MIC2174 datasheet(11/27 Pages) MICREL

11 / 27 page

Micrel, Inc.

MIC2174/MIC2174C

September 2010

M9999-091310-C

Functional Description

The MIC2174/MIC2174C is an adaptive on-time

synchronous buck controller built for low cost and high

performance. It is designed for a wide input voltage

range from 3V to 40V and for high output power buck

converters. An estimated-ON-time method is applied in

MIC2174/MIC2174C 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/MIC2174C is an adaptive on-time

synchronous buck controller. Further, Figure 1 illustrates

the block diagram for the control loop. The output

voltage

variation

will

sensed

the

MIC2174/MIC2174C 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, which improves the

MIC2174/MIC2174C converter output voltage regulation.

If the FB 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,

where in DH pin is logic high and DL pin is logic low. The

ON-time period length is predetermined by 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 an ON-time period, the MIC2174/MIC2174C goes

into the OFF-time period. This is when the 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, then the ON-time period is

triggered and the OFF-time period ends. If the OFF-time

period determined by the FB voltage is less than the

minimum OFF time TOFF(min), which is about 363ns

typical, then the MIC2174/MIC2174C 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 363ns

TOFF(min):

OFF(min)

363ns

Dmax

−

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

use MIC2174/MIC2174C 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 MIC2174 should be

limited to 5.5V for up to 28V VHSD and 3.6V for VHSD

higher than 28V. If a higher output voltage is required,

use the MIC2176 instead. Please refer to “Setting Output

Voltage” subsection in “Application Information” for more

details.

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, then 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 varies with the different rising and falling times

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 high VHSD

and low VOUT applications, such as 36V to 1.0V. The

minimum TON measured on the MIC2174/MIC2174C

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/MIC2174C control loop timing

during steady-state operation. During 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 ON-

time is predetermined by the estimation. The ending of

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

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