MIC2174 datasheet(10/24 Pages) MICREL

10 / 24 page

Micrel, Inc.

MIC2174

September 2009

M9999-090409-B

Functional Description

The MIC2174 is an adaptive on-time synchronous buck

controller built for low cost and high performance. It is

designed for wide input voltage range from 3V to 40V

and for high output power buck converters. An

estimated-ON-time method is applied in MIC2174 to

obtain a constant switching frequency and to simplify the

control compensation. The over-current protection is

implemented without the use of an external sense

resistor. It 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 MIC2174 is an adaptive on-time buck controller.

Figure 1 illustrates the block diagram for the control loop.

The output voltage variation will be sensed by the

MIC2174 feedback pin FB via the voltage divider R1 and

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

the

error

comparator

through

low

gain

transconductance (gm) amplifier, which improves the

MIC2174 converter output voltage regulation. If the FB

voltage decreases and the output of the gm amplifier is

below 0.8V, the error comparator will trigger the control

logic and generate an ON-time period, in which DH pin is

logic high and DL pin is logic low. The ON-time period

length

predetermined

the

“FIXED

TON

ESTIMATION” circuitry:

300kHz

HSD

OUT

ed)

ON(estimat

(1)

where VOUT is the output voltage, VHSD is the power

stage input voltage.

After ON-time period, the MIC2174 goes into the OFF-

time period. In which DH pin is logic low and DL pin is

logic high. The OFF-time period length depends upon

the FB voltage in most cases. When the FB voltage

decreases and the output of the gm amplifier is below

0.8V, the ON-time period is trigger and the OFF-time

period ends. If the OFF-time period decided by the FB

voltage is less than the minimum OFF time TOFF(min),

which is about 363ns typical, the MIC2174 control logic

will apply the TOFF(min) instead. TOFF(min) is required by the

BST charging. The maximum duty cycle is obtained from

the 363ns TOFF(min):

OFF(min)

363ns

Dmax

−

where Ts = 1/300kHz = 3.33μs. It is not recommended to

use MIC2174 with a OFF time close to TOFF(min) at the

steady state.

The power stage input voltage VHSD is fed into the Fixed

Ton Estimation block through a 6:1 divider and 5V

voltage clamper. Therefore, if the VHSD is higher than

30V, the Fixed TON Estimation block uses 30V to

estimate Ton instead of the real VHSD. As a result, the

switching frequency will be less than 300kHz:

300kHz

30V

HSD

30V)

SW(VHDS

(2)

The estimated-ON-time method results in a constant

300kHz switching frequency up to 30V VHSD. The actual

ON time is varied with the different rising and falling time

of the external MOSFETs. Therefore, the type of the

external MOSFETs, the output load current, and the

control circuitry power supply VIN will modify the actual

ON time and the switching frequency. Also, the minimum

Ton results in a lower switching frequency in the high

VHSD and low VOUT applications, such as 36V to 1.0V

application. The minimum Ton measured on the

MIC2174 evaluation board with Si7148DP MOSFETs is

about 184ns. During the load transient, the switching

frequency is changed due to the varying OFF time.

To illustrate the control loop, the steady-state scenario

and the load transient scenario are analyzed. 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 FB voltage. Figure 2

shows the MIC2174 control loop timing during the

steady-state. During the steady-state, the gm amplifier

senses the FB voltage ripple, which is proportional to the

output voltage ripple and the inductor current ripple, to

trigger

the

ON-time

period.

The

time

predetermined by the estimation. The ending of OFF

time is controlled by the FB voltage. At the valley of the

FB voltage ripple, which is below than VREF, OFF period

ends and the next ON-time period is triggered through

the control logic circuitry.

Figure 2. MIC2174 Control Loop Timing

Figure 3 shows the load transient scenario of the

MIC2174 converter. The output voltage drops due to the

sudden load increasing, which would cause the FB

voltage to be less than VREF. This will cause the error

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