BD9123MUV_14 Datasheet, PDF (10/28 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD9123MUV
Datasheet
Application Information
1. Operation
BD9123MUV is a synchronous rectifying step-down switching regulator that achieves faster transient response by
employing current mode PWM control system. Its switching operation utilizes PWM (Pulse Width Modulation) mode for
heavier load, while SLLM^TM (Simple Light Load Mode) operation for lighter load to improve efficiency.
(1) Synchronous Rectifier
Integrated synchronous rectification using two MOSFETS reduces power dissipation and increases efficiency when
compared to converters using external diodes. Internal shoot-through current limiting circuit further reduces power
dissipation.
(2) Current Mode PWM Control
The PWM control signal of this IC depends on two feedback loops, the voltage feedback and the inductor current
feedback.
(a) PWM (Pulse Width Modulation) Control
The clock signal coming from OSC has a frequency of 1MHz. When OSC sets the RS latch, the P-Channel
MOSFET is turned ON and the N-Channel MOSFET is turned OFF, causing an inductor current IL to increase.
The opposite happens when the current comparator (Current Comp) resets the RS latch i.e. the P-Channel
MOSFET is turned OFF and the N-Channel MOSFET is turned ON. Current Comp’s output is a comparison of two
signals, the current feedback control signal “SENSE” which is a voltage proportional to the current IL and the
voltage feedback control signal, FB.
(b) SLLM^TM (Simple Light Load Mode) Control
When the control mode is shifted by PWM from heavier load to the one for lighter load or vice versa, the switching
pulse is designed to turn OFF with the device held operating in normal PWM control loop. This allows linear
operation without voltage drop or deterioration in transient response during sudden load changes. .
Although the PWM control loop continues to operate with a SET signal from OSC and a RESET signal from
Current Comp, it is designed such that the RESET signal is kept constant when shifted to the light load mode
where the switching is tuned OFF and the switching pulses disappear. Activating the switching discontinuously
reduces the switching dissipation and improves the efficiency.
VOUT
SENSE
Level
Shift
Gm Amp
Current
Comp
RESET
RQ
FB
SET _S
RITH
OSC
Driver
Logic
IL
SW
Load
VOUT
Current
Comp
SET
RESET
SW
IL
VOUT
Figure 22. Diagram of Current Mode PWM Control
PVCC
SENSE
FB
GND
Current
Comp
SET
GND
GND
RESET
SW
IL(AVE)
IL
VOUT(AVE)
VOUT
PVCC
SENSE
FB
GND
GND
GND
0A
VOUT(AVE)
Figure 23. PWM Switching Timing Chart
Not switching
Figure 24. SLLM^TM Switching Timing Chart
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02.Oct.2014 Rev.002