MIC2102YML datasheet(20/37 Pages) MICREL

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Micrel, Inc.

MIC2101/02

August 2012

M9999-080712-A

Functional Description

The MIC2101/02 are adaptive on-time synchronous buck

controllers built for high-input voltage to low output

voltage applications. It is designed to operate over a

wide input voltage range from, 4.5V to 38V and the

output is adjustable with an external resistive divider. An

adaptive on-time control scheme is employed to obtain a

constant switching frequency and to simplify the control

compensation. Over-current protection is implemented

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

features internal soft-start, enable, UVLO, and thermal

shutdown.

Theory of Operation

Figure 1 illustrates the block diagram of the MIC2101/02.

The output voltage is sensed by the MIC2101/02

feedback pin FB via the voltage divider R1 and R2, 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

ED)

ON(ESTIMAT





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 MIC2101/02 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):

OFF(MIN)

MAX

200ns









Eq. 2

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

MIC2101/02 with a OFF-time close to tOFF(min) during

steady-state operation.

The adaptive ON-time control scheme results in a

constant switching frequency in the MIC2101/02. 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 2 shows the MIC2101/02 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 2. MIC2101/02 Control Loop Timing

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