RT6310ASLASHAH datasheet(10/54 Pages) RICHTEK

10 / 54 page

RT6310

is a registered trademark of Richtek Technology Corporation.

www.richtek.com

DS6310-04

August 2023

Operation

The RT6310 is a high efficiency synchronous step-down

converter with integrated MOSFETs. The RT6310

utilizes the proprietary Advanced Constant On-Time

(ACOT® ) control architecture providing vary fast

transient response. The ultra-fast ACOT®

control

enables the use of small output capacitance and

optimizes

the component size

without additional

compensation network.

During normal operation, the high-side MOSFET turns

on with a fixed one-shot on-time timer after the

beginning of each clock cycle. The inductor current

linearly increases when high-side MOSFET turns on

and low-side MOSFET turns off. Similarly, the inductor

current linearly decreases when high-side MOSFET

turns off and low-side MOSFET turns on. The voltage

ripple on the output has similar shape to the inductor

current due to the output capacitor ESR.

The feedback voltage ripple comparing with an internal

reference is caught by feedback resistor network. When

a fixed minimum off-time timer is timeout and the

inductor valley current is below the valley current-limit

threshold, the fixed one-shot one-time timer is triggered

if the feedback voltage falls below the feedback

reference voltage. Therefore, the output voltage is

regulated through the previously mentioned principle.

ACOT® Control Architecture

In order to achieve good stability with low-ESR ceramic

capacitors, ACOT® uses a virtual inductor current ramp

generated inside the IC. The 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.

Conventional COT control implements the on-time timer

proportional to VOUT and inversely proportional to VIN to

achieve pseudo-fixed frequency with wide VIN range. A

fixed on-time timer of conventional COT control has no

compensation for the voltage drop of the MOSFETs and

inductor during higher load condition.

In order to compensate the voltage drop of MOSFETs

and inductor without influencing the fast transient

behavior of the COT topology, a frequency locked loop

system with slowly adjusting on-time timer is further

added to the ACOT® control.

Average Output Voltage Control Loop

In continuous conduction mode, conventional COT

control has DC offset between VFB(average) and VREF as

shown in Figure 1. In order to cancel the DC offset, the

RT6310 provides an average output voltage control

loop to adjust the comparator input VREF. Hence, the

VFB(average) always follows the designed value. The

control loop efficiently improves the load and line

regulation without affecting the transient performance.

DC offset

VFB(average)

VREF

Figure 1. Conventional COT Control Loop Operation

High Voltage Conversion Ratio Function

In conventional COT control, the maximum duty cycle is

limited by the minimum off-time. RT6310 provides a

feature of increasing the on-time function (up to 15

s)

to extend the maximum duty cycle of 2S battery

applications.

Diode Emulation Mode (DEM)

Diode emulation mode is selected by the EN/MODE

voltage level. The device enters diode emulation mode

when EN/MODE voltage is greater than 2.3V. In diode

emulation

mode,

the

RT6310

automatically

and

smoothly reduces switching frequency at light-load

conditions. As the output current decreases from heavy

load to light load, the inductor current is naturally

reduced. Once the valley point of inductor current

touches to zero during decreasing output current, the

behavior is boundary mode between continuous

conduction and discontinuous conduction mode. In

order to emulate the behavior of free-wheeling diode,

the device only allows partial negative current flow from

drain to source of the low-side MOSFET when inductor

free-wheeling current becomes negative.

During decreasing output current, the discharge time of

the output capacitor is gradually longer. When the

voltage on output capacitor is lower than the reference

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