2 Channel 5.5V 1.5A 1.5MHz DC-DC Step down PMU

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where

•

: Inductor saturation current at operating temperature

•

: Peak inductor current during worst case conditions

•

: Maximum average inductor current

•

: Peak-to-Peak inductor current

•

: Output voltage

•

: Input voltage

•

L

•

: Switching frequency, Hertz

•

D

: Estimated duty factor

•

EFF

: Estimated power supply efficiency

may not be exceeded during any operation, including transients, startup, high temperature, worst case conditions,

etc.

Output and Input Capacitors Characteristics

Special attention should be paid when selecting these components. The DC bias of these capacitors can result in a

capacitance value that falls below the minimum value given in the recommended capacitor specifications table.

The ceramic capacitor’s actual capacitance can vary with temperature. The capacitor type X7R, which operates over

a temperature range of −55°C to +125°C, will only vary the capacitance to within ±15%. The capacitor type X5R

has a similar tolerance over a reduced temperature range of −55°C to +85°C. Many large value ceramic capacitors,

larger than 1uF are manufactured with Z5U or Y5V temperature characteristics. Their capacitance can drop by more

than 50% as the temperature varies from 25°C to 85°C. Therefore X5R or X7R is recommended over Z5U and Y5V

in applications where the ambient temperature will change significantly above or below 25°C.

Tantalum capacitors are less desirable than ceramic for use as output capacitors because they are more expensive

when comparing equivalent capacitance and voltage ratings in the 0.47uF to 44uF range. Another important

consideration is that tantalum capacitors have higher ESR values than equivalent size ceramics. This means that

while it may be possible to find a tantalum capacitor with an ESR value within the stable range, it would have to be

larger in capacitance (which means bigger and more costly) than a ceramic capacitor with the same ESR value. It

should also be noted that the ESR of a typical tantalum will increase about 2:1 as the temperature goes from 25°C

down to −40°C, so some guard band must be allowed.

BUCK Output Capacitor Selection

The output capacitor of a switching converter absorbs the AC ripple current from the inductor and provides the

initial response to a load transient. The ripple voltage at the output of the converter is the product of the ripple

current flowing through the output capacitor and the impedance of the capacitor. The impedance of the capacitor

can be dominated by capacitive, resistive, or inductive elements within the capacitor, depending on the frequency

of the ripple current. Ceramic capacitors have very low ESR and remain capacitive up to high frequencies. Their

inductive component can be usually neglected at the frequency ranges the switcher operates.

The output-filter capacitor smoothest out the current flow from the inductor to the load and helps maintain a steady

output voltage during transient load changes. It also reduces output voltage ripple. These capacitors must be selected

with sufficient capacitance and low enough ESR to perform these functions.

Note that the output voltage ripple increases with the inductor current ripple and the Equivalent Series Resistance

of the output capacitor (ESRCOUT).Also note that the actual value of the capacitor’s ESRCOUT is frequency and

temperature dependent, as specified by its manufacturer. The ESR should be calculated at the applicable switching

frequency and ambient temperature.

HM5213