MIC45116 datasheet(19/42 Pages) MICROCHIP

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DS20005571B-page 19

MIC45116

4.0

FUNCTIONAL DESCRIPTION

The MIC45116 is an adaptive ON-time synchronous

buck regulator module built for high-input voltage to

low-output voltage conversion applications. The

MIC45116 is designed to operate over a wide input

voltage range, from 4.75V to 20V, and the output is

adjustable with an external resistor divider.

An adaptive ON-time control scheme is employed to

obtain a constant switching frequency in steady state

and to simplify the control compensation. Hiccup mode

over-current protection is implemented by sensing

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

internal soft-start, enable, UVLO, and thermal

shutdown. The module has integrated switching FETs,

inductor, bootstrap diode, and bypass capacitors.

4.1

Theory of Operation

Figure 4-1, in association with Equation 4-1, shows the

output voltage is sensed by the MIC45116 feedback pin

(FB) via the voltage divider RFB1 and RFB2 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 falls 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:

FIGURE 4-1:

Output Voltage Sense via

FB Pin.

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 falls 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 250 ns, the MIC45116 control logic will apply the

tOFF(MIN) instead. tOFF(MIN) is required to maintain

enough energy in the internal boost capacitor (CBST) to

drive the high-side MOSFET.

The maximum duty cycle is obtained from the 250 ns

tOFF(MIN):

EQUATION 4-2:

It is not recommended to use MIC45116 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 MIC45116 during

steady state operation. The actual ON-time and

resulting switching frequency will vary with the different

rising and falling times of the 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, we will 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 4-2 shows the MIC45116 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.

INTERNAL

RIPPLE

INJECTION

INJ

COMP

COMPENSATION

M EA

FB1

FB2

REF

0.8V

tON ESTIMATED





VOUT

VIN fSW



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

Where:

VOUT

Output Voltage

VIN

Power Stage Input Voltage

fSW

Switching Frequency

DMAX

tS tOFF MIN





–

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

1 250ns

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

–

Where:

1/fSW

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