RT6318B datasheet(18/25 Pages) RICHTEK

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RT6318B/C

DS6318B/C-01 December 2021

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The actual on-time will be slightly longer as the IC

compensates for voltage drops in the circuit, but we can

neglect both of these since the on-time increases

compensations for the voltage losses. Calculate the output

voltage SAG as :

()

OUT

SAG

OUT

IN(MIN)

MAX

OUT











The amplitude of the capacitive SOAR is a function of the

load step, the output capacitor value, the inductor value

and the output voltage :

()

OUT

SOAR

OUT







Most applications never experience instantaneous full load

steps and the IC's high switching frequency and fast

transient response can easily control voltage regulation

at all times. Therefore, sag and soar are seldom an issue

except in very low-voltage CPU core or DDR memory

supply applications, particularly for devices with high clock

frequencies and quick changes into and out of sleep

modes. In such applications, simply increasing the amount

of ceramic output capacitor (sag and soar are directly

proportional to capacitance) or adding extra bulk

capacitance can easily eliminate any excessive voltage

transients.

In any application with large quick transients, it should

calculate soar and sag to make sure that over-voltage

protection and under-voltage protection will not be triggered.

Feedforward Capacitor CFF Design

For saving time to design compensator and reducing the

layout area through external components, the components

of compensator are integrated in IC. However, this

integrated compensator might not suit to every load

transient spec. Hence, for the RT6318B/C to be more

adaptable, the feedforward capacitor CFF is used in the

feedback loop to improve transient response, as shown in

Figure 4. Figure 5 shows the comparison result of bode

plot with different feedback loop conditions. Referring to

Figure 5, through connecting a CFF in feedback network,

the gain and phase are raised in mid-frequency that not

only can extend the bandwidth, but boost the phase margin

as well. Moreover, there is also a high frequency pole to

eliminate high frequency noise. Consequently, those

features of feedforward feedback network allow the

RT6318B/C have faster response to handle different load

transient.

Figure 4. Feedback Loop with Feedforward Capacitor

RT6318B/C

VOUT

CFF

COUT

PGND

In addition to voltage ripple at the switching frequency,

the output capacitor and its ESR also affect the voltage

sag (undershoot) and soar (overshoot) when the load steps

up and down abruptly. The ACOT® transient response is

very quick and output transients are usually small.

However, the combination of small ceramic output

capacitors (with little capacitance), low output voltages

(with little stored charge in the output capacitors), and

low duty cycle applications (which require high inductance

to get reasonable ripple currents with high input voltages)

increases the size of voltage variations in response to

very quick load changes. Typically, load changes occur

slowly with respect to the IC's switching frequency.

However, some modern digital loads can exhibit nearly

instantaneous load changes and the following section

shows how to calculate the worst-case voltage swings in

response to very fast load steps.

The amplitude of the ESR step up or down is a function of

the load step and the ESR of the output capacitor :

ESR_STEP

OUT

ESR





The amplitude of the capacitive sag is a function of the

load step, the output capacitor value, the inductor value,

the input-to-output voltage differential, and the maximum

duty cycle. The maximum duty cycle during a fast transient

is a function of the on-time and the minimum off-time since

the ACOT® control scheme will ramp the current using

on-times spaced apart with minimum off-times, which is

as fast as allowed. Calculate the approximate on-time

(neglecting parasitics) and maximum duty cycle for a given

input and output voltage as :

OUT

MAX

OFF_MIN

and D

+ t





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