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TPS53129

www.ti.com

SLVSAE6B – OCTOBER 2009 – REVISED JULY 2010

DETAILED DESCRIPTION

PWM Operation

The main control loop of the TPS53129 is an adaptive on-time pulse width modulation (PWM) controller using a

proprietary D-CAP2™ mode control. D-CAP2™ mode control combines constant on-time control with an internal

compensation circuit for pseudo-fixed frequency and low external component count configuration with both low

ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output.

At the beginning of each cycle, the synchronous high-side MOSFET is turned on. After an internal one-shot timer

expires, this MOSFET is turned off. When the feedback voltage falls below the reference voltage, the one-shot

timer is reset and the high-side MOSFET is turned back on. The one shot is set by the converter input voltage

VIN, and the output voltage VO, to maintain a pseudo-fixed frequency over the input voltage range. An internal

ramp is added to the reference voltage to simulate output ripple, eliminating the need for ESR induced output

ripple from D-CAP mode control. The low-side MOSFET is turned off when the inductor current information

detects zero level. This enables seamless transition to the reduced frequency operation at light-load conditions

so that high efficiency is kept over a broad range of load current.

Light-Load Condition

TPS53129 automatically reduces switching frequency at light-load conditions to maintain high efficiency. This

reduction of frequency is achieved smoothly and without increase of Vout ripple or load regulation. Detail

operation is described as follows. 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 condition modes. The low-side MOSFET is turned

off when this zero inductor current is detected. As the load current is further decreased, the converter runs in

discontinuous conduction mode and it takes longer and longer to discharge the output capacitor to the level that

requires the next ON cycle. The ON time is kept the same as that in the heavy-load condition. 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 conduction. The transition load point to the light load operation,

(1)

Drivers

Each channel of the TPS53129 contains two high-current resistive MOSFET gate drivers. The low-side driver is a

MOSFET whose source is connected to PGND. The high-side driver is a floating SWx referenced VBST powered

voltage during the high-side driver ON time, a capacitor is placed from SWx to VBSTx. Each driver draws

To prevent cross-conduction, there is a narrow dead-time when both high-side and low-side drivers are OFF

between each driver transition. During this time the inductor current is carried by one of the MOSFETs body

diodes.

PWM Frequency and Adaptive On-Time Control

TPS53129 employs adaptive on-time control scheme and does not have a dedicated on board oscillator.

TPS53129 runs with pseudo-constant frequency by using the input voltage and output voltage to set the on-time

one-shot timer. The on-time is inversely proportional to the input voltage and proportional to the output voltage.

Therefore, when the duty ratio is VOUT/VIN, the frequency is constant.

Copyright © 2009–2010, Texas Instruments Incorporated

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