“PicoBK™” MYRGM-W／MYRGM-B series
1.5A Inductor Built-in Step-Down DC/DC Converters
■OPERATIONAL EXPLANATION (Continued)
<CurrentLimit>
The current limiter circuit of the MYRGM-W/MYRGM-B series monitors the current flowing through the P-channel MOS driver
transistor connected to the Lx pin. When the driver current is greater than a specific level, the current limit function operates to
turn off the pulses from the Lx pin at any given timing. When the over current state is eliminated, the IC resumes its normal
operation.
<Reference voltage source, soft-start function>
The reference voltage forms a reference that is used to stabilize the output voltage of the IC. After chip enable of the IC, the
reference voltage connected to the error amp increases linearly during the soft-start interval.
This allows the voltage divided by the internal R1 and R2 resistors and the reference voltage to be controlled in a balanced
manner, and the output voltage rises in proportion to the rise in the reference voltage. This operation prevents rush input current
and enables the output voltage to rise smoothly.
If the output voltage does not reach the set output voltage within the soft start time, such as when the load is heavy or a large
capacity output capacitor is connected, the balancing of the voltage divided by the internal resistors R1 and R2 and the reference
voltage is lost, however, the current restriction function activates to prevent an excessive increase of input current, enabling a
smooth rise of the output voltage.
<PWM/PFM selection circuit>
PWM control is a continuous conduction mode, and operates at a stable switching frequency by means of an on time (ton) that is
determined by the input voltage and output voltage regardless of the load.
PWM/PFM auto switching control is a discontinuous conduction mode at light loads, and lowers the switching frequency to
reduce switching loss and improve efficiency.
The MYRGM-W series is internally fixed to PWM control.
The MYRGM-B series is internally fixed to PWM/PFM auto switching control.
<CEfunction>
Operation starts when “H” voltage is input into the CE pin. The IC can be put in the shutdown state by inputting “L” voltage into
the CE pin. In the shutdown state, the supply current of the IC is 0μA (TYP.), and the Pch MOS driver Tr. and Nch MOS switch Tr.
for synchronous rectification turn off. The CE pin is a CMOS input and the sink current is 0μA.
<UVLO>
When the VIN voltage becomes 2.00V (TYP.) or lower, the P-ch MOS driver transistor output driver transistor is forced OFF to
prevent false pulse output caused by unstable operation of the internal circuitry. When the VIN pin voltage becomes 2.10V (TYP.)
or higher, switching operation takes place. By releasing the UVLO function, the IC performs the soft start function to initiate output
startup operation. The UVLO circuit does not cause a complete shutdown of the IC, but causes pulse output to be suspended;
therefore, the internal circuitry remains in operation.
<ThermalShutdown>
For protection against heat damage of the ICs, thermal shutdown function monitors chip temperature. The thermal shutdown
circuit starts operating and the P-ch MOS driver and N-ch MOS driver transistor will be turned off when the chip’s temperature
reaches 150℃. When the temperature drops to 120℃ (TYP.) or less after shutting of the current flow, the IC performs the soft-
start function to initiate output startup operation.
<Short-circuit protectionfunction>
The short-circuit protection circuit protects the device that is connected to this product and to the input/output in
situations such as when the output is accidentally shorted to GND. The short-circuit protection circuit monitors the output voltage,
and when the output voltage falls below the short-circuit protection threshold voltage, it turns off the Pch MOS driver Tr and
latches it. Once in the latched state, operation is resumed by turning off the IC from the CE pin and then restarting, or by re-input
into the VINpin.
MYRGM-W/B series A01 Page 9 of 17
