MIC4721 Datasheet, PDF (10/19 Pages) Micrel Semiconductor – 1.5A 2MHz Integrated Switch Buck Regulator

Micrel, Inc.
Ton = ^D
f_S
therefore, peak to peak ripple current is:
( ) _{IPK −PK}
= I_OUT
+
V_IN
− V_OUT
× ^VOUT
V_IN
f_S × L
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.
( ) _IPK
= I_OUT
+
V_IN
− V_OUT
× ^VOUT
V_IN
2 × f_S × L
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:
− ^{(VOUT + VD )}
L
The total off time can be calculated as:
Toff = ^{1 − D}
f_S
Figure 4. Off-Time
MIC4721
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 in 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.
May 2007
10
M9999-052907-A