BD91361MUV_14 Datasheet, PDF (9/27 Pages) Rohm – Synchronous Buck Converter with Integrated FET

BD91361MUV
Datasheet
Application Information
1. Operation
BD91361MUV is a synchronous step-down switching regulator that achieves fast transient response by employing
current mode PWM control system. It utilizes switching operation in PWM (Pulse Width Modulation) mode for heavier
load, while it utilizes 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. 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 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 the 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 so
designed such that the RESET signal is continuously sent even if the load is changed to light mode where the
switching is tuned OFF and the switching pulses disappear. Activating the switching discontinuously reduces
the switching dissipation and improves the efficiency.
SENSE
VOUT
Current
Comp
RESET
Level
RQ
Shift _FB
Gm Amp
SET _S
RITH
OSC
Driver
Logic
IL
SW
Load
VOUT
Figure 21. Diagram of Current Mode PWM Control
Current
Comp
SET
RESET
SW
IL
VOUT
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 22. PWM Switching Timing Diagram
Not switching
Figure 23. SLLM^TM Switching Timing Diagram
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06.Oct.2014 Rev.002