RT2702 datasheet(12/20 Pages) RICHTEK

12 / 20 page

RT2702

www.richtek.com

DS2702-00 October 2016

is a registered trademark of Richtek Technology Corporation.

Application Information

The RT2702 is a high-efficiency and high-current

synchronous buck controller designed to drive a DrMOS

module. A COT (Constant-On-Time) PWM controller and

PWM signal for DrMOS are integrated so that the external

circuit is easily designed and the component count is

reduced. The RT2702 adopts FCOT control technology

that responds instantly to input voltage change. Besides,

the RT2702 includes a 5V (VDRV) linear regulator, which

can provide 5V voltage and at least 100mA current loading.

The topology circumvents the poor load transient timing

problems of fixed-frequency current mode PWMs while

avoiding the problems caused by widely varying switching

frequency in conventional constant-on-time and constant-

off-time PWM schemes.

The IC supports two modes transition function with various

operating states, which include audio skipping mode

(ASM) and forced-CCM mode (FCCM). These different

operating eliminates audio-frequency modulation and

states make the system efficiency as high as possible.

The device can operate at wide input voltage range from

4.5 V to 19V. It is capable of producing an output voltage

as low as 0.6V. Other features include power good

indication, external soft start setting, soft-off discharge

function, OVP, UVP, Current Limit, precise DCR sense,

and enable/disable control.

PWM Operation

The RT2702 integrates a Constant-On-Time PWM

controller, and the controller provides the PWM signal

which relies on the FB voltage comparing with internal

reference voltage as shown in Figure 1. Refer to the

function block diagram of the RT2702, the synchronous

high side MOSFET of DrMOS will be turned on at the

beginning of each cycle. After the internal one shot timer

expires, the MOSFET will be turned off. The pulse width

of this one shot is determined by the converter’s input

voltage and the output voltage to keep the frequency

constant in a specific input voltage range. Another one

shot sets a minimum off-time (275ns typ.). The on-time

one shot will be triggered if the PWM comparator output

is high, the low side switch current is below the current

limit threshold and the minimum off-time one shot has

timed out.

Figure 1. Constant On-Time PWM Control

On-Time Control

A resistor, RTON, is connected between VIN and TON

generates on-time current forward to TONGen. block. With

different VIN voltage and RTON value, the on-time current

has different value. This on-time current charges an internal

capacitor, CTON, which is about 3.8pF, then the on-time

will be the voltage on CTON from 0V to VOUT. Besides, the

switching frequency, fSW, is roughly determined by RTON

and CTON without a clock generator.

TON

OUT

3.8p

V1.17







And the specific switching frequency fSW is :

OUT





The recommended operating frequency is between 200kHz

and 1.2MHz.

Mode Selection

The RT2702 operates under audio skipping mode (ASM)

or force-CCM (FCCM) according to MODE pin voltage

setting. If MODE voltage is pulled below 0.4V, controller

turns on ASM function. If MODE voltage is higher than

1.2V, controller enters forced-CCM.

VFB

VPEAK

VVALLEL

VREF

tON

RT2702 Datasheet(PDF) 12 Page - Richtek Technology Corporation

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