Datasheet
TPS51123A
SLUSAA6C –APRIL 2011–REVISED SEPTEMBER 2012
www.ti.com
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
IN
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, V
VFBx
, 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
TPS51123A 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 is
kept as V
OUT
/V
IN
technically with the same cycle time. The frequencies are set by the TONSEL pin as shown in
Table 3.
Table 3. TONSEL Connection and Switching Frequency
SWITCHING FREQUENCY (kHz)
TONSEL CONNECTION
CH1 CH2
GND 200 250
VREF 245 305
VREG3 300 375
VREG5 365 460
20 Submit Documentation Feedback Copyright © 2011–2012, Texas Instruments Incorporated
Product Folder Links: TPS51123A