APPLICATION NOTES CONT'D

RF HIGH:

It is very important that the DC voltage returned to the RF high pin from the output be as noise and oscillation free as possible.

This voltage helps to determine the final output and therefore must be a clean voltage. Excessive noise or oscillation can cause the

device to have an incorrect output voltage. Proper PC board layout techniques can help to achieve a noise free voltage at the RF

high pin.

Under heavy loads, the MSK 5040 operates in full PWM mode. Each pulse from the oscillator sets the internal PWM latch that

turns on the high-side MOSFET. As the high-switch turns off, the synchronous rectifier latch is set. 60ns later the low-side

MOSFET turns on until the start of the next clock cycle or until the inductor current crosses zero. Under fault conditions the current

exceeds the ±100mV current-limit threshold and the high-side switch turns off.

At light loads the inductor current does not exceed the 30mV threshold set by the minimum-current comparator. When this

occurs, the MSK 5040 goes into idle mode, skipping most of the oscillator pulses in order to reduce the switching frequency and

cut back gate-charge losses. The oscillator is gated off at light loads because the minimum-current comparator immediately resets

the high-side latch at the start of each cycle. Refer to Table 1 for the operational characteristics.

DEVICE DISABLED

PULSE SKIPPING MODE DISCONTINUOUS INDUCTOR CURRENT

PULSE SKIPPING MODE CONTINUOUS INDUCTOR CURRENT

CONSTANT FREQ. PWM MODE CONTINUOUS INDUCTOR CURRENT

DESCRIPTION

0

1

1

1

ENABLE

LOAD

X

LOW <10%

MED <30%

HIGH >30%

OPERATIONAL CHARACTERISTICS

TABLE 1

TYPICAL 2.5V APPLICATION CIRCUIT

Rev. F 2/06

4

MODES OF OPERATION: