MIC4721 datasheet(10/19 Pages) MICREL

10 / 19 page

Micrel, Inc.

MIC4721

May 2007

M9999-052907-A

Ton

therefore, peak to peak ripple current is:

()

OUT

−

Peak-to-peak ripple current is used in calculating output

voltage ripple.

The peak current (or maximum inductor current) is equal

to the output current plus ½ the peak-to-peak current.

()

OUT

−

The peak inductor current is used when selecting a

suitable output inductor.

Figure 4 demonstrates circuit activity during the off-time.

When the high-side internal P-channel MOSFET turns

off, current must flow through the free-wheeling diode,

since inductor current must remain continuous. In this

case, the inductor discharge rate is:

()

OUT +

−

The total off time can be calculated as:

Toff

−

Figure 4. Off-Time

Discontinuous Operation

Discontinuous operation is when the inductor current

discharges to zero at sometime during the off cycle.

Figure 5 demonstrates the switch voltage and inductor

currents during discontinuous operation.

Figure 5. Discontinuous Operation

When

the

inductor

current

(IL)

has

completely

discharged, the voltage on the switch node rings at the

frequency determined by the parasitic capacitance and

the inductor value. In figure 5, it is drawn as a DC

voltage, but to see actual operation (with ringing) refer to

the functional characteristics.

Discontinuous mode of operation has the advantage

over full PWM in that at light loads, the MIC4721 will skip

pulses as necessary, reducing gate drive losses,

drastically improving light load efficiency.

Duty Cycle Considerations

The P-Channel MOSFET inside the MIC4721 allows the

FET to remain in the on-state indefinitely (100% duty

cycle). This feature is useful for maintaining output

voltage

regulation

battery

powered

and

other

applications where the input voltage drops close to the

output voltage.

When the MOSFET is operating at the 2MHz switching

frequency, the maximum operating duty cycle is typically

82%. In situations where a duty cycle greater than the

maximum operating duty cycle is required, the MIC4721

will keep the FET turned on for additional cycles to

maintain output voltage regulation. This effectively

decreases the switching frequency, allowing a larger

duty cycle. The output voltage ripple increases slightly

when the MIC4721 is in pulse skipping mode.

MIC4721 Datasheet(PDF) 10 Page - Micrel Semiconductor

MIC4721 Datasheet(HTML) 10 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL