MIC2202 datasheet(8/18 Pages) MICREL

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MIC2202

Micrel

M9999-052104

May 2004

Efficiency Loss

1–

100

OUT

×+





















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.

100

OUTPUT CURRENT (mA)

1.8V

OUT

4.7

µH

Efficiency

vs. Inductance

2.2

µH

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 2.2

µH inductor

and a 1

µF ceramic (X5R) output capacitor. These values can

be interchanged (i.e. 1

µH 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

follows:

– 1

OUT

REF









Where V

REF is 0.5V and VOUT is the desired output voltage.

A 10k

Ω 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.

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