SC1480ITSTRT datasheet(10/22 Pages) SEMTECH

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SC1480

POWER MANAGEMENT

Applications Information (Cont.)

The selection of the ripple current is based on the maxi-

mum output current and tends to be between 20% to

50% of the maximum load current. Again, cost, size and

efficiency all play a part in the selection process.

Design Example

The following design example is for the evaluation board

schematic shown on page 16. While most DDR supplies

have a maximum load of 3A, the following design was

meant for applications beyond DDR memory. Therefore,

this design will have an input voltage from 5V to 19V,

with an output voltage of 1.25V at 5A of load current.

Inductor Selection

The switching frequency is set to 300kHz which yields a

good trade-off of size and efficiency. RTON is chosen to

be 1M Ohm for a switching frequency of 300kHz. Be-

cause ripple voltage is used as the feedback mechanism

of this device, this leads to the choice of the ripple cur-

rent being 10% of load current. This will give a nice ripple

voltage waveform for ensuring proper PWM triggering for

this type of controller.

0.5A

0.1

∆IL

•

OUT

∆I

)

•

−

•

Setting the Current Limit

The minimum current-limit threshold must be high enough

to support the maximum load current. The valley of the

inductor current occurs at:

ILoad(Max) ∆

−

, (see Figure 2) therefore:

ILoad(Max)

ILIM(Low)

∆

The inductor must not saturate under all conditions of

operation. If the current limit is set to 6.5A the maximum

current through the inductor will be:

∆

Setting the over current to 6.5A is calculated as follows:

ILOC

RDS

RILIM

•

RDSON of the MOSFET is a nominal 0.013 Ohms, ac-

counting for increased temperature effects use 0.015

Ohm.

015

RILIM

•

The inductor chosen was a Panasonic 4µH, 11A induc-

tor.

Similarly, using a Sense resistor to obtain a more accu-

rate current limit would make use of the valley current

equation. Thus, for a 0.015 Ohm resistor the RILIM would

calculate to the same 10k Ohm ILIM resistor will be

015

•

effecting the efficiency budget.

Output Capacitor Selection

The output filter capacitor must have low effective se-

ries resistance (ESR) to meet the output ripple and load

transient requirements, at the same time have high

enough ESR to satisfy stability requirements. In addition,

the value of output capacitance must be high enough to

absorb the inductor energy going from full-load to no-

load without tripping the overvoltage protection circuit.

For CPU load transients, how much ESR is needed de-

pends upon output voltage variation limits under a CPU

load transient. The ESR for this condition is given:

LOAD(MAX)

OUT

ESR

∆

In non CPU applications, the output capacitor size de-

pends on how much ESR is needed to maintain an ac-

ceptable level of output voltage ripple. Under these con-

ditions the ESR value is given:

LOAD(MAX)

OUT(p

ESR ∆

However, for most CPU applications the minimum capaci-

tance required is limited by the energy absorption of the

output capacitor. The equation for determining the mini-

mum capacitance can be found by the following equa-

tion:

OUT

MIN

−

•

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