Datasheet
Table Of Contents
- FEATURES
- APPLICATIONS
- DESCRIPTION
- ABSOLUTE MAXIMUM RATINGS
- DISSIPATION RATINGS
- RECOMMENDED OPERATING CONDITIONS
- ELECTRICAL CHARACTERISTICS
- DEVICE INFORMATION
- TYPICAL CHARACTERISTICS
- APPLICATION INFORMATION
- PWM Operations
- Adaptive On-Time Control and PWM Frequency
- Loop Compensation
- Ramp Signal
- Light Load Condition in Auto-Skip Operation
- Out-of-Audio™ Light-Load Operation
- Enable and Soft Start
- VREG5/VREG3 Linear Regulators
- VREG5 Switch Over
- VREG3 Switch Over
- Powergood
- Output Discharge Control
- Low-Side Driver
- High-Side Driver
- Current Protection
- Overvoltage and Undervoltage Protection
- UVLO Protection
- Thermal Shutdown
- External Parts Selection
- Layout Considerations
- Application Circuit
- Revision History
TPS51123
www.ti.com
SLUS890E –DECEMBER 2008–REVISED JANUARY 2013
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, 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
TPS51123 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 (kHz)
TONSEL CONNECTION
CH1 CH2
GND 200 250
VREF 245 305
VREG3 300 375
VREG5 365 460
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