MIC2128 datasheet(16/32 Pages) MICROCHIP

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MIC2128

DS20005620A-page 16

 2016 Microchip Technology Inc.

4.0

FUNCTIONAL DESCRIPTION

The MIC2128 is an adaptive on-time synchronous buck

controller designed to cover a wide range of input

voltage applications ranging from 4.5V to 75V. An

adaptive on-time control scheme is employed to get

fast transient response and to obtain high voltage

conversion ratios at constant switching frequency.

Overcurrent protection is implemented by sensing

low-side MOSFET's RDS(ON) which eliminates lossy

current sense resistor. The device features external

soft-start, enable input, UVLO, power good output

(PG), secondary bootstrap LDO and thermal shutdown.

4.1

Theory of Operation

The MIC2128 is an adaptive on-time synchronous buck

controller which operates based on ripple at feedback

node. The output voltage is sensed by the MIC2128

feedback pin (FB) and is compared to a 0.6V reference

voltage (VREF) at the low-gain transconductance error

amplifier (gm) as shown in the Functional Block

Diagram. Figure 4-1 shows the MIC2128 control loop

timing during steady-state operation.

The error amplifier behaves as short circuit for the

ripple voltage frequency on the FB pin which causes

the error amplifier output voltage ripple to follow the

feedback voltage ripple. When the transconductance

error amplifier output (VgM) is below the reference

voltage of the comparator, which is same as the error

amplifier reference (VREF), the comparator triggers and

generates an on-time event. The on-time period is

predetermined by the fixed tON estimator circuitry

which is given by the following Equation 4-1:

EQUATION 4-1:

At the end of the ON time, 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 of the

high-side MOSFET depends on the feedback voltage.

When the feedback voltage decreases, the output of

the gm amplifier (VgM) also decreases. When the output

of the gm amplifier (VgM) is below the reference voltage

of the comparator (which is same as the error amplifier

reference (VREF)) the OFF time ends and ON time is

triggered. If the OFF time determined by the feedback

voltage is less than the minimum OFF time

(tOFF(MIN))of the MIC2128, which is about 230 ns

(typical), the MIC2128 control logic applies the

tOFF(MIN) instead.

The maximum duty cycle can be calculated using the

following Equation 4-2:

EQUATION 4-2:

It is not recommended to use the MIC2128 with an OFF

time close to tOFF(MIN) during steady-state operation.

The adaptive on-time control scheme results in a

constant switching frequency over wide range of input

voltage and load current. The actual ON time and

resulting switching frequency varies with the different

rising and falling times of the external MOSFETs. The

minimum controllable ON time (tON(MIN)) results in a

lower switching frequency than the target switching

frequency in high VIN to VOUT ratio applications.

The equation below shows the output-to-input voltage

ratio, below which the MIC2128 lowers the switching

frequency in order to regulate the output-to-set value.

EQUATION 4-3:

FIGURE 4-1:

MIC2128 Control Loop

Timing.

ON ESTIMA TED



OUT

VIN



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

Where:

VOUT

= Output voltage

VVIN

= Power stage input voltage

fSW

= Switching frequency

MAX

OFF MIN



–

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

230ns

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

–

Where:

tSW

= Switching period, equal to 1/fSW

OUT

-------------t

ON MIN





Where:

VOUT

= Output voltage

VIN

= Input voltage

fSW

= Switching frequency

tON(MIN) = Minimum controllable ON time (80 ns typ.)

VOUT

VFB

VgM

VDH

VFB

VREF

VOUT = ESR* IL

VFB = VOUT *(VREF/VOUT)

MIC2127 Triggers ON-Time event if the error

amplifier output (VgM) is below VREF

Estimated ON-Time

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