSING CIRCUITRY
(
DS(ON)
–
6
)
⋅
FL
LGATE
I
------- -
ISEN(n)
FL
N
UGATE
of the lower MOSFETs; the full-load
; and the number of phases, N using
CHANNEL N
LOWER MOSFET
R
ISEN
19
R
G2
R
G
G1
G
V
R
C
IN
GI2
GD
R
C
C
GI1
GD
GS
CHANNEL N
UPPER MOSFET
+
C
DS(ON)
-
GS
I
L
x
S
r
S
DS ON
I
L
, as shown in
D
(
D
Q2
C
)
Q1
DS
C
(EQ. 23)
DS
ISL8102
In certain circumstances, it may be necessary to adjust the
value of one or more ISEN resistors. When the components of
one or more channels are inhibited from effectively dissipating
their heat so that the affected channels run hotter than
desired, choose new, smaller values of R
phases (see the section entitled Channel Current Balance).
Choose R
temperature rise in order to cause proportionally less current
to flow in the hotter phase.
In Equation 24, make sure that ∆T
rise above the ambient temperature, and ∆T
temperature rise above the ambient temperature. While a
single adjustment according to Equation 24 is usually
sufficient, it may occasionally be necessary to adjust R
two or more times to achieve optimal thermal balance
between all channels.
Load Line Regulation Component Selection (DCR
Current Sensing)
For accurate load line regulation, the ISL8102 senses the
total output current by detecting the voltage across the
output inductor DCR of each channel (As described in the
Load Line Regulation section). As Figure 18 illustrates, an
R-C network is required to accurately sense the inductor
DCR voltage and convert this information into a “droop”
voltage, which is proportional to the total output current.
Choosing the components for this current sense network is a
two step process. First, R
chosen so that the time constant of this R
network matches the time constant of the inductor L/DCR.
Then the resistor R
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C network.
R
1. Choose an arbitrary value for C
2. Plug the inductor L and DCR component values, and the
3. Use the new value for R
ISEN 2 ,
value is 0.01µF.
values for C
calculate the value for R
as well as the desired full load current, I
voltage, V
calculate the value for R
R
R
COMP
S
=
=
ISEN,2
------------------------ - R
V
R
DROOP
=
ISEN
I
FL
DROOP
---------------------------------------
DCR C
COMP
in proportion to the desired decrease in
⋅
∆T
----------
∆T
⋅
S
⋅
L
must be chosen to set the current
, and inductor DCR in Equation 26 to
COMP
2
1
COMP
chosen in steps 1, into Equation 25 to
COMP
COMP
⋅
COMP
S
DCR
.
and C
2
obtained from Equation 25,
COMP
.
is the desired temperature
COMP
ISEN
. The recommended
COMP
FL
1
is the measured
, full load droop
must be
for the affected
-C
October 19, 2005
COMP
(EQ. 24)
(EQ. 25)
(EQ. 26)
ISEN
FN9247.0