ISL6322G Datasheet, PDF (33/39 Pages) Intersil Corporation – Two-Phase Buck PWM Controller with Integrated MOSFET Drivers, I2C Interface and Phase Dropping

ISL6322G
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 19 and 20 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 with Equation 29.
Inductor DCR Current Sensing Component
Selection
The ISL6322G senses each individual channel’s inductor
current by detecting the voltage across the output inductor
DCR of that channel (as described in “Continuous Current
Sampling” on page 12). As Figure 21 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.
The R-C network across the inductor also sets the
overcurrent trip threshold for the regulator. Before the R-C
components can be selected, the desired overcurrent
protection level should be chosen. The minimum overcurrent
trip threshold the controller can support is dictated by the
DCR of the inductors and the number of active channels. To
calculate the minimum overcurrent trip level, I_OCP,min, use
Equation 30, where N is the number of active channels, and
DCR is the individual inductor’s DCR.
I_{OCP, min}
=
0_----._--0_---3_---6_-----⋅_---N_---
DCR
(EQ. 30)
MOSFET
DRIVER
V_IN
UGATE(n)
LGATE(n)
ISL6322G INTERNAL CIRCUIT
IL
L
DCR
INDUCTOR
V_L(s)
V_C(s)
R₁
C₁
R_2*
V_OUT
C_OUT
I_n
SAMPLE
+
-
I_SEN
V_C(s)
R_ISEN
ISEN-(n)
ISEN+(n)
*R₂ is OPTIONAL
FIGURE 21. DCR SENSING CONFIGURATION
If the desired overcurrent trip level is equal to or less
than the minimum overcurrent trip level, follow the steps
below to choose the component values for the R-C
current sensing 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 31 to
calculate the value for R₁.
R₁
=
------------^L-------------
DCR ⋅ C₁
I_OCP = I_{OCP, min}
(EQ. 31)
3. Resistor R₂ should be left unpopulated.
If the desired overcurrent trip level, I_OCP, is greater than the
minimum overcurrent trip level, I_OCP,min, then a resistor
divider R-C circuit should be used to set the desired trip
level.
Take the following steps to choose the component
values for the resistor divider R-C current sensing
network:
1. Choose an arbitrary value for C₁. The recommended
value is 0.1µF.
2. Plug the inductor L and DCR component values, the
value for C₁ chosen in step 1, the number of active
channels N, and the desired overcurrent protection level
I_OCP into Equations 32 and 33 to calculate the values for
R₁ and R₂.
R₁
=
-------^L-----^â‹---^I--^O-----^C----^P---------
C₁ ⋅ 0.036 ⋅ N
I_OCP > I_{OCP, min}
(EQ. 32)
.
R₂
=
-----------------------------^L-----^â‹---^I--^O----^C-----^P------------------------------
C₁ ⋅ (I_OCP ⋅ DCR – 0.036 ⋅ N)
(EQ. 33)
IOUT Pin Resistor
A copy of the total channel sense current, I_SEN1+I_SEN2,
flows out of the IOUT pin, and a resistor, R_IOUT, placed from
this pin to ground can be used to set the overcurrent
protection trip level. Based on the desired overcurrent trip
threshold, I_OCP, the IOUT pin resistor, R_IOUT, can be
calculated from Equation 34 or Equation 35, depending on
the R-C current sense circuitry being employed. If a basic
R-C sense circuit consisting of C₁ and R₁ is being used, use
Equation 36. If a resistor divider R-C sense circuit consisting
of R₁, R₂, and C₁ is being used, use Equation 36.
R_IOUT
=
------------⁶---⁰---⁰-------------
DCR ⋅ I_OCP
I_OCP ≤ I_{OCP, min} (EQ. 34)
R_IOUT
=
D^-----C^-----R^--6---0-⋅^--0-I^--_O^----_C^----_P^--
⋅
⎛
⎜
⎝
-^R----¹--_R--+---₂--^R-----²-_{⎠⎟⎞}
I_OCP > I_{OCP, min (EQ. 35)}
33
FN6715.0
May 22, 2008