RT6318B datasheet(13/25 Pages) RICHTEK

13 / 25 page

RT6318B/C

DS6318B/C-01 December 2021

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT6318B/C is high-performance 8A step-down

regulators with internal power switches and synchronous

rectifiers. They feature an Advanced Constant On-Time

(ACOT®) control architecture that provides stable operation

for ceramic output capacitors without complicated external

compensation, among other benefits. The input voltage

range is from 4.5V to 23V. The output voltage are fixed

3.3V (RT6318B) or 5.1V (RT6318C).

The proprietary ACOT® control scheme improves

conventional constant on-time architectures, achieving

nearly constant switching frequency over line, load, and

output voltage ranges. Since there is no internal clock,

response to transients is nearly instantaneous and inductor

current can ramp quickly to maintain output regulation

without large bulk output capacitance.

The RT6318B and the RT6318 include 3.3V and 5V linear

regulator(LDO), respectively. The linear regulator provides

an automatic saving power function, when VOUT rises

above 3.1V (RT6318B) / 4.7V (RT6318C), an automatic

circuit will change the power source of linear regulator

from VIN path to VOUT path, therefore the power

dissipation of linear regulator will be decrease efficiently.

ACOT®Control Architecture

The conventional CFCOT (constant frequency constant

on-time) control which making the on-time proportional to

VOUT and inversely proportional to VIN is not sufficient to

achieve good constant-frequency behavior. Because

voltage drops across the MOSFET switches and inductor

cause sensing mismatch as sensing input and output

voltage from LX pin. When the load change, the voltage

drops across the MOSFET switches and inductor cause

a switching frequency variation with load current. One way

to reduce these effects is to measure the actual switching

frequency and compare it to the desired range. This has

the added benefit eliminating the need to sense the actual

output voltage, potentially saving one pin connection.

ACOT®uses this method, measuring the actual switching

frequency and modifying the on-time with a feedback loop

to keep the average switching frequency in the desired

range.

In order to achieve good stability with low-ESR ceramic

capacitors, ACOT® uses a virtual inductor current ramp

generated inside the IC. This internal ramp signal replaces

the ESR ramp normally provided by the output capacitor’s

ESR. The ramp signal and other internal compensations

are optimized for low-ESR ceramic output capacitors.

ACOT®One-Shot Operation

The RT6318B/C control algorithm is simple to understand.

The feedback voltage, with the virtual inductor current ramp

added, is compared to the reference voltage. When the

combined signal is less than the reference, the on-time

one-shot is triggered as long as the minimum off-time one-

shot is clear and the measured inductor current (through

the synchronous rectifier) is below the current limit. The

on-time one-shot turns on the high-side switch and the

inductor current ramps up linearly. After the on-time,

the high-side switch is turned off and the synchronous

rectifier is turned on and the inductor current ramps down

linearly. At the same time, the minimum off-time one-shot

is triggered to prevent another immediate on-time during

the noisy switching time and allow the feedback voltage

and current sense signals to settle. The minimum off-time

is kept short (200ns typical) so that rapidly-repeated on-

times can raise the inductor current quickly when needed.

Average Output Voltage Control Loop

In continuous conduction mode, the RT6318B/C provides

a average output voltage control loop to cancel the DC

error between VFB(average) and VREF by adjusting the

comparator input VREF to make VFB(average) always follow

designed value. This loop can efficiently improves the load

and line regulation without affecting the transient

performance. The operation figure is shown in Figure 1.

Figure 1. Average output voltage control loop operation

DC error

VFB(average)

VREF

RT6318B 数据表(PDF) 13 Page - Richtek Technology Corporation

RT6318B 数据表(HTML) 13 Page - Richtek Technology Corporation

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

数据表下载