Micrel, Inc.

MIC2250

November 2008

9

M9999-111708-A

Application Information

Overview

The MIC2250 Boost Regulator utilizes a combination of

PFM & Current Mode Control to achieve very high

efficiency over a wide range of output load. This

innovative design is the basis for the regulator’s high

efficiency, excellent stability, and self compensation

technique. The boost regulator performs a power

conversion that results in an output voltage that is

greater than the input. Operation starts with activating an

internal MOSFET switch which draws current through

the inductor (L1). While one end of the inductor is fixed

switch is on, the current through the inductor increases.

When the switch is off the inductor current continues to

flow through the output diode.

The current flow imposes a voltage across the inductor,

At low power levels (typically less than 1W), the period

varies between switching cycles, indicative of Pulse

Frequency Modulation (PFM). As the output power

increases beyond approximately 1W, the period between

switching cycles continues to decrease and the power

(switch current) delivered with each cycle increases

indicative of Current Mode control.

PFM Regulation

The error amplifier compares the regulator’s reference

voltage with the feedback voltage obtained from the

output resistor voltage divider network. The resulting

error voltage acts as a correction input signal to the

control block. The control block generates two signals

that turn on and off the output MOSFET switch. An

decrease in value. The control loop then changes the

switching frequency to increase the energy transferred to

the output capacitor to regulate the output voltage. A

the control loop compensates by reducing the effective

switching frequency, thus reducing the amount of energy

delivered to the output capacitor in order to keep the

output voltage within regulation.

Current Mode Regulation

The control block’s oscillator starts the cycle by setting

the MOSFET switch control flip flop. The switch then

turns on. This flip flop is reset when the switch current

ramp reaches the threshold set by the error amplifier. If

too low, then the threshold for the comparator measuring

back to within regulation limits. The level of the error

signal also sets the off time of the switch. A higher error

signal (output voltage is low) will reduce off time to

increase energy transfer to the output. A lower error

signal (output voltage is high) will conversely, increase

off time to reduce energy transfer to the output.

Component Selection

Resistors

An external resistive divider network (R1 and R2) with its

center tap connected to the feedback pin sets the output

voltage. The appropriate R1 and R2 values for the

desired output voltage are calculated by:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

=

1

1.24V

V

R1

R2

OUT

Large resistor values are recommended to reduce light

load operating current, and improve efficiency. The table

below gives a good compromise between quiescent

current and accuracy. Additionally, a feedforward

to improve transient performance. Recommended values

are suggested below:

Suggested R1

5V to 10V

100k

4.7nF

10V to 15V

240k

2.2nF

15V to 32V

1M

470pF

Figure 1. Typical Application Circuit

Inductor

Inductor selection is a balance between efficiency,

stability, cost, size, and rated current. For most

applications, inductors in the range 4.7uH to 22uH are

recommended. Larger inductance values reduce the

peak-to-peak ripple current, thereby reducing both the

DC losses and the transition losses for better efficiency.

The inductor’s DC resistance (DCR) also plays an

important role. Since the majority of the input current

(minus the MIC2250 operating current) is passed

through the inductor, higher DCR inductors will reduce

efficiency at higher load currents. Figure 2 shows the

comparison of efficiency between a 140mΩ DCR, 4.7uH

inductor and a 190mΩ DCR, 10uH inductor. The switch

current limit for the MIC2250 is typically 2A. The