MMM5062 datasheet(12/22 Pages) FREESCALE

12 / 22 page

Micrel, Inc.

MIC22950

February 2010

M9999-021910-B

The tradeoff 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.

The integration of one pole-zero pair within the control

loop greatly simplifies compensation. The optimum

values for CCOMP (in series with a 1k resistor) are shown

below:

10-22µF

47-100µF

120-470µF

0.39 - 0.47µH

10*-20pF

†

25-47pF

0.56 – 1.0µH

‡-15pF*

†-39pF

56-100pF

1.2 – 2.2µH

10-22pF

22-68pF

† VOUT > 1V; * VOUT > 1.4V; ‡ VOUT > 1.8V

Feedback

The MIC22950 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

0.7V reference voltage and adjusts the output voltage to

maintain regulation. To calculate the resistor divider

network for the desired output is as follows:

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

REF

OUT

Where VREF is 0.7V and VOUT is the desired output

voltage. A 10kΩ or lower resistor value from the output

to the feedback is recommended since large feedback

resistor values increase the impedance at the feedback

pin, making the feedback node more susceptible to

noise pick-up. A small decoupling capacitor (22pF –

100pF) across the lower resistor can reduce noise pick-

up by providing a low impedance path to ground.

PWM Operation

The

MIC22950

voltage-mode,

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 MIC22950 will run at 100% duty

cycle.

The MIC22950 provides switching frequency at 400kHz

to 2MHz with synchronous internal MOSFETs. The

internal MOSFETs include a high-side 11m

Ω P-Channel

MOSFET from the input supply to the switch pin and an

Ω N-Channel MOSFET from the switch pin-to-ground.

Since the low-side N-Channel MOSFET provides the

current during the off cycle, a freewheeling Schottky

diode from the switch node-to-ground is not required.

PWM control provides fixed-frequency operation. By

maintaining a constant switching frequency, predictable

fundamental and harmonic frequencies are achieved.

Other methods of regulation, such as burst and skip

modes, have frequency spectrums that change with load

that

can

interfere

with

sensitive

communication

equipment.

CF Pin

Each switching cycle begins immediately following the

discharge of the CF pin capacitor. From this point, a

current source of 200µA flows from CF pin into the

external capacitor connected to the CF pin. This creates

a linear voltage ramp rising towards a threshold voltage

of 1V. When this capacitor reaches the 1V threshold, it

triggers the end of the switching cycle by discharging CF

pin to ground via an internal N-Channel; at which point,

the next cycle begins. The actual switching frequency

can be approximated by using the following equation

which accounts for internal delays and capacitance:

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅

RAMP

DELAY

Where:

FSW = Switching Frequency

TDELAY = CF pin Discharge time ~ 85ns

VRAMP = Voltage ramp amplitude ~ 0.9V

CCF = External CF capacitor = 68pF to 560pF

CPIN = Internal Pin capacitance ~ 15pF

ICF = CF current source = 200µA

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