UP9303ASAC datasheet(15/18 Pages) UPI

15 / 18 page

uP9303

uP9303-DS-F0101, Aug. 2017

www.upi-semi.com

specified in two methods: permissible DC current and

saturation current. Permissible DC current is the allowable

DC current that causes 40OC temperature raise. The

saturation current is the allowable current that causes 10%

inductance loss. Make sure that the inductor will not

saturate over the operation conditions including

temperature range, input voltage range, and maximum

output current.

The size requirements refer to the area and height

requirement for a particular design. For better efficiency,

choose a low DC resistance inductor. DCR is usually

inversely proportional to size.

Different core materials and shapes will change the size/

current and price/current relationship of an inductor. Toroid

or shielded pot cores in ferrite or permalloy materials are

small and don’t radiate much energy, but generally cost

more than powdered iron core inductors with similar

electrical characteristics. The choice of which style

inductor to use often depends more on the price vs. size

requirements and any radiated field/EMI requirements.

Input Capacitor Selection

The synchronous-rectified buck converter draws pulsed

current with sharp edges from the input capacitor resulting

in ripples and spikes at the input supply voltage. Use a

mix of input bypass capacitors to control the voltage

overshoot across the MOSFETs. Use small ceramic

capacitors for high frequency decoupling and bulk

capacitors to supply the current needed each time upper

MOSFET turns on. Place the small ceramic capacitors

physically close to the MOSFETs and between the drain

of upper MOSET and the source of lower MOSFET to

avoid the stray inductance along the connection trace.

The important parameters for the bulk input capacitor are

the voltage rating and the RMS current rating. For reliable

operation, select the bulk capacitor with voltage and current

ratings above the maximum input voltage and largest RMS

current required by the circuit. The capacitor voltage rating

should be at least 1.25 times greater than the maximum

input voltage and a voltage rating of 1.5 times is a

conservative guideline. The RMS current rating

requirement for the input capacitor of a buck converter is

calculated as:

OUT

)

MAX

(

OUT

)

RMS

(

)

(

−

This formula has a maximum at VIN = 2VOUT, where

IIN(RMS) = IOUT(RMS)/2. This simple worst-case condition

is commonly used for design because even significant

deviations do not offer much relief. Note that the capacitor

manufacturer’s ripple current ratings are often based on

2000 hours of life. This makes it advisable to further derate

the capacitor, or choose a capacitor rated at a higher

temperature than required. Always consult the

manufacturer if there is any question.

For a through-hole design, several electrolytic capacitors

may be needed. For surface mount designs, solid tantalum

capacitors can also be used, but caution must be exercised

with regard to the capacitor surge current rating. These

capacitors must be capable of handling the surge-current

at power-up. Some capacitor series available from

reputable manufacturers are surge current tested.

Output Capacitor Selection

An output capacitor is required to filter the output and

supply the load transient current. The selection of COUT

is primarily determined by the ESR required to minimize

voltage ripple and load step transients. The output ripple

∆

VOUT is approximately bounded by:

)

ESR

(

OUT

OSC

OUT

∆

≤

∆

Since ∆IL increases with input voltage, the output ripple is

highest at maximum input voltage. Typically, once the ESR

requirement is satisfied, the capacitance is adequate for

filtering and has the necessary RMS current rating. Multiple

capacitors placed in parallel may be needed to meet the

ESR and RMS current handling requirements. Dry

tantalum, special polymer, aluminum electrolytic and

ceramic capacitors are all available in surface mount

packages. Special polymer capacitors offer very low ESR

but have lower capacitance density than other types.

The load transient requirements are a function of the slew

rate (di/dt) and the magnitude of the transient load current.

These requirements are generally met with a mix of

capacitors and careful layout. Modern components and

loads are capable of producing transient load rates above

1A/ns. High frequency capacitors initially supply the

transient and slow the current load rate seen by the bulk

capacitors. The bulk filter capacitor values are generally

determined by the ESR (Effective Series Resistance) and

voltage rating requirements rather than actual capacitance

requirements.

High frequency decoupling capacitors should be placed

as close to the power pins of the load as physically

possible. Be careful not to add inductance in the circuit

board wiring that could cancel the usefulness of these

low inductance components. Consult with the

manufacturer of the load on specific decoupling

requirements.

Use only specialized low-ESR capacitors intended for

Application Information

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