MIC2126 datasheet(17/34 Pages) MICROCHIP

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 2015 Microchip Technology Inc.

DS20005459B-page 17

MIC2125/6

4.0

FUNCTIONAL DESCRIPTION

The MIC2125 and MIC2126 are adaptive on-time

synchronous buck controllers built for high input

voltage to low output voltage applications. They are

designed to operate over a wide input voltage range

from 4.5V to 28V and their output is adjustable with an

external resistive divider. An adaptive ON-time control

scheme is employed to obtain a constant switching

frequency and to simplify the control compensation.

Overcurrent protection is implemented when sensing

low-side MOSFET’s RDS(ON). The device features

internal soft-start, enable, UVLO, and thermal

shutdown.

4.1

Theory of Operation

The MIC2125/6 Functional Block Diagram appears on

page two. The output voltage is sensed by the

MIC2125/6 feedback pin (FB), and is compared to a

0.6V reference voltage (VREF) at the low gain

transconductance error amplifier (gm). Figure 4-1

shows the MIC2125/6 control loop timing during

steady-state operation. When the feedback voltage

decreases and the amplifier output is below 0.6V, the

comparator triggers and generates an ON-time period.

The ON-time period is predetermined by the fixed tON

estimator circuitry value from Equation 4-1:

EQUATION 4-1:

At the end of the ON-time, the internal high-side driver

turns off the high-side MOSFET and the low-side driver

turns on the low-side MOSFET. The OFF-time depends

upon the feedback voltage. When the feedback voltage

decreases and the output of the gm amplifier is below

0.6V, the ON-time period is triggered and the OFF-time

period ends. If the OFF-time period determined by the

feedback voltage is less than the minimum OFF-time

tOFF(min), which is about 220 ns, the MIC2125/6 control

logic applies the tOFF(min) instead. tOFF(min) is required

to maintain enough energy in the boost capacitor

(CBST) to drive the high-side MOSFET.

The maximum duty cycle is obtained from the 220 ns

tOFF(MIN):

EQUATION 4-2:

It is not recommended to use MIC2125/6 with an

OFF-time close to tOFF(MIN) during steady-state

operation.

The adaptive ON-time control scheme results in a

constant switching frequency in the MIC2125/6. The

actual ON-time and resulting switching frequency

varies with the different rising and falling times of the

external MOSFETs. Also, the minimum tON results in a

lower switching frequency in high VIN to VOUT

applications.

FIGURE 4-1:

MIC2125/6 Control Loop

Timing

Figure 4-2 shows the operation of the MIC2125/6

during load transient. The output voltage drops due to

a sudden increase in load, which results in the VFB

falling below VREF. This causes the comparator to

trigger an ON-time period. At the end of the ON-time, a

minimum OFF-time tOFF(min) is generated to charge

CBST if the feedback voltage is still below VREF. The

next ON-time is triggered immediately after the

tOFF(min) due to the low feedback voltage. This

operation results in higher switching frequency during

load transients. The switching frequency returns to the

nominal set frequency once the output stabilizes at new

load current level. The output recovery time is fast and

the output voltage deviation is small in MIC2125/6

converter due to the varying duty cycle and switching

frequency.

ON ESTIMATED



OUT



-----------------------

MAX

OFF MIN



–

-----------------------------------

220ns

---------------

–

Where:

1/fSW

Where:

VOUT

Output Voltage

VIN

Power Stage Input Voltage

fSW

Switching Frequency

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