MIC2174 datasheet(11/24 Pages) MICREL

11 / 24 page

Micrel, Inc.

MIC2174

September 2009

M9999-090409-B

comparator 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 FB voltage is still

below the VREF. Then, the next ON-time period is

triggered due to the low FB voltage. Therefore, the

switching frequency changes during the load transient.

With the varying duty cycle and switching frequency, the

output recovery time is fast and the output voltage

deviation is small in MIC2174 converter.

Figure 3. MIC2174 Load-Transient Response

Unlike the current-mode control, MIC2174 uses the

output voltage ripple, which is proportional to the

inductor current ripple if the ESR of the output capacitor

is large enough, to trigger an ON-time period. The

predetermined ON time makes MIC2174 control loop

has the advantage as the adaptive on-time mode

control. Therefore, the slope compensation, which is

necessary for the current-mode control, is not required in

the MIC2174.

The MIC2174 has its own stability concern: the FB

voltage ripple should be in phase with the inductor

current ripple and large enough to be sensed by the gm

amplifier and the error comparator. The recommended

minimum FB voltage ripple is 20mV. If a low ESR output

capacitor is selected, the FB voltage ripple may be too

small to be sensed by the gm amplifier and the error

comparator. Also, the output voltage ripple and the FB

voltage ripple are not in phase with the inductor current

ripple if the ESR of the output capacitor is very low.

Therefore, the ripple injection is required for a low ESR

output capacitor. Please refer to “Ripple Injection”

subsection in “Application Information” for more details

about the ripple injection.

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.

MIC2174 implements an internal digital soft-start by

making the 0.8V reference voltage VREF ramp from 0 to

100% in about 6ms with a 9.7mV step. Therefore, the

output voltage is controlled to increase slowly by a stair-

case VREF ramp. Once the soft-start ends, the related

circuitry is disabled to reduce the current consumption.

VIN should be powered up no earlier than VHSD to make

the soft-start function behavior correctly.

Current Limit

The MIC2174 uses the RDS(ON) of the low-side power

MOSFET to sense over-current conditions. The lower-

side MOSFET is used because it displays much lower

parasitic oscillations during switching then the high-side

MOSFET. Using the low-side MOSFET RDS(ON) as a

current sense is an excellent method for circuit

protection. This method will avoid adding cost, board

space and power losses taken by discrete current sense

resistors.

In each switching cycle of the MIC2174 converter, the

inductor current is sensed by monitoring the low-side

MOSFET in the OFF period. The sensed voltage is

compared with a current-limit threshold voltage VCL after

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

VCL, which is 130mV typical at 0.8V feedback voltage,

the MIC2174 turns off the high-side MOSFET and a soft-

start sequence is trigged. This mode of operation is

called the “hiccup mode” and its purpose is to protect the

down stream load in case of a hard short. The current

limit threshold VCL has a fold back characteristics 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 MIC2174 current limiting

circuit.

Figure 4. MIC2174 Current Limiting Circuit

Using the typical VCL value of 130mV, the current limit

value is roughly estimated as:

DS(ON)

130mV

≈

For designs where the current ripple is significant

compared to the load current IOUT, or for low duty cycle

operation, calculating the current limit ICL should take

into account that one is sensing the peak inductor

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