MIC28512 datasheet(18/34 Pages) MICROCHIP

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MIC28512

DS20005524A-page 18

 2016 Microchip Technology Inc.

4.0

FUNCTIONAL DESCRIPTION

The MIC28512 is an adaptive on-time synchronous

buck regulator with integrated high-side and low-side

MOSFETs suitable for high-input voltage to low-output

voltage conversion applications. It is designed to

operate over a wide input voltage range (4.6V to 70V)

which is suitable for automotive and industrial

applications. The output is adjustable with an external

resistive divider. An adaptive on-time control scheme is

employed to produce a constant switching frequency in

continuous-conduction mode and reduced switching

frequency in discontinuous-operation mode, improving

light load efficiency. Overcurrent protection is

implemented by sensing low-side MOSFET’s RDS(ON).

The device features internal soft-start, enable, UVLO,

and thermal shutdown.

4.1

Theory of Operation

As illustrated in the Functional Diagram, the output

voltage of the MIC28512 is sensed by the feedback

(FB) pin via the voltage dividers R1 and R2, and

compared to a 0.8V reference voltage VREF at the error

comparator through a low-gain transconductance (gM)

amplifier. If the feedback voltage decreases and the

amplifier output is below 0.8V, the error comparator will

trigger the control logic and generate an ON-time

period. The ON-time period length is predetermined by

the “Fixed tON Estimator” circuitry:

EQUATION 4-1:

At the end of the ON-time period, 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

period length depends upon the feedback voltage in

most cases. When the feedback voltage decreases

and the output of the gM amplifier is below 0.8V, then

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 200 ns (typical), the

MIC28512 control logic will apply the tOFF(MIN) instead.

The tOFF(MIN) is required to maintain enough energy in

the boost capacitor (CBST) to drive the high-side

MOSFET.

The

maximum

duty

cycle

obtained

from

Equation 4-2.

EQUATION 4-2:

It is not recommended to use MIC28512 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 MIC28512. The

actual ON-time and resulting switching frequency will

vary 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. During load transients, the switching

frequency is changed due to the varying OFF-time.

Figure 4-1 shows the allowable range of the output

voltage versus the input voltage. The minimum output

voltage is 0.8V which is limited by the reference

voltage. The maximum output voltage is 24V which is

limited by the internal circuitry.

FIGURE 4-1:

Allowable Output Voltage

Range vs. Input Voltage.

To illustrate the control loop operation, both the

steady-state and load transient scenarios will be

analyzed.

Figure 4-2 shows the MIC28512 control loop timing

during steady-state operation. During steady-state, the

gM amplifier senses the feedback voltage ripple, which

is proportional to the output voltage ripple and the

inductor current ripple, to trigger the ON-time period.

The ON-time is predetermined by the tON estimator.

The termination of the OFF-time is controlled by the

feedback voltage. At the valley of the feedback voltage

ripple, which occurs when VFB falls below VREF, the

OFF period ends and the next ON-time period is

triggered through the control logic circuitry.

ON ESTIMATED



OUT



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

Where:

VOUT

Output Voltage

VIN

Power Stage Input Voltage

fSW

Switching Frequency

MAX

OFF MIN



–



0.0

5.0

10.0

15.0

20.0

25.0

30.0

1831445770

INPUT VOLTAGE (V)

Fsw = 600kHz

Fsw = 400kHz

Fsw = 200kHz

0.8V min.

Allowable Range

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