MIC2204_05 Datasheet, PDF (9/11 Pages) Micrel Semiconductor – High-Efficiency 2MHz Synchronous Buck Converter

MIC2204
Figure 2 shows an efficiency curve. On the non-shaded
portion, from 0 to 200mA, efficiency losses are dominated by
quiescent current losses, gate drive and transition losses. In
this case, lower supply voltages yield greater efficiency in that
they require less current to drive the MOSFETs and have
reduced input power consumption.
Efficiency
vs. Output Current
100
95
90
4.2V_IN
85
80
5V_IN
75
3.6V_IN
70
65
60
55
3.3V_OUT
500 _{100 200 300 400 500}
OUTPUT CURRENT (A)
Figure 2.
On the shaded region, 200mA to 500mA, efficiency loss is
dominated by MOSFET R_DSON and inductor losses. Higher
input supply voltages will increase the Gate-to-Source thresh-
old on the internal MOSFETs, reducing the internal R_DSON.
This improves efficiency by reducing DC losses in the device.
All but the inductor losses are inherent to the device, making
inductor selection even more critical in efficiency calcula-
tions. As the inductors are reduced in size, the DC resistance
(DCR) can become quite significant. The DCR losses can be
calculated as follows:
L_PD=I_OUT2 x DCR
From that, the loss in efficiency due to inductor resistance can
be calculated as follows:
Efficiency
Loss
=

1±



V_OUT × I_OUT
V_OUT × I_OUT + L_PD





×
100
Efficiency loss due to DCR is minimal at light loads and gains
significance as the load is increased. Inductor selection
becomes a trade-off between efficiency and size in this case.
Compensation
The MIC2204 is an internally compensated, voltage-mode
buck regulator. Voltage mode is achieved by creating an
internal 2MHz ramp signal and using the output of the error
amplifier to pulsewidth modulate the switch node, maintain-
Micrel, Inc.
ing output voltage regulation. With a typical gain bandwidth of
200kHz, the MIC2204 is capable of extremely fast transient
responses.
The MIC2204 is designed to be stable with a 4.7µH inductor
and a 4.7µF ceramic (X5R) output capacitor for output
voltages greater than 1.6V. For output voltages less than
1.6V, a 10µF capacitor is required. Also, when a feed forward
capacitor is used, the gain bandwidth is increased to unity
gain. This will also require increasing the output capacitor to
10µF.
Feedback
The MIC2204 provides a feedback pin to adjust the output
voltage to the desired level. This pin connects internally to an
error amplifier. The error amplifier then compares the voltage
at the feedback to the internal 1V reference voltage and
adjusts the output voltage to maintain regulation. To calculate
the resistor divider network for the desired output is as
follows:
R2 =
R1
 V_OUT
 ^V_REF

± 1_
Where V_REF is 1.0V and V_OUT is the desired output voltage.
A 10kΩ or lower resistor value from the output to the feedback
is recommended. Larger resistor values require an additional
capacitor (feed-forward) from the output to the feedback. The
large high-side resistor value and the parasitic capacitance
on the feedback pin (~10pF) can cause an additional pole in
the loop. The additional pole can create a phase loss at
high-frequency. This phase loss degrades transient response
by reducing phase margin. Adding feed-forward capacitance
negates the parasitic capacitive effects of the feedback pin.
A minimum 1000pF capacitor is recommended for feed-
forward capacitance.
Also, large feedback resistor values increase the impedance,
making the feedback node more susceptible to noise pick-up.
A feed-forward capacitor would also reduce noise pick-up by
providing a low impedance path to the output.
When using a feed-forward capacitor, the gain bandwidth of
the device reaches unity gain at high-frequency. Therefore,
output capacitance will need to be increased to a minimum
10µF. For more information on output capacitor selection for
stability, see the “Compensation ” section.
April 2005
9
M9999-042205