FUNCTIONAL DECRIPTIONS

Loop Operation

The LR9641 is a wide input range, high-efficiency, DC-to-DC step-down switching regulator, capable of delivering up to

0.7A of output current, integrated with a 400mΩ high side MOSFET. It uses a PWM current-mode control scheme. An

error amplifier integrates error between the FB signal and the internal reference voltage. The output of the integrator is

then compared to the sum of a current-sense signal and the slope compensation ramp. This operation generates a PWM

signal that modulates the duty cycle of the power MOSFETs to achieve regulation for output voltage.

Light Load Operation

Traditionally, a fixed constant frequency PWM DC-DC regulator always switches even when the output load is small.

When energy is shuffling back and forth through the power MOSFETs, power is lost due to the finite RDSONs of the

MOSFETs and parasitic capacitances. At light load, this loss is prominent and efficiency is therefore very low. LR9641

employs a proprietary control scheme that improves efficiency in this situation by enabling the device into a power save

mode during light load, thereby extending the range of high efficiency operation.

APPLICATION INFORMATION

Setting Output Voltages

Output voltages are set by external resistors. The FB threshold is 0.8V.

RTOP = RBOTTOM x [(VOUT / 0.8) - 1]

Inductor Selection

The peak-to-peak ripple is limited to 30% of the maximum output current. This places the peak current far enough from

the minimum overcurrent trip level to ensure reliable operation while providing enough current ripples for the current

mode converter to operate stably. In this case, for 0.7A maximum output current, the maximum inductor ripple current

is 300 mA. The inductor size is estimated as following equation:

LIDEAL=(VIN(MAX)-VOUT)/IRIPPLE*DMIN*(1/FOSC)

Therefore, for VOUT=5V, The inductor values is calculated to be L = 13μH. Chose 10μH or 15μH

For VOUT =3.3V,The inductor values is calculated to be L = 9.2μH. Chose 10μH

Output Capacitor Selection

For most applications a nominal 22μF or larger capacitor is suitable. The LR9641 internal compensation is designed for a

fixed corner frequency that is equal to FC= 8.7Khz

For example, for VOUT=5V, L=15μH, COUT=22μF.

The output capacitor keeps output ripple small and ensures control-loop stability. The output capacitor must also have

low impedance at the switching frequency. Ceramic, polymer, and tantalum capacitors are suitable, with ceramic

5ms/div

5ms/div

5ms/div

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