mic4782 Micrel Semiconductor, mic4782 Datasheet - Page 11

• Manufacturer Part Number: mic4782
• Description: 1.8 Mhz Dual 2a Integrated Switch Buck Regulator
• Manufacturer: Micrel Semiconductor

Micrel, Inc.
Figure 4 demonstrates the off-time. During the off-time,
the high-side internal P-channel MOSFET turns off.
Since the current in the inductor has to discharge, the
current flows through the free-wheeling Schottky diode
to the output. In this case, the inductor discharge rate is
(where V
The total off time can be calculated as;
Discontinuous Operation
Discontinuous operation is when the inductor current
discharges to zero during the off cycle. Figure 5
demonstrates the switch voltage and inductor currents
during discontinuous operation.
When
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.
August 2009
the
−
T
D
OFF
(
is the diode forward voltage);
V
OUT
=
inductor
1
fsw
L
−
+
D
V
Figure 4. Off-Time
D
)
current
(IL)
has
completely
11
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC4782 will skip
pulses as necessary, reducing gate drive losses,
drastically improving light load efficiency.
Efficiency Considerations
Calculating the efficiency is as simple as measuring
power out and dividing it by the power in;
Where input power (P
and output power (P
The Efficiency of the MIC4782 is determined by several
factors.
R
the high side P-Channel MOSFET. The amount of power
loss can be approximated by;
Where D is the duty cycle.
Since the MIC4782 uses an internal P-Channel
MOSFET, R
supply voltage. Higher supply voltage yields a higher
gate to source voltage, reducing the R
MOSFET conduction losses. A graph showing typical
DSON
•
•
•
•
losses are caused by the current flowing through
R
Diode conduction losses
Inductor Conduction losses
Switching losses
Efficiency
P
P
P
IN
OUT
DSON
SW
Figure 5. Discontinuous Operation
=
DSON
=
V
(Internal P-channel Resistance)
=
R
IN
V
DSON
OUT
×
losses are inversely proportional to
=
I
OUT
IN
P
IN
P
OUT
×
) is;
IN
) is calculated as;
×
I
I
OUT
OUT
×
100
2
×
D
DSON
M9999-081709-D
, reducing the
MIC4782