SC2453_05 datasheet(17/22 Pages) SEMTECH

17 / 22 page

 2005 Semtech Corp.

www.semtech.com

SC2453

POWER MANAGEMENT

Where, T is switching cycle period.

Because the inductor current ramps up according to Vin

–Vo, and down with Vo, a falling output voltage in over

current condition moves the inductor “ramp up” current

up faster and the peak current I

P larger but the “ramp

down” current slower. As long as the current is still larger

than the current limiting threshold, the top switch is held

off. The switching frequency is folded back lower but

average current is relatively still high.

An example is shown in Typical Characteristics section

with input voltage 12V and output voltage 5V with out-

put current 3.5A.

The preset valley current limit is 5A, which has to be

greater than full load current 3.5. When the output is

shorted, the inductor peak current and the load current,

both are a function of the input voltage and the output

inductance, would be higher than the current limit. There-

fore, proper thermal management and inductor satura-

tion current rating have to be considered at the worst

conditions.

Loop Compensation

SC2453 is a voltage mode buck controller with high gain

bandwidth error amplifier utilizing external network com-

pensation to regulate output voltage.

For a DC/DC converter, it is usually required that the

converter has a loop gain of a high crossover frequency

for fast load response, high DC and low frequency gain

for low steady state error, and enough phase margin for

its operating stability. Often one can not have all these

properties at the same time. The purpose of the loop

compensation is to arrange the poles and zeros of the

compensation network to meet the requirements for a

specific application.

The power stage of a buck converter control-to-output

transfer function is as shown below:

)

(

where,

O =

L - Output inductance

C - Output capacitance

C - Output capacitor ESR

IN - Input voltage

The transfer function of the error amplifier with external

compensation network is as follows:

)

)(

(

)

)(

(

)

(

COMP

⋅

where,

)

(

)

(

Vref

Figure 7. Voltage Mode Buck Converter Compensation

Network

With the compensation, the converter total loop gain is

as follows:

)

(

)

(

)

(

COMP

PWM

⋅

Where:

PWM = PWM gain

M = 3.0V, ramp peak to valley voltage of SC2453

The design guidelines are as following:

1. Set the loop gain crossover frequency

C less than1/

5th the switching frequency.

2. Place an integrator in the origin to increase DC and

low frequency gains.

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