AL8853 datasheet(8/15 Pages) DIODES

8 / 15 page

AL8853

Document number: DS42211 Rev. 1 - 2

8 of 15

www.diodes.com

September 2019

AL8853

Function Description (continued)

ΔI(n)

ΔI(n+1)

ΔI(n+2)

VSENSE

Time

Input of PWM Comparator Waveform

Figure 2 Sub-Harmonic Oscillation at Duty>50% and Compensation Slope to Avoid Sub-Harmonic Oscillation

In Figure 2,

ΔI(n), ΔI(n+1) and ΔI(n+2) represent the inductor current increments when there is disturbance in the inductor current. And the

relationship of the inductor current increment can be calculated with Equation 2.

……………………………………………………… (2)

Where Sn is the slope of the inductor current when rising, Sf is the slope of the inductor current when falling and Se is the slope of control signal.

If the control signal has no compensation, the inductor current will ramp up till slope reaches the control signal, and then ramps down. If the PWM

duty is higher than 50%, the distribution will not be converged from duty to duty, and system runs into sub-harmonic oscillation. To avoid sub-

harmonic oscillation, a control compensation is needed and (Sf - Se) / (Sn + Se) should always be smaller than 1. For boost applications, the

slopes Se, Sf and Sn can be calculated with the equations below.

…………………………………………………………… (3)

…………………………………………………………… (4)

…………………………………………………………… (5)

Where RSEN is the current sense resistance and typical value of VSL is 100mV.

From the equations, the factor (Sf - Se) / (Sn + Se) will decrease if Se increases. When the duty cycle is greater than 50%, lower inductance can

also result in the factor increase. And for more flexibility, slope compensation amplitude can be increased by adding one external resistor, RSL, in

the CS path. Figure 3 shows the circuit setup. The external generated slope compensation is then added to the internal slope compensation of the

AL8853. When using external slope compensation, the equation for Se becomes:

…………………………………………………………… (6)

Where typical value of IS is 50A.

AL8853

GATE

RSL

RSEN

Figure 3. Adding External Slope Compensation

It is a good design practice to only add as much slope compensation as needed to avoid sub-harmonic oscillation. Additional slope compensation

minimizes the influence of the sensed current in the control loop. With very large slope compensation, the control loop characteristics are similar to

a voltage mode regulator which compares the error voltage to a saw tooth waveform rather than the inductor current.

AL8853 Datasheet(PDF) 8 Page - Diodes Incorporated

AL8853 Datasheet(HTML) 8 Page - Diodes Incorporated

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Datasheet Download

Link URL