MIC45205 datasheet(14/40 Pages) MICROCHIP

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MIC45205

DS20005798B-page 14



2017 - 2022 Microchip Technology Inc.

4.0

FUNCTIONAL DESCRIPTION

The MIC45205 is an adaptive on-time synchronous

buck regulator module built for high-input voltage to

low-output voltage conversion applications. The

MIC45205 is designed to operate over a wide input

voltage range, from 4.5V to 26V, 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

overcurrent 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, resistor, and capacitor.

4.1

Theory of Operation

As shown in Figure 4-1 (in association with

Equation 4-1), the output voltage is sensed by the

MIC45205 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 200 ns, the MIC45205 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 MIC45205 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 MIC45205 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 MIC45205 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.

COMPENSATION

COMP

FB1

FB2

REF

0.8V

OUT

M EA

tON ESTIMATED





VOUT

VIN fSW



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

Where:

VOUT Output Voltage

VIN

Power Stage Input Voltage

fSW

Switching Frequency

DMAX

tS tOFF MIN





–

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

1 200ns

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

–

Where:

1/fSW

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