ISL6566A Datasheet, PDF (21/28 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with Two Integrated MOSFET Drivers and One External Driver Signal

ISL6566A
dissipation in the controller itself, P_DR, can be roughly
estimated as:
P_DR = P_{DR_UP} + P_{DR_LOW} + P_BOOT + (I_Q • VCC) (EQ. 23)
P_BOOT
=
-^P----^Q----^g----^_--^Q-----¹-
3
P _{D R _UP}
=



-_R----_H----_I--₁--^R---+--^H--_R--^I--¹-_E----_X----_T---₁--
+
R^-----_L^---_O^-----₁^{R----+-L---O-}R^----1-_E^----_X^----_T^---₁^- _{}
•
P_{-----Q----g----_--Q-----1-}
3
P _{D R _LOW}
=



-_R----_H----_I--₂--^R---+--^H--_R--^I--²-_E----_X----_T---₂--
+
R^-----_L^---_O^-----₂^{R----+-L---O-}R^----2-_E^----_X^----_T^---₂^- _{}
• ^P-----^Q----^g----^_--^Q-----²-
2
R_EXT1
=
R_G
1
+
R_{-----G-----I-1--}
N_Q1
R_EXT2
=
R_G2
+
R_{-----G-----I-2--}
N_Q2
Current Balancing Component Selection
The ISL6566A senses the channel load current by sampling
the voltage across the lower MOSFET r_DS(ON), as shown in
Figure 17. The ISEN pins are denoted ISEN1, ISEN2, and
ISEN3. The resistors connected between these pins and the
respective phase nodes determine the gains in the channel-
current balance loop.
ISL6566A
V_IN
CHANNEL N
UPPER MOSFET
I_L
ISEN(n)
R_ISEN
-
IL rDS_(ON)
+
CHANNEL N
LOWER MOSFET
FIGURE 17. ISL6566A INTERNAL AND EXTERNAL CURRENT-
SENSING CIRCUITRY
Select values for these resistors based on the room
temperature r_DS(ON) of the lower MOSFETs; the full-load
operating current, I_FL; and the number of phases, N using
Equation 24.
R_ISEN
=
5-r--^D-0---S--×^{--(-1-O--0---N-–--6-)-}
I--_F----L-
N
(EQ. 24)
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_ISEN for the affected
phases (see the section entitled Channel-Current Balance).
Choose R_ISEN,2 in proportion to the desired decrease in
temperature rise in order to cause proportionally less current
to flow in the hotter phase.
R _{I S E N ,2}
=
R_ISEN
∆_-----T----2-
∆T₁
(EQ. 25)
In Equation 25, make sure that ∆T₂ is the desired temperature
rise above the ambient temperature, and ∆T₁ is the measured
temperature rise above the ambient temperature. While a
single adjustment according to Equation 25 is usually
sufficient, it may occasionally be necessary to adjust R_ISEN
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 ISL6566A 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_COMP and C_COMP must be
chosen so that the time constant of this R_COMP-C_COMP
network matches the time constant of the inductor L/DCR.
Then the resistor R_S must be chosen to set the current
sense network gain, obtaining the desired full load droop
voltage. Follow the steps below to choose the component
values for this R-C network.
1. Choose an arbitrary value for C_COMP. The recommended
value is 0.01µF.
2. Plug the inductor L and DCR component values, and the
values for C_COMP chosen in steps 1, into Equation 26 to
calculate the value for R_COMP.
R_COMP
=
-------------------^L--------------------
DCR ⋅ C_COMP
(EQ. 26)
3. Use the new value for R_COMP obtained from Equation
26, as well as the desired full load current, I_FL, full load
droop voltage, V_DROOP, and inductor DCR in Equation
27 to calculate the value for R_S.
R_S
=
---------^I--^F---^L----------
V_DROOP
⋅
R_COM
P
⋅
D
C
R
(EQ. 27)
21
FN9200.2
July 27, 2005