Datasheet

TPS51125

www.ti.com

SLUS786G –OCTOBER 2007–REVISED JUNE 2012

APPLICATION INFORMATION

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 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 top MOSFET is turned on, or becomes ‘ON’ state. This

MOSFET is turned off, or becomes ‘OFF’ state, after internal one shot timer expires. This one shot is determined

by V

and V

OUT

to keep frequency fairly constant over input voltage range, hence it is called adaptive on-time

control. The MOSFET is turned on again when the feedback point voltage, VFB, decreased to match with internal

2-V reference. The inductor current information is also monitored and should be below the over current threshold

to initiate this new cycle. Repeating operation in this manner, the controller regulates the output voltage. The

synchronous bottom or the “rectifying” MOSFET is turned on at the beginning of each ‘OFF’ state to keep the

conduction loss minimum.The rectifying MOSFET is turned off before the top MOSFET turns on at next switching

cycle or when inductor current information detects zero level. In the auto-skip mode or the OOA skip mode, this

enables seamless transition to the reduced frequency operation at light load condition so that high efficiency is

kept over broad range of load current.

Adaptive On-Time Control and PWM Frequency

TPS51125 does not have a dedicated oscillator on board. However, the part runs with pseudo-constant

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

controlled inverse proportional to the input voltage and proportional to the output voltage so that the duty ratio will

be kept as VOUT/VIN technically with the same cycle time. The frequencies are set by TONSEL terminal

connection as Table 2.

Table 2. TONSEL Connection and Switching Frequency

SWITCHING FREQUENCY

TONSEL CONNECTION

CH1 CH2

GND 200 kHz 250 kHz

VREF 245 kHz 305 kHz

VREG3 300 kHz 375 kHz

VREG5 365 kHz 460 kHz

Product Folder Link(s): TPS51125