MIC2124YMM datasheet(10/24 Pages) MICREL

10 / 24 page

Micrel, Inc.

MIC2124

June 2010

M9999-060810-D

Figure 3. MIC2124 Load-Transient Response

Unlike in current-mode control, the MIC2124 uses

adaptive ON-time current mode control. The MIC2124

predetermined ON-time control loop has the advantage

of constant ON-time mode control and eliminates the

need for the slope compensation.

Soft-Start

Soft-start reduces the power supply input surge current

at startup by controlling the output voltage rise time. The

input surge appears while the output capacitor is

charged up. A slower output rise time will draw a lower

input surge current.

The MIC2124 implements an internal digital soft-start by

making the 0.8V reference voltage VREF ramp from 0 to

100% in about 4ms. Therefore, the output voltage is

controlled to increase slowly by a stair-case VREF ramp.

Once the soft-start cycle ends, the related circuitry is

disabled to reduce current consumption. VIN must be

powered up no earlier than VHSD to make the soft-start

function behavior correctly.

Current Limit

The MIC2174/MIC2174C uses the RSD(ON) of the low-

side power MOSFET to sense over-current conditions.

This method will avoid adding cost, board space and

power losses taken by a discrete current sense resistor.

The low-side MOSFET is used because it displays much

lower parasitic oscillations during switching than the

high-side MOSFET.

In each switching cycle of the MIC2124 converter, the

inductor current is sensed by monitoring the low-side

MOSFET in the OFF period. The sensed voltage VLX is

compared with a current-limit threshold voltage VCL after

a blanking time of 150ns. If the sensed voltage VLX is

under VCL, which is -127mV typical at 0.8V feedback

voltage, the MIC2124 keeps the low-side MOSFET on

until VLX > -127mV, and then goes into the ON status

with minimum ON-time. The current limit threshold VCL

has a fold back characteristic related to the FB voltage.

Please refer to the “Typical Characteristics” for the curve

of VCL vs. FB voltage. The circuit in Figure 4 illustrates

the MIC2124 current limiting circuit.

Figure 4. MIC2124 Current Limiting Circuit

Using the typical VCL value of -127mV, the current limit

value in the inductor is roughly estimated as:

DS(ON)

127mV

≈

For designs where the inductor current ripple is

significant compared to the load current IOUT, or for low

duty cycle operation, calculating the load current limit

ICL(LOAD) should take into account that one is sensing the

peak inductor current.

ΔI

127mV

L(pp)

DS(ON)

CL(LOAD)

−

(2)

ΔI

OUT

L(pp)

−

(3)

where:

VOUT = The output voltage

ΔIL(pp) = Inductor current ripple peak-to-peak value

D = Duty Cycle

fSW = Switching frequency

The MOSFET RDS(ON) varies 30% to 40% with

temperature; therefore, it is recommended to add 50%

margin to ICL(LOAD) in the above equation to avoid false

current limiting due to increased MOSFET junction

temperature rise. It is also recommended to connect LX

pin directly to the drain of the low-side MOSFET to

accurately sense the MOSFETs RDS(ON)

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