FN8808 datasheet(32/44 Pages) RENESAS

32 / 44 page

ISL78227

FN8808 Rev.6.02

Page 32 of 43

Feb 28, 2025

The typical scenario when fast overloading is applied is described

as follows: When a 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 due to the filter delay of RIMON and CIMON. When VIMON

reaches 1.6V, the CC loop starts to limit and control the average

current to be constant, which lowers 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 CIMON. When a longer time delay is needed, larger

CIMON can be used. Refer to “Constant Current Control (CC)” on

page 35 for a more detailed description.

Adjustable Slope Compensation

For a boost 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% because 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, RSLOPE, from the SLOPE pin to ground. This

function eases the compensation design and provides 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 mSL needs to be larger than 50% of the inductor current

down ramp slope slew rate mb.

Equation 15 shows the resistor value, RSLOPE, at the SLOPE pin

to create a compensation ramp:

where KSLOPE is the selected gain of compensation slope over

inductor down slope. For example, KSLOPE = 1 gives the RSLOPE

value generating a compensation slope equal to inductor current

down ramp slope. Theoretically, the KSLOPE 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 VOUT

and minimum of VIN should be used in Equation 15 to calculate

the RSLOPE.

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. Refer to Table 2 on page 33 for the three

configuration modes.

1. When DE/PHDRP = VCC, 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.

3. When DE/PHDRP = GND, both Diode Emulation and Phase

Drop functions are disabled. The part is set in Continuous

Conduction Mode (CCM).

RSLOPE

6.67 105



Lx RSETx



KSLOPE VOUT VIN

–







SENx

---------------------------------------------------------------------------------------- 

 

(EQ. 15)

FIGURE 61. SLOPE COMPENSATION BLOCK DIAGRAM

RSLOPE

SLOPE

VRAMP

RRAMP

ISL

CSL

VSL

ISLOPE = k2*0.5V/RSLOPE

LGx

VRAMP = (ISENx+ISL)*RRAMP

ISL

VRAMP

ISL0

ISENx

mSL

ma1 = ma + mSL

0.5V

VOUT

RSENx

RBIASx

RSETx

ISENxP

ISENxN

CSA

VIN

k1*ISENx

FN8808 bảng dữ liệu(PDF) 32 Page - Renesas Technology Corp

FN8808 bảng dữ liệu(HTML) 32 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

bảng dữ liệu tải về

Link URL