MIC2124YMM datasheet(9/24 Pages) MICREL

9 / 24 page

Micrel, Inc.

MIC2124

June 2010

M9999-060810-D

Functional Description

The MIC2124 is an adaptive on-time current mode

synchronous buck controller built for low cost and high

performance. It is designed for wide input voltage range

from 3V to 18V and for high output power buck

converters. An estimated-ON-time method is applied in

MIC2124 to obtain a constant switching frequency and to

simplify the control compensation. The over-current

protection is implemented without the use of an external

sense resistor. It includes an internal soft-start function

which reduces the power supply input surge current at

start-up by controlling the output voltage rise time.

Theory of Operation

The MIC2124 is an adaptive on-time current mode buck

controller. Figure 1 illustrates the block diagram for the

control loop. The output voltage variation due to load or

line changes will be sensed by the inverting input of the

transconductance error amplifier via the feedback

resistors (RFB1 and RFB2 in “Typical Application”), and

compared to a reference voltage at the non-inverting

input. This will cause a small change in the DC voltage

level at the output of the error amplifier, or VCOMP.

Meanwhile, the inductor current is sensed through the

bottom MOSFET RDS(ON) and “Bottom Current Sense

Circuit” as VIL. If VIL is lower than VCOMP, an ON-time

period is triggered, in which DH pin is logic high and DL

pin is logic low. The ON-time period length is

predetermined by the “Fixed Ton Estimator” circuitry:

300kHz

HSD

OUT

ed)

ON(estimat

⋅

(1)

where VOUT is the output voltage, VHSD is the power

stage input voltage.

After an ON-time period, the MIC2124 goes into the

OFF-time period, in which DH pin is logic low and DL pin

is logic high. The inductor current and VIL decrease

during OFF time. If VIL is above VCOMP, the OFF status is

maintained. When VIL is below VCOMP, the ON-time

period is triggered and the OFF-time period ends. If the

OFF-time period determined by the inductor current and

VCOMP is less than the minimum OFF time TOFF(min), which

is 350ns typical, the MIC2124 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 350ns TOFF(min):

OFF(min)

MAX

350ns

−

where TS = 1/300kHz = 3.33μs. It is not recommended to

use MIC2124 with a OFF-time close to TOFF(min) during

steady state operation.

The estimated ON-time method results in a constant

300kHz switching frequency. The actual ON-time varies

a little with the different rising and falling times of the

external MOSFETs. Therefore, the type of the external

MOSFETs, the output load current, and the control

circuitry power supply VIN will slightly modify the actual

ON-time and the switching frequency. Also, the minimum

TON, which is 140ns typical, results in a lower switching

frequency in high VHSD and low VOUT applications, such

as 18V to 0.8V. During the load transient, the switching

frequency is changed due to the varying OFF-time.

To illustrate the control loop, the steady-state scenario

and the load transient scenario are analyzed. VCOMP is

defined as the output of the error amplifier. Figure 2

shows the MIC2124 control loop timing during the

steady-state

operation

continuous

mode.

VIL

represents the inductor current sensing voltage via the

bottom MOSFET RDS(ON) and “Bottom Current Sense

Circuit”. When VIL is below VCOMP, which means that the

inductor current reaches the valley value, the OFF-time

ends and ON-time is triggered. The ON-time is

predetermined by the estimation.

Figure 2. MIC2124 Control Loop Timing

Figure 3 shows the load transient operation of the

MIC2124 converter. Assume the output voltage drops

due to sudden load increase, which would cause the

inverting input of the error amplifier, which is divided

down version of VOUT, to be slightly less than the

reference voltage, causing the output voltage of the error

amplifier VCOMP to go high. This will cause “CONTROL

LOGIC” to trigger ON-time period. At the end of the ON-

time period, a minimum OFF-time TOFF(min) is generated

to charge BST since the inductor current VIL is still below

VCOMP. Then, the next ON-time period is triggered due to

the high VCOMP. Therefore, the switching frequency

changes during the load transient. Also the load

regulation and transient load recovery is done by

modulating the OFF-time. With the varying duty cycle

and switching frequency, the output recovery time is fast

and the output voltage deviation is small in MIC2124

converter.

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