April 2005

9

M9999-042205

MIC2204

Micrel, Inc.

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.

50

55

60

65

70

75

80

85

90

95

100

0

100

200

300

400

500

OUTPUT CURRENT (A)

3.3V

OUT

4.2V

IN

3.6V

IN

Efficiency

vs. Output Current

5V

IN

Figure 2.

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:

L

From that, the loss in efficiency due to inductor resistance can

be calculated as follows:

Efficiency Loss

1±

VI

VI

L

100

OUT

OUT

OUT

OUT

PD

=

×

×+





























×

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-

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

V

±1

OUT

REF

=













Where V

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.