MIC28304 datasheet(22/39 Pages) MICREL

22 / 39 page

Micrel, Inc.

MIC28304

Functional Description

The MIC28304 is an adaptive on-time synchronous buck

regulator module built for high-input voltage to low-output

voltage

conversion

applications.

The

MIC28304

designed to operate over a wide input voltage range,

from 4.5V to 70V, 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.

Theory of Operation

Per the Functional Diagram of the MIC28304 module, the

output voltage is sensed by the MIC28304 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:

OUT

)

ESTIMATED

(

Eq. 1

where VOUT is the output voltage, VIN is the power stage

input voltage, and fSW is the switching frequency.

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

200ns, the MIC28304 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 200ns

tOFF(MIN):

)

MIN

(

OFF

MAX

200

−

Eq. 2

Where:

tS = 1/fSW. It is not recommended to use MIC28304 with

OFF-time

tOFF(MIN) during steady-state

operation.

The adaptive ON-time control scheme results in a

constant switching frequency in the MIC28304. 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, 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 1 shows the MIC28304 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.

March 25, 2014

Revision 1.1

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