MIC2103 Datasheet, PDF(18/36 Page) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

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MIC2103 Datasheet, PDF (18/36 Pages) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

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Micrel, Inc.

Functional Description

The MIC2103/04 are adaptive on-time synchronous buck

controllers built for high-input voltage to low-output

voltage conversion applications. They are designed to

operate over a wide input voltage range, from 4.5V to

75V, 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

MOSFETâs RDS(ON). The device features internal soft-

start, enable, UVLO, and thermal shutdown.

Theory of Operation

Figure 1 illustrates the block diagram of the MIC2103/04.

The output voltage is sensed by the MIC2103/04

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, thenthe error comparator

will trigger the control logic and generate an ON-time

period. The ON-time period length is predetermined by

the âFixed tON Estimatorâ circuitry:

f t ON(estimated)

ï½

VOUT

VIN ï´ SW

(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 MIC2103/04 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.

MIC2103/04

The maximum duty cycle is obtained from the 200ns

tOFF(min):

DMAX ï½

ï t OFF(MIN)

ï½

1ï

200ns

(Eq. 2)

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

MIC2103/04 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 MIC2103/04. 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 MIC2103/04 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.

August 2012

Figure 2. MIC2103/04 Control Loop Timing

M9999-080712-A

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