MIC28303 datasheet(21/38 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005464A-page 21

MIC28303

4.0

FUNCTIONAL DESCRIPTION

The MIC28303 is an adaptive on-time synchronous

buck regulator module built for high-input voltage to

low-output voltage conversion applications. The

MIC28303 is designed to operate over a wide input

voltage range, from 4.5V to 50V, and the output is

adjustable with an external resistor divider. An adaptive

on-time control scheme is employed to obtain a

constant switching frequency 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, resistor and

capacitor.

4.1

Theory of Operation

Per the Functional Diagram of the MIC28303 module,

the output voltage is sensed by the MIC28303

feedback pin FB via the voltage divider R1 and R11,

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 MIC28303 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 MIC28303 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 MIC28303. 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, both the

steady-state and load transient scenarios were

analyzed. 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 MIC28303 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:

MIC28303 Control Loop

Timing

Figure 4-2 shows the operation of the MIC28303 during

a load transient. The output voltage drops due to the

sudden load increase, which causes the VFB to be less

than VREF. This will cause the error comparator to

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

period, a minimum OFF-time tOFF(MIN) is generated to

ON ESTIMATED



OUT



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

Where:

VOUT

Output Voltage

VIN

Power Stage Input Voltage

fSW

Switching Frequency

MAX

OFF MIN



–

-----------------------------------1

200ns

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

–

Where:

1/fSW

OUT

REF

ΔI

L(PP)

ΔV

OUT(PP) = ESRCOUT × ΔIL(PP)

ESTIMATED ON-TIME

ΔV

FB(PP) = ΔVOUT(PP) ×

R1+R2

TRIGGER ON-TIME IF V

FB IS BELOW VREF

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