MIC2207 Datasheet, PDF (11/21 Pages) Micrel Semiconductor – 3mmx3mm 2MHz 3A PWM Buck Regulator

Micrel
MIC2207
Therefore, peak to peak ripple current is;
I_{pk −pk}
=
(V _IN − V_OUT
)×
V_OUT
V_IN
2MHz × 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 ½ the peak to peak
current.
I_pk
= I_OUT
+
(V_IN
−
V_OUT )×
V_OUT
V_IN
2 × 2MHz × 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
2MHz
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) refer to the functional characteristics.
Discontinuous mode of operation has the advantage
over full PWM in that at light loads, the MIC2207 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 MIC2207 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;
P_SW = R_DSON × I_OUT ² × D
April 2005
11
M9999-040705
www.micrel.com