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TPS51285A

TPS51285B

www.ti.com

SLVSBX0 – APRIL 2013

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

compensation circuits 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 the

detecting inductor current decreases to zero. 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

Because the device 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. To achieve higher duty operation for

lower input voltage application (2-cell battery), the target switching frequency is varied according to the input

voltage. The switching frequency of CH1 (5-V output) is 400kHz during continuous conduction mode (CCM)

To improve load transient performance and load regulation in lower input voltage condition, device can extend

the on-time. The maximum on-time extension of CH1 is 5 times. For CH2, it is 2 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. 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

The device automatically reduces switching frequency during light-load conditions to maintain high efficiency.

This reduction of frequency is achieved smoothly. 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. In reverse, when the output current

increase from light load to heavy load, the switching frequency increases to the preset value as the inductor

the threshold between continuous and discontinuous conduction mode) can be calculated as shown in n

Equation 1.

where

•

(1)

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

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