MIC26601 datasheet(15/30 Pages) MICREL

15 / 30 page

Micrel, Inc.

MIC26601

July 2011

M9999-071311-A

Functional Description

The MIC26601 is an adaptive ON-time synchronous

step-down DC-DC regulator with an internal 5V linear

regulator and a Power Good (PG) output. It is designed

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

28V and provides a regulated output voltage at up to 7A

of output current. An adaptive ON-time control scheme is

employed in to obtain a constant switching frequency

and to simplify the control compensation. Over-current

protection is implemented without the use of an external

sense resistor. The device 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 MIC26601 operates in a continuous mode as shown

in Figure 1.

Continuous Mode

In continuous mode, the output voltage is sensed by the

MIC26601 feedback pin FB via the voltage divider R1

and R2, and compared to a 0.8V reference voltage VREF

the

error

comparator

through

low

gain

transconductance (gm) amplifier. If the feedback voltage

decreases and the output of the gm amplifier 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

ESTIMATION” circuitry:

kHz

600

OUT

)

ESTIMATED

(

Eq. 1

where VOUT is the output voltage and VIN is the power

stage input voltage.

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

300ns, the MIC26601 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 300ns

tOFF(min):

)

MIN

(

OFF

MAX

300

−

Eq. 2

where tS = 1/600kHz = 1.66μs.

It is not recommended to use MIC26601 with a OFF-time

close to tOFF(min) during steady-state operation. Also, as

VOUT increases, the internal ripple injection will increase

and reduce the line regulation performance. Therefore,

the maximum output voltage of the MIC26601 should be

limited to 5.5V and the maximum external ripple injection

should be limited to 200mV. Please refer to “Setting

Output Voltage” subsection in Application Information for

more details.

The actual ON-time and resulting switching frequency

will vary with the part-to-part variation in the rise and fall

times of the internal MOSFETs, the output load current,

and variations in the VDD voltage. Also, the minimum tON

results in a lower switching frequency in high VIN to VOUT

applications, such as 24V to 1.0V. The minimum tON

measured on the MIC26601 evaluation board is about

100ns. 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.

Figure 2 shows the MIC26601 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 and the inductor

current 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.

MIC26601 Datasheet(PDF) 15 Page - Micrel Semiconductor

MIC26601 Datasheet(HTML) 15 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL