MIC2267 datasheet(9/19 Pages) MICREL

9 / 19 page

Micrel Inc.

MIC2267

January 2011

M9999-011711-A

Functional Diagram

Figure 1. MIC2267 Functional Block Diagram

Functional Description

The MIC2267 is an input current limited, 2A synchronous

buck regulator. The part offers control of the input

average current limit and the time constant of the current

limit response, allowing full control of the input current

profile during load steps and plug in events. The P-

Channel high side switch allows for 100% duty cycle

operation. Optimization of the loop bandwidth can be

achieved using the available connections to the slope

compensation ramp and error amplifier nodes. For logic

control and error flagging, the MIC2267 has an enable

function and a Power Good function.

VIN

The input supply (VIN) provides power to the internal

power MOSFETs and the analog control circuitry. The

VIN operating range is 3V to 5.5V. A minimum bypass

capacitor of 10µF should be placed close to the input

(VIN) pin and the ground (PGND) pin. Refer to the Layout

Recommendations

section for details on placing the

input capacitor.

LIMIT

The current limit as set by the RLIMIT resistor (Nominally,

ILIMIT = 75k/RLIMIT) is converted to the total charge

allowed in one cycle (QTOT). So if the current limit is 1A

and the frequency is 1MHz, since Q = I x T, the total

charge allowed QTOT = 1µC. If for example, a 1µF

capacitor CTOT is used to store this charge, since Q = C x

V, the voltage would be 1V.

If a replica of the input current is then integrated and the

resultant input charge per cycle:

∫

(

O IN

Is stored on a similar 1µF capacitor CINT, then input

current limit is reached when the voltage on the CINT

capacitor reaches 1V, i.e. QIN ≥ QTOT. Once this

condition is satisfied and the FILTER delay time has

elapsed, the P-Channel switch cycle is terminated. At

the beginning of each cycle the integration storage

capacitors are discharged. In the actual circuit the

storage capacitor is in the order of 5pF and the replica

current is scaled down accordingly.

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