SC417 datasheet(22/30 Pages) SEMTECH

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SC417/SC427

Applications Information (continued)

Using the On-chip LDO to Bias the SC417/SC427

The following steps must be followed when using the on-

chip LDO to bias the device.

Connect V5V to VLDO before enabling the LDO.

The LDO has an initial current limit of 40mA at

start-up, therefore, do not connect any external

load to VLDO during start-up.

When VLDO reaches 90% of its final value, the

LDO current limit increases to 200mA. At this

time the LDO may be used to supply the required

bias current to the device.

Attempting to operate in self-powered mode in any other

configuration can cause unpredictable results and may

damage the device.

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.

= 12V + 10%

OUT

= 1.05V + 4%

•

= 250kHz

Load = 10A 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 250kHz which results

from using components selected for optimum size and

cost .

A resistor (R

TON

) is used to program the on-time (indirectly

setting the frequency) using the following equation.

OUT

TON

)

(

To select R

TON

, use the maximum value for V

, and for T

use the value associated with maximum V

INMAX

OUT

ON = 318 ns at 13.2VIN, 1.05VOUT, 250kHz

Substituting for R

TON

results in the following solution.

TON = 154.9kΩ, use RTON = 154kΩ

Inductor Selection

In order to determine the inductance, the ripple current

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 directly

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 4A then

Power-save operation will typically start for loads less than

2A. If ripple current is set at 40% of maximum load current,

then power-save will start for loads less than 20% of

maximum current.

•

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