MIC4782 Datasheet, PDF (11/23 Pages) Micrel Semiconductor – 1.8 MHz Dual 2A Integrated Switch Buck Regulator

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_D is the diode forward voltage);
− ^{(VOUT + VD )}
L
The total off time can be calculated as;
T_OFF
=
1− D
fsw
MIC4782
Figure 4. Off-Time
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 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.
Figure 5. Discontinuous Operation
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;
Efficiency = ^POUT × 100
P_IN
Where input power (P_IN) is;
P_IN = V_IN × I_IN
and output power (P_OUT) is calculated as;
P_OUT = V_OUT × I_OUT
The Efficiency of the MIC4782 is determined by several
factors.
• R_DSON (Internal P-channel Resistance)
• Diode conduction losses
• Inductor Conduction losses
• Switching losses
R_DSON losses are caused by the current flowing through
the high side P-Channel MOSFET. The amount of power
loss can be approximated by;
P_SW = R_DSON × I_OUT ² × D
Where D is the duty cycle.
Since the MIC4782 uses an internal P-Channel
MOSFET, R_DSON losses are inversely proportional to
supply voltage. Higher supply voltage yields a higher
gate to source voltage, reducing the R_DSON, reducing the
MOSFET conduction losses. A graph showing typical
August 2009
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M9999-081709-D