ISL6333_14 Datasheet, PDF (33/40 Pages) Intersil Corporation – Three-Phase Buck PWM Controller with Integrated MOSFET Drivers and Light Load Efficiency Enhancements for Intel VR11.1 Applications

ISL6333, ISL6333A, ISL6333B, ISL6333C
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 20 and 21 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 calculated in Equation 32:
P_DR = P_{DR_UP} + P_{DR_LOW} + P_BOOT + (I_Q ⋅ VCC)
P_BOOT
=
-^P----^Q----^g----^_--^Q-----¹-
3
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
(EQ. 32)
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_G2
+
R_{-----G-----I-2--}
N_Q2
Inductor DCR Current Sensing Component
Selection
The controllers sense each individual channel’s inductor current
by detecting the voltage across the output inductor DCR of that
channel (As described in the “Continuous Current Sensing” on
page 21). As Figure 22 illustrates, an R-C network is required to
accurately sense the inductor DCR voltage and convert this
information into a current, which is proportional to the total
output current. The time constant of this R-C network must
match the time constant of the inductor L/DCR.
Follow the steps below to choose the component values for
this RC network.
1. Choose an arbitrary value for C₁. The recommended
value is 0.1µF.
2. Plug the inductor L and DCR component values, and the
value for C₁ chosen in step 1, into Equation 33 to
calculate the value for R₁.
R₁ = _D-----_C-----_R--^L----_â‹---_C-----₁-
(EQ. 33)
Once the R-C network components have been chosen, the
effective internal R_ISEN resistance must then be set. The
R_ISEN resistance sets the gain of the load line regulation loop
as well as the gain of the channel-current balance loop and the
overcurrent trip level. The effective internal R_ISEN resistance is
set through a single resistor on the R_SET pin, R_SET.
MOSFET
DRIVER
I_n
UGATE
IL
L
DCR
LGATE
INDUCTOR
V_L(s)
V_C(s)
R₁
C₁
ISL6333 INTERNAL
CIRCUIT
SENSE
+
-
V_C(s)
ISEN-
R_ISEN
ISEN+
V_OUT
C_OUT
I_SEN
RSET
R_SET
VCC
FIGURE 22. DCR SENSING CONFIGURATION
Use Equation 34 to calculate the value of R_SET. In
Equation 34, DCR is the DCR of the output inductor at room
temperature, I_OCP is the desired overcurrent trip level, and
N is the number of phases. It is recommended that the
desired overcurrent trip level, I_OCP, be chosen so that it’s
30% larger then the maximum load current expected.
R_SET
=
--------^D-----^C-----^R----------
100 × 10^–6
⋅
-^I-^O-----^C----^P--
N
⋅
4_----0_---0_--
3
(EQ. 34)
Due to errors in the inductance or DCR it may be necessary
to adjust the value of R₁ 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
initial load transient spike, as shown in Figure 23. Follow the
steps below to ensure the R-C and inductor L/DCR time
constants are matched accurately.
1. Capture a transient event with the oscilloscope set to
about L/DCR/2 (sec/div). For example, with L = 1µH and
DCR = 1mΩ, set the oscilloscope to 500µs/div.
2. Record ΔV1 and ΔV2 as shown in Figure 23.
3. Select new values, R_1(NEW), for the time constant
resistor based on the original value, R_1(OLD), using
Equation 35.
R_1(NEW) = R_1(OLD) ⋅ ^ΔΔ----_VV----¹²--
(EQ. 35)
4. Replace R₁ with the new value and check to see that the
error is corrected. Repeat the procedure if necessary.
33
FN6520.3
October 8, 2010