ISL78226 Datasheet, PDF (38/94 Pages) Intersil Corporation – Cycle-by-cycle peak current limiting

ISL78226
Equation 6, where I_Lx is the per phase current flowing through
R_SENx.
I_SENx = I_Lx  ^R-R----SS----^EE----^NT----_xx- + 56A
(EQ. 6)
Since R_SETx is fixed to 1kΩ, R_SENx must be selected by
considering the desired cycle-by-cycle peak current limiting level
OC1.
Current Monitoring — IMON
The ISL78226 continuously monitors the inductor current of each
phase at the current sense resistor, which is connected in-series to
the inductor. A current which is proportional to the total inductor
current of each phases comes out at IMON pin. By connecting an
RC filter network at IMON pin, the voltage at IMON pin reflects the
averaged total inductor current, which will be the averaged output
current for Buck mode and averaged input current for Boost mode.
Equation 7 shows the relation between IMON output current and
averaged total current.
nmax
 ^{IIMON =}
12_{--  ILx } R_-R----s_-s---e_-e---nt_{- + 56A}
n=1
(EQ. 7)
where;
I_IMON = Output Current at IMON
n_max = Maximum operating phase count,
I_Lx = Inductor current at phase x,
Rsen = Sense resistor value
Rset = Gain setting resistor (to be fixed to 1kΩ for ISL78226)
So, the IMON voltage (V_IMON), which will be proportional to the
averaged total output current for Buck mode or input current for
Boost mode, will be:
VIMON = RIMON  IIMON
(EQ. 8)
where R_IMON is the Resistor value connected from IMON to GND.
Since the IMON voltage (VIMON) will be used for the Constant
Current Control Loop (CCL), Average Current Limit (ACL) control,
and Phase Drop/Add control, it is recommended to use 0.1µF to
1µF filter capacitor to filter out the ripples at IMON pin.
For details about CCL, ACL, and Phase Drop/Add functions, refer
to the corresponding sections.
Adjustable Slope Compensation
For a converter with peak current mode control, slope
compensation is needed when the duty cycle is larger than 50%.
It is advised to add slope compensation when the duty cycle is
approximately 30 to 40% since a transient load step can push
the duty cycle higher than the steady state level. When slope
compensation is too low, the converter suffers from subharmonic
oscillation, which may result in noise emissions at half the
switching frequency. On the other hand, overcompensation of the
slope may reduce the phase margin. Therefore, proper design of
the slope compensation is needed.
The ISL78226 features adjustable slope compensation by
setting the resistor value R_{SLP_BT} and R_{SLP_BK} from the
SLOPE_BT and SLOPE_BK pin to ground, for Boost and Buck
mode, respectively.
Figure 33 shows the block diagram related to slope compensation.
For current mode control, in theory, the compensation slope slew
rate ms_L, needs to be larger than 50% of the inductor current
down ramp slope slew rate m_b.
Equation 11 shows the resistor value, K_SLOPE, at the SLOPE pin
to create a compensation ramp.
R_SLPBT = -_K----_S----_L---_O---²-_P--^.--¹_E---⁴--_---^---_V1---⁰-_O---⁵--_U--^--_T-^L---–-^x----_V^---^R-_I--_N--^S--_--^E--_-^T--_R--^x--_S----_E----_N----_x- 
(EQ. 9)
R_SLPBK = -_K---²-_S---^.-_L¹---_O-⁴---_P--^--_E-¹---⁰-_--⁵-_V----^-_O--^L--_U-^x---_T--^---^R_---_R-^S----_SE---_E-^T---_N-^x---_x--
(EQ. 10)
Where K_SLOPE is the selected gain of compensation slope over
inductor down slope. For example, K_SLOPE = 1 gives the R_SLOPE
value generating a compensation slope equal to inductor current
down ramp slope. Theoretically, the K_SLOPE needs to be larger
than 0.5, but practically more than 1.0 is used as shown in
Equation 11 for Boost mode and minimum of V_OUT, and in
Equation 12 for Buck mode and maximum of V_IN.
VIN ⁺
RSE_-Nx
IL
L
VOUT
RSETx
RBIASx
ISENxN
ISENxP
SLOPE
RSLOPE
0.5V
CSA
k1*ISENx
ISLOPE = k2*0.5V/RSLOPE
VSL
CSL
VRAMP
RRAMP
ISL
LGx
I_SL0
m_a
m_b
I_SENx
I_SL
m_SL
m_a1 = m_a + m_SL
V_RAMP
V_RAMP = (I_SENx+I_SL)*R_RAMP
FIGURE 33. SLOPE COMPENSATION BLOCK DIAGRAM
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November 7, 2016