Buck Driver Example 2
7 Application Information
Input Voltage Regulator
The MG39U301 integrates one high voltage regulator to generate the power for internal core logic. The
internal power is taken from the AC line directly. Extra auxiliary transformer is not needed for the supply. When a
voltage (7~500V) is applied at the VIN pin, the regulator maintains a constant 5.6V at the VDD pin. The VDD pin
must be bypassed by a low ESR capacitor to provide a low impedance path for the high frequency current of the
output date driver.
Since the current driving capability of the regulator is limited to around 0.7mA. It’s not suitable for driving the
external components. User must make sure the switching frequency should not be too high and the load at the
GATE pin should not be too heavy.
It’s recommended to add a series resistor between VINDC and VIN pin to protect the IC. Basically, several
ten K to 100K ohms are recommended. The voltage drop on the resistor can be calculated as VRDROP = IOP x RVIN
.
For large ILED application, it’s recommended to add a pull-high resistor (RP) between VINDC and VDD pin to
provide more driving current.
Selection of the Switching Control Scheme
The MG39U301 provides two control schemes, constant frequency and constant off time. For a buck
converter, the duty of PWM control signal is determined as D = VO / VIN, where VO is the voltage drop on LED
strings (VLED). For a heavy load application (large VLED drop, D>0.45), user should select the constant off time
mode.
Selection of the Switching Frequency (FS)
Rule of thumb, the range of 20KHz ~ 100KHz is recommended.
For detail frequency setting, please check application note “AN_MG39U301_ROSC SEL”.
GATE Output
The GATE output of the MG39U301 is used to drive an external MOSFET. It is recommended that the gate
charge QG of the external MOSFET should satisfy the equation as below:
MEGAWIN
MG39U301 Data Sheet
7