MIC26901 datasheet(17/39 Pages) MICROCHIP

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DS20007047B-page 17

MIC26901

4.0

FUNCTIONAL DESCRIPTION

The MIC26901 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 9A

of output current. An adaptive ON-time control scheme

is employed in to obtain a constant switching frequency

and to simplify the control compensation. Overcurrent

protection is implemented without the use of an

external sense resistor. The device includes an internal

soft-start function that reduces the power supply input

surge current at start-up by controlling the output

voltage rise time.

4.1

Theory of Operation

The MIC26901 operates in a continuous mode as

shown in the Functional Block Diagram.

4.2

Continuous Mode

In continuous mode, the output voltage is sensed by

the MIC26901 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 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:

EQUATION 4-1:

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 300 ns, the MIC26901 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 300 ns

tOFF(MIN):

EQUATION 4-2:

It is not recommended to use MIC26901 with an

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 MIC26901 should be limited to 5.5V and the

maximum external ripple injection should be limited to

200 mV. Please refer to the Setting Output Voltage

section 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 MIC26901 evaluation board is

about 100 ns. 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 4-1 shows the MIC26901 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.

FIGURE 4-1:

Control Loop Timing.

tON ESTIMATED





VOUT

VIN 600kHz



-----------------------------------

Where:

VOUT = The desired output voltage.

VIN = The power stage input voltage.

DMAX

tS tOFF MIN





–

-----------------------------------

1 300ns

----------------

–

Where:

tS = 1/600 kHz = 1.66 µs

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