MIC2208 Datasheet, PDF (10/19 Pages) Micrel Semiconductor – 3mmx3mm 1MHz 3A PWM Buck Regulator

Micrel, Inc.
MIC2208
Therefore, peak to peak ripple current is;
I_{pk −pk}
=
(V_IN
) - V_OUT
× ^VOUT
V_IN
1MHz × L
Since the average peak-to-peak current is equal to
the load current. The actual peak (or highest current
the inductor will see in a steady state condition) is
equal to the output current plus half the peak-to-
peak current. For those of you who have not had
enough formulas already, here it is;
I_pk
=
I_OUT
+
(V_IN
-
V_OUT
)×
V_OUT
V_IN
2 ×1MHz × L
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
1MHz
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.
Figure 5. Discontinuous Operation
When the inductor current (I_L) 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 and all) refer to the functional
characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC2208 will
skip pulses as nessasary, 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 MIC2208 is determined by
several factors.
• Rdson (Internal P-channel Resistance)
• Diode conduction losses
• Inductor Conduction losses
• Switching losses
Rdson losses are caused by the current flowing
through the high side P-channel MOSFET. The
amount of power loss can be approximated by;
September 2005
10
M9999-092905
www.micrel.com