SC173AEVB datasheet(14/26 Pages) SEMTECH

14 / 26 page

Design Procedure

When designing a switch mode supply the input voltage

range, load current, switching frequency, and inductor

ripple current must be specified.

The maximum input voltage (V

INMAX) is the highest speci-

fied input voltage. The minimum input voltage ( V

INMIN) is

determined by the lowest input voltage after evaluating

the voltage drops due to connectors, fuses, switches, and

PCB traces.

The following parameters define the design.

Nominal output voltage (V

OUT)

Static or DC output tolerance

Transient response

Maximum load current (I

OUT)

There are two values of load current to evaluate — con-

tinuous load current and peak load current. Continuous

load current relates to thermal stresses which drive the

selection of the inductor and input capacitors. Peak load

current determines instantaneous component stresses and

filtering requirements such as inductor saturation, output

capacitors, and design of the current limit circuit.

The following values are used in this design.

IN = 5V + 10%

OUT = 1.0V + 4%

SW = 800kHz

Load = 3A maximum

Frequency Selection

Selection of the switching frequency requires making a

trade-off between the size and cost of the external filter

components (inductor and output capacitor) and the

power conversion efficiency.

The desired switching frequency is 800kHz which results

from using components selected for optimum size and

cost .

A resistor (R

TON) is used to program the on-time (indirectly

•

Applications Information (continued)

setting the frequency) using the following equation.

Calculating R

TON results in the following solution.

TON=50kW, we use RTON=49.9kW in real application.

Inductor Selection

In order to determine the inductance, the ripple cur-

rent must first be defined. Low inductor values result in

smaller size but create higher ripple current which can

reduce efficiency. Higher inductor values will reduce the

ripple current/voltage and for a given DC resistance are

more efficient. However, larger inductance translates di-

rectly into larger packages and higher cost. Cost, size,

output ripple, and efficiency are all used in the selection

process.

The ripple current will also set the boundary for power

save operation. The switching will typically enter power

save mode when the load current decreases to 1/2 of the

ripple current. For example, if ripple current is 3A then

power save operation will typically start for loads less

than 1.5A. If ripple current is set at 40% of maximum load

current, then power save will start for loads less than 20%

of maximum current.

The inductor value is typically selected to provide a rip-

ple current that is between 25% to 50% of the maximum

load current. This provides an optimal trade-off between

cost, efficiency, and transient performance.

During the DH on-time, voltage across the inductor is

IN - VOUT).

The equation for determining inductance is

shown next.

TON

25pF

⋅

RIPPLE

OUT

)

INMAX

OUT

⋅

SC173A

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