ISL6308A Datasheet, PDF (20/28 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with High Current Integrated MOSFET Drivers

ISL6308A
The total gate drive power losses are dissipated among the
resistive components along the transition path and in the
bootstrap diode. The portion of the total power dissipated in
the controller itself is the power dissipated in the upper drive
path resistance, P_{DR_UP}, the lower drive path resistance,
P_{DR_UP}, and in the boot strap diode, P_BOOT. The rest of the
power will be dissipated by the external gate resistors (R_G1
and R_G2) and the internal gate resistors (R_GI1 and R_GI2) of
the MOSFETs. Figures 15 and 16 show the typical upper
and lower gate drives turn-on transition path. The total power
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)
P_BOOT
=
-^P----^Q----^g----^_--^Q-----¹-
3
(EQ. 22)
P _{D R _UP}
=
⎛
⎜
⎝
-------------^R-----^H----^I--¹--------------
R_HI1 + R_EXT1
+
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_HI2 + R_EXT2
+
-_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_G
2
+
R_{-----G-----I-2--}
N_Q2
Current Balancing Component Selection
The ISL6308A 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.
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 23.
R_ISEN
=
-r--^D----^S----⁽--^O-----^N----⁾- ⋅
50 ⋅ 10^–6
-I-^F----^L-
N
(EQ. 23)
V_IN
CHANNEL N
UPPER MOSFET
I_L
ISEN(n)
ISL6308A
R_ISEN
CHANNEL N
LOWER MOSFET
-
ILx rDS_(ON)
+
FIGURE 17. ISL6308A INTERNAL AND EXTERNAL CURRENT-
SENSING CIRCUITRY
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 “Channel Current Balance” on
page 11). 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 ⋅
Δ_-----T----2-
ΔT₁
(EQ. 24)
In Equation 24, make sure that ΔT₂ is the desired
temperature rise above the ambient temperature, and ΔT₁ is
the measured 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_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 ISL6308A senses the
total output current by detecting the voltage across the output
inductor DCR of each channel (as described in “Load-Line
(Droop) Regulation” on page 13). As Figure 7 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 25 to
calculate the value for R_COMP.
R_COMP = _D-----_C-----_R------_â‹---^LC-----_C----_O----_M-----_P--
(EQ. 25)
3. Use the new value for R_COMP obtained from Equation 25,
as well as the desired full load current, I_FL, full load droop
voltage, V_DROOP, and inductor DCR in Equation 26 to
calculate the value for R_S.
R_S
=
---------^I--^F---^L----------
V_DROOP
⋅
R_COM
P
⋅
D
C
R
(EQ. 26)
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R_COMP to match the time constants
correctly. The effects of time constant mismatch can be seen
in the form of droop overshoot or undershoot during the
20
FN6669.0
September 9, 2008