RT8129A

Copyright © 2015 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

DS8129A-01

October

2015

www.richtek.com

13

Application Information

The RT8129A is a single-phase synchronous buck

PWM controller with integrated drivers which is

optimized for high performance graphic microprocessor

and computer applications. A COT (Constant-On-Time)

PWM controller and two MOSFET drivers with internal

bootstrap diodes are integrated so that the external

circuit is easily designed and the component count is

reduced.

The topology solves the poor load transient timing

problems of fixed-frequency current-mode PWM and

avoids

the

problems

caused

by

widely

varying

switching frequencies in conventional constant-on-time

and constant off-time PWM schemes.

RT8129A also features complete fault protection

functions including OVP, UVP and Current Limit.

PWM Operation

The RT8129A 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. Referring to

the function block diagram of TON generator, the

synchronous UGATE driver will be turned on at the

beginning of each cycle. After the internal one-shot

timer expires, the UGATE driver 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 fairly constant over the input voltage

range. Another one-shot sets a minimum off-time.

t

Figure 1. Constant On-Time PWM Control

Diode-Emulation Mode

In diode-emulation mode, the RT8129A automatically

reduces switching frequency at light-load conditions to

maintain

high

efficiency.

As

the

output

current

decreases from heavy-load condition, the inductor

current is also reduced, and eventually comes to the

point that its valley touches zero current, which is the

boundary

between

continuous

conduction

and

discontinuous conduction modes. By emulating the

behavior of diodes, the low-side MOSFET allows only

partial

of

negative

current

when

the

inductor

freewheeling current reach negative level. As the load

current is further decreased, it takes longer and longer

to discharge the output capacitor to the level that

requires the next “ON” cycle. In reverse, when the

output current increases from light load to heavy load,

the switching frequency increases to the preset value

as the inductor current reaches the continuous

condition.

The switching waveforms may appear noisy and

asynchronous

when

light

loading

causes

diode-emulation operation,

but this is a normal

operating condition that results in high light-load

efficiency. Trade-offs in DEM noise vs. light-load

efficiency is made by varying the inductor value.

Generally, low inductor values produce a broader

efficiency vs. load curve, while higher values result in

higher full-load efficiency (assuming that the coil

resistance remains fixed) and less output voltage ripple.

The disadvantages for using higher inductor values

include larger physical size and degrade load-transient

response (especially at low input-voltage levels).

Enable and Disable

The EN pin allows for power sequencing between the

controller bias voltage and another voltage rail. The

RT8129A remains in shutdown if the EN pin is lower

than 800mV. When EN pin rises above the 2V, the

RT8129A will begin a new initialization and soft-start

cycle.

Power-On Reset (POR), UVLO

Power-on reset (POR) occurs when VCC rises above

to approximately 4.4V (typical), the RT8129A will reset

the fault latch and preparing the PWM for operation.

Below 4V (typical), the VCC under voltage-lockout

(UVLO) circuitry inhibits switching by keeping UGATE

and LGATE low.