MIC2204 Micrel Semiconductor, MIC2204 Datasheet - Page 9

• Manufacturer Part Number: MIC2204
• Description: High-Efficiency 2MHz Synchronous Buck Converter
• Manufacturer: Micrel Semiconductor

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.
On the shaded region, 200mA to 500mA, efficiency loss is
dominated by MOSFET R
input supply voltages will increase the Gate-to-Source thresh-
old on the internal MOSFETs, reducing the internal R
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:
From that, the loss in efficiency due to inductor resistance can
be calculated as follows:
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-
January 2004
MIC2204
L
Efficiency Loss
PD
=I
OUT
2
x DCR
100
95
90
85
80
75
70
65
60
55
50
0
vs. Output Current
OUTPUT CURRENT (A)
100
5V
4.2V
1–
Figure 2.
IN
Efficiency
DSON
IN
200
V
OUT
300
V
and inductor losses. Higher
OUT
3.6V
3.3V
400
I
IN
OUT
OUT
I
OUT
500
L
PD
100
DSON
.
9
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 4.7 H inductor
and a 4.7 F ceramic (X5R) output capacitor for output
voltages greater than 1.6V. For output voltages less than
1.6V, a 10 F 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
10 F.
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:
Where V
A 10k 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.
the device reaches unity gain at high-frequency. Therefore,
output capacitance will need to be increased to a minimum
10 F. For more information on output capacitor selection for
stability, see the “Compensation ” section.
When using a feed-forward capacitor, the gain bandwidth of
R2
REF
V
is 1.0V and V
V
OUT
REF
R1
– 1
OUT
is the desired output voltage.
M0214-012904
Micrel