MIC45205 datasheet(13/31 Pages) MICREL

13 / 31 page

Micrel, Inc.

MIC45205

Functional Description

The MIC45205 is an adaptive on-time synchronous buck

regulator module built for high-input voltage to low-output

voltage

conversion

applications.

The

MIC45205

designed to operate over a wide input voltage range,

from 4.5V to 26V, and the output is adjustable with an

external resistor divider. An adaptive on-time control

scheme is employed to obtain a constant switching

frequency in steady state and to simplify the control

compensation. Hiccup mode over-current protection is

implemented by sensing low-side MOSFET’s RDS(ON). The

device features internal soft-start, enable, UVLO, and

thermal shutdown. The module has integrated switching

FETs, inductor, bootstrap diode, resistor, and capacitor.

Theory of Operation

As shown in Figure 1 (in association with Equation 1), the

output voltage is sensed by the MIC45205 feedback pin

(FB) via the voltage divider RFB1 and RFB2 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 amplifier output

falls 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

Estimator” circuitry:

Figure 1. Output Voltage Sense via FB Pin

OUT

)

ESTIMATED

(

Eq. 1

where VOUT is the output voltage, VIN is the power stage

input voltage, and fSW is the switching frequency.

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 falls 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

200ns, the MIC45205 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 200ns

tOFF(MIN):

)

MIN

(

OFF

MAX

200

−

Eq. 2

Where:

tS = 1/fSW. It is not recommended to use MIC45205 with

OFF-time

tOFF(MIN) during steady-state

operation.

The adaptive ON-time control scheme results in a

constant switching frequency in the MIC45205 during

steady state operation. The actual ON-time and resulting

switching frequency will vary with the different rising and

falling times of the MOSFETs. Also, the minimum tON

results in a lower switching frequency in high VIN to VOUT

applications.

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. For

easy analysis, the gain of the gm amplifier is assumed to

be 1. With this assumption, the inverting input of the error

comparator is the same as the feedback voltage.

Figure 2 shows the MIC45205 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 plus injected

voltage 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.

April 15, 2014

Revision 1.0

MIC45205 Datasheet(PDF) 13 Page - Micrel Semiconductor

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