ISL78227 Datasheet, PDF (31/43 Pages) Intersil Corporation – 2-Phase Boost Controller with Integrated Drivers

ISL78227
phase will be used for peak current mode control loop, phase
current balance, individual phase cycle-by-cycle peak current
limiting (OC1), individual phase overcurrent fault protection
(OC2_PEAK), input average Constant Current (CC) control and
average overcurrent protection (OC_AVG), diode emulation and
phase drop control. The internal circuitry shown in Figure 60
represents a single phase. This circuitry is repeated for each
phase.
CURRENT SENSE FOR INDIVIDUAL PHASE - I_SENX
V_IN
+ ^RSENx -
I_L
L
+
+
R_SETxA
-
I_SENx+112µA
+
R_SETxB
-
R_BIASxA
C_ISENx ^-
112µA
+
R_BIASxB
-
ISENxP
ISENxN
V_OUT
112µA
I_BIAS
112µA
I_SENx
CSA
I_SENx
IC INTERNAL CIRCUITS
FIGURE 60. CURRENT-SENSING BLOCK DIAGRAM
The RC network between R_SENx and ISENxP/N pins as shown in
Figure 60 is the recommended configuration. The ISENxP pin
should be connected to the positive potential of the R_{SEN_CHx}
through resistor R_SETx, where in Figure 60 R_SETx is composed by
R_SETxA plus R_SETxB. R_SET is used to set the current sense gain
externally.
R_SETx = R_SETxA + R_SETxB
(EQ. 8)
Since there is an 112µA bias current sinking to each of the
ISENxP and ISENxN pins, R_BIASx with same value to R_SETx should
be placed between the ISENxN pin to the low potential of the
R_SENx, where in Figure 60 R_BIASx is composed by R_BIASxA plus
R_BIASxB.
R_BIASx = R_BIASxA + R_BIASxB
(EQ. 9)
R_BIASx = R_SETx
(EQ. 10)
It is recommended to have R_SETxA = R_BIASxA and
R_SETxB = R_BIASxB, and insert a capacitor C_ISENx between them
as shown in Figure 60. This will form a symmetric noise filter for
the small current sense signals. The differential filtering time
constant equals to (R_SETxA+R_BIASxA)*C_ISENx. This time constant
is typically selected in range of tens of ns depending on the
actual noise levels.
CSA generates the sensed current signal I_SENx by forcing ISENxP
voltage to be equal to ISENxN voltage. Since R_SETx equals to
R_BIASx, the voltage drop across R_SETx and R_BIASx incurred by the
fixed 112µA bias current cancels each other. Therefore, the
resulting current at CSA output I_SENx is proportional to each
phase inductor current I_Lx. I_SENx per phase can be derived in
Equation 11, where I_Lx is the per phase current flowing through
R_SENx.
I_SENx = I_Lx  ^R-R----SS----^EE----^NT----_xx-
(EQ. 11)
R_SENx is normally selected with smallest resistance to minimize
the power loss on it. With R_SENx selected, R_SETx is selected by
the desired cycle-by-cycle peak current limiting level OC1 (refer to
“Peak Current Cycle-by-Cycle Limiting (OC1)” on page 35).
AVERAGE CURRENT SENSE FOR 2 PHASES - IMON
The IMON pin serves to monitor the total average input current of
the 2-phase boost. As shown in Figure 3 on page 7, the individual
current sense signals (I_SENx) are divided by 8 and summed
together. A 17µA offset current is added to form a current source
output at the IMON pin with the value calculated as shown by
Equation 12.
IMON
=



I_{--L----1-R---}_--S-R_{---E--S-T---E-1---N----1-}
+
-^I-^L----²-_R---^--_S-^R---_E--^S--_T--^E-₂---^N----²-_{}
 0.125 + 17  10^–6
(EQ. 12)
Assume R_SEN1 = R_SEN2, R_SET1 = R_SET2, and I_IN = I_L1+I_L2 (which
is the total boost input average current):
IMON = I_IN  ^R-_R----^SS----^EE----^NT--  0.125 + 17  10^–6
(EQ. 13)
As shown in Figure 4 on page 8, a resistor R_IMON is placed
between the IMON pin and ground, which turns the current sense
output from the IMON pin to a voltage V_IMON. A capacitor C_IMON
should be used in parallel with R_IMON to filter out the ripple such
that V_IMON represents the total average input current of the
2-phase boost. V_IMON can be calculated using Equation 14.
V_IMON = IMON  R_IMON
(EQ. 14)
As shown in Figure 3 on page 7, V_IMON is sent to inputs of Gm2
and comparators of CMP_PD and CMP_OCAVG for the following
functions:
1. V_IMON is compared with 1.6V (VREF_CC) at error amplifier
Gm2 inputs to achieve constant current control function. The
CC control threshold for the boost input current is typically set
in a way that the per phase average inductor current (when CC
control) is lower than the per phase cycle-by-cycle peak
current limiting (OC1) threshold. Please refer to “Constant
Current Control (CC)” on page 35 for detailed descriptions.
2. V_IMON is compared with phase dropping thresholds (1.1V
falling to drop phase2, 1.15V rising to add phase2). Please
refer to “Automatic Phase Dropping/Adding” on page 33 for
detailed descriptions.
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FN8808.2
February 24, 2016