ISL78227 Datasheet, PDF (32/43 Pages) Intersil Corporation – 2-Phase Boost Controller with Integrated Drivers

ISL78227
3. V_IMON is compared with 2V for OC_AVG fault protections.
Please refer to “Average Overcurrent Fault (OC_AVG)
Protection” on page 36 for detailed descriptions.
The typical scenario when fast overloading is applied is described
as the following. When large overload is suddenly applied at
boost output, the phase inductor peak currents are initially
limited by OC1 cycle-by-cycle, during which time the IMON
voltage slowly rises up due to the filter delay of R_IMON and
C_IMON. When V_IMON reaches 1.6V, the CC loop starts to limit and
control the average current to be constant, which lowers down
the inductor current (as described previously, CC threshold
normally is set lower than the OC1 cycle-by-cycle limiting
threshold). Typically tens of nF are used for C_IMON. In the case
when a longer time delay is needed, larger C_IMON can be used.
“Constant Current Control (CC)” on page 35 has a more detailed
description.
Adjustable Slope Compensation
For a boost converter with peak current mode control, slope
compensation is needed when 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 ISL78227 features adjustable slope compensation by setting
the resistor value R_SLOPE from the SLOPE pin to ground. This
function will ease the compensation design and provide more
flexibility in choosing the external components.
Figure 61 shows the block diagram related to slope compensation.
For current mode control, in theory, the compensation slope slew
rate m_SL needs to be larger than 50% of the inductor current
down ramp slope slew rate m_b.
Equation 15 shows the resistor value R_SLOPE at the SLOPE pin to
create a compensation ramp.
R_SLOPE = -_K----_S----_L---_O---⁶-_P--^.--⁶_E---⁷--_---^---_V1---⁰-_O---⁵-_U---^--_T-^L---–-^x----_V^---^R-_I--_N--^S--_--^E--_-^T--_R--^x--_S----_E----_N----_x- 
(EQ. 15)
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 in the actual
application. To cover the operating range, the maximum of V_OUT
and minimum of V_IN should be used in Equation 15 to calculate
the R_SLOPE.
VIN ⁺
RSETx
RSE_-Nx
IL
L
RBIASx
VOUT
ISENxN
CSA
k1*ISENx
ISENxP
SLOPE
RSLOPE
0.5V
ISLOPE = k2*0.5V/RSLOPE
VSL
CSL
VRAMP
RRAMP
ISL
LGx
ISL0
ma
mb
ISENx
ISL
mSL
ma1 = ma + mSL
VRAMP
VRAMP = (ISENx+ISL)*RRAMP
FIGURE 61. SLOPE COMPENSATION BLOCK DIAGRAM
Light-Load Efficiency Enhancement
For switching mode power supplies, the total loss is related to
conduction loss and switching loss. The conduction loss
dominates at heavy load, while the switching loss dominates at
light load condition. Therefore, if a multiphase converter is
running at a fixed phase number for the entire load range, the
efficiency starts to drop significantly below a certain load current.
The ISL78227 has selectable automatic phase dropping,
cycle-by-cycle diode emulation and pulse skipping features to
enhance the light-load efficiency. By observing the total input
current on-the-fly and dropping an active phase, the system can
achieve optimized efficiency over the entire load range.
The phase dropping (PH_DROP) and Diode Emulation (DE)
functions can be selected to be active or inactive by setting the
DE/PHDRP pin. Please refer to Table 2 for the 3 configuration
modes.
1. When DE/PHDRP = V_CC, Diode Emulation function is enabled,
and Phase Drop function is disabled.
2. When DE/PHDRP = FLOAT, both Diode Emulation and Phase
Drop functions are enabled.
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FN8808.2
February 24, 2016