(

)

(

)

-

´

=

´

´

´

IN

OUT

OUT

OUT LL

SW

IN

V

V

V

1

I

2 L f

V

TPS51275, TPS51275C

SLUSB45 – JUNE 2012

www.ti.com

DETAILED DESCRIPTION

PWM Operations

The main control loop of the switch mode power supply (SMPS) is designed as an adaptive on-time pulse width

modulation (PWM) controller. It supports a proprietary D-CAP™ mode. D-CAP™ mode does not require external

conpensation circuit and is suitable for low external component count configuration when used with appropriate

amount of ESR at the output capacitor(s).

At the beginning of each cycle, the synchronous high-side MOSFET is turned on, or enters the ON state. This

MOSFET is turned off, or enters the ‘OFF state, after the internal, one-shot timer expires. The MOSFET is turned

current information is also monitored and should be below the overcurrent threshold to initiate this new cycle. By

repeating the operation in this manner, the controller regulates the output voltage. The synchronous low-side

(rectifying) MOSFET is turned on at the beginning of each OFF state to maintain a minimum of conduction loss.

The low-side MOSFET is turned off before the high-side MOSFET turns on at next switching cycle or when

inductor current information detects zero level. This enables seamless transition to the reduced frequency

operation during light-load conditions so that high efficiency is maintained over a broad range of load current.

Adaptive On-Time/ PWM Frequency Control

Bacause the TPS51275/C does not have a dedicated oscillator for control loop on board, switching cycle is

controlled by the adaptive on-time circuit. The on-time is controlled to meet the target switching frequency by

feed-forwarding the input and output voltage into the on-time one-shot timer. The target switching frequency is

varied according to the input voltage to achieve higher duty operation for lower input voltage application. The

switching frequency of CH1 (5-V output) is 300 kHz during continuous conduction mode (CCM) operation when

To improve load transient performance and load regulation in lower input voltage conditions, TPS51275/C can

extend the on-time. The maximum on-time extension of CH1 is 4 times for CH2 is 3 times. To maintain a

reasonable inductor ripple current during on-time extension, the inductor ripple current should be set to less than

half of the OCL (valley) threshold. (See Step 2. Choose the Inductor). The on-time extension function provides

high duty cycle operation and shows better DC (static) performance. AC performance is determined mostly by

the output LC filter and resistive factor in the loop.

Light Load Condition in Auto-Skip Operation (TPS51275/C)

The TPS51275/C automatically reduces switching frequency during light-load conditions to maintain high

efficiency. This reduction of frequency is achieved smoothly and without an increase in output voltage ripple. A

more detailed description of this operation is as follows. As the output current decreases from heavy-load

condition, the inductor current is also reduced and eventually approaches valley zero current, which is the

boundary between continuous conduction mode and discontinuous conduction mode. The rectifying MOSFET is

turned off when this zero inductor current is detected. As the load current further decreases, 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 maintained the same as that in the heavy-load condition. In reverse,

when the output current increase from light load to heavy load, the switching frequency increases to the preset

value as the inductor current reaches to the continuous conduction. The transition load point to the light load

as shown in Equation 1.

where

•

(1)

Switching frequency versus output current during light-load conditions is a function of inductance (L), input

10

Submit Documentation Feedback

Copyright © 2012, Texas Instruments Incorporated