MIC2104 datasheet(22/42 Pages) MICROCHIP

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MIC2103/4

DS20005899A-page 22

 2017 Microchip Technology Inc.

4.0

FUNCTIONAL DESCRIPTION

The MIC2103/4 are adaptive on-time synchronous

buck controllers built for high-input voltage to

low-output voltage conversion applications. They are

designed to operate over a wide input voltage range,

from 4.5V to 75V, and the 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 by sensing

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

internal soft-start, enable, UVLO, and thermal

shutdown.

4.1

Theory of Operation

The Functional Block Diagram illustrates the block

diagram of the MIC2103/4. The output voltage is

sensed by the MIC2103/4 feedback pin FB via the

voltage divider 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, then 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, 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, the MIC2103/4 control

logic will apply 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 200 ns

tOFF(min):

EQUATION 4-2:

It is not recommended to use MIC2103/4 with a

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

operation.

The adaptive ON-time control scheme results in a

constant switching frequency in the MIC2103/4. 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.

To illustrate the control loop operation, one must

analyze both the steady-state and load transient

scenarios. For easy analysis, the gain of the gm

amplifier is assumed to be 1. With this assumption, the

inverting input of the error comparator is the same as

the feedback voltage.

Figure 4-1 shows the MIC2103/4 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 plus injected

voltage 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.

FIGURE 4-1:

MIC2103/4 Control Loop

Timing.

Figure 4-2 shows the operation of the MIC2103/4

during a load transient. The output voltage drops due to

the sudden load increase, which causes the VFB to be

ON ESTIMATED



OUT



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

Where:

VOUT = Output voltage.

VIN = Power stage input voltage.

fSW = Switching frequency.

MAX

OFF MIN



–

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

200ns

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

–

Where:

tS = 1/fSW.

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