MIC2202 Datasheet, PDF (8/18 Pages) Micrel Semiconductor – HIGH EFFICIENCY 2MHZ SYNCHRONOUS BUCK CONVERTER 1UP STABLE PWM REGULATOR

MIC2202
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.
Alternatively, under lighter loads, the ripple current due to the
inductance becomes a significant factor. When light load
efficiencies become more critical, a larger inductor value may
be desired. Larger inductances reduce the peak-to-peak
ripple current which minimize losses. The following graph
illustrates the effects of inductance value at light load.
Efficiency
vs. Inductance
100
4.7µH
80
1µH
60
2.2µH
40
20
1.8V_OUT
0
0
25 50 75 100
OUTPUT CURRENT (mA)
Figure 3. Efficiency vs. Inductance
Compensation
The MIC2202 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 pulse width modulate the switch node, maintain-
ing output voltage regulation. With a typical gain bandwidth of
200kHz, the MIC2202 is capable of extremely fast transient
responses.
The MIC2202 is designed to be stable with a 2.2µH inductor
and a 1µF ceramic (X5R) output capacitor. These values can
be interchanged (i.e. 1µH inductor and a 2.2µF capacitor).
The trade off between changing these values is that with a
larger inductor, there is a reduced peak-to-peak current
which yields a greater efficiency at lighter loads. A larger
output capacitor will improve transient response by providing
a larger hold up reservoir of energy to the output.
Feedback
The MIC2202 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 0.5V reference voltage and
adjusts the output voltage to maintain regulation. To calculate
the resistor divider network for the desired output is as
Micrel
follows:
R2 = ^R1
 V_OUT
 ^V_REF

– 1_
Where V_REF is 0.5V 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.
PWM Operation
The MIC2202 is a pulse width modulation (PWM) controller.
By controlling the ratio of on-to-off time, or duty cycle, a
regulated DC output voltage is achieved. As load or supply
voltage changes, so does the duty cycle to maintain a
constant output voltage. In cases where the input supply runs
into a dropout condition, the MIC2202 will run at 100% duty
cycle.
The MIC2202 provides constant switching at 2MHz with
synchronous internal MOSFETs. The internal MOSFETs
include a high-side P-Channel MOSFET from the input
supply to the switch pin and an N-Channel MOSFET from the
switch pin to ground. Since the low-side N-Channel MOSFET
provides the current during the off cycle, a free wheeling
Schottky diode from the switch node to ground is not required.
PWM control provides fixed frequency operation. By main-
taining a constant switching frequency, predictable funda-
mental and harmonic frequencies are achieved. Other meth-
ods of regulation, such as burst and skip modes, have
frequency spectrums that change with load that can interfere
with sensitive communication equipment.
M9999-052104
8
May 2004