RT5758 datasheet(15/26 Pages) RICHTEK

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RT5758

DS5758-00 April 2018

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

For the 0.47

μH inductance value, the inductor saturation

current and thermal rating should exceed 9.851A.

Input Capacitor Selection

Input capacitors are needed to smooth out the RMS ripple

current (IRMS) imposed by the switching currents and

drawn from the input power source, by reducing the ripple

voltage amplitude seen at the input of the converters. The

voltage rating of the input filter capacitors must be greater

than the maximum input voltage. It's also important to

consider the ripple current capabilities of capacitors.

The RMS ripple current (IRMS) of the regulator can be

determined by the input voltage (VIN), output voltage

(VOUT), and rated output current (IOUT) as the following

equation :

OUT

RMS

OUT

= I





From the above, the maximum RMS input ripple current

occurs at maximum output load, which will be used as

the requirements to consider the current capabilities of

the input capacitors. Furthermore, for a single phase buck

converter, the duty cycle is approximately the ratio of

output voltage to input voltage. The maximum ripple voltage

usually occurs at 50% duty cycle, that is, VIN = 2 x VOUT.

The maximum IRMS

, as IRMS (Max), can be approximated

as 0.5 x IOUT_MAX, where IOUT_MAX is the maximum rated

output current. Besides, the variation of the capacitance

value with temperature, DC bias voltage, switching

frequency, and allowable peal-to-peak ripple voltage that

reflects back to the input, also need to be taken into

consideration. For example, the capacitance value of a

capacitor decreases as the DC bias across the capacitor

increases; also, higher switching frequency allows the use

of input capacitors of smaller capacitance values.

Ceramic capacitors are most commonly used to be placed

right at the input of the converter to reduce ripple voltage

amplitude because only ceramic capacitors have

extremely low ESR which is required to reduce the ripple

voltage. Note that the capacitors need to be placed as

close as to the input pins as possible for highest

effectiveness. Ceramic capacitors are preferred also due

to their low cost, small size, high RMS current ratings,

robust inrush surge current capabilities, and low parasitic

inductance, which helps reduce the high-frequency ringing

on the input supply.

However, care must be taken when ceramic capacitors

are used at the input, and the input power is supplied by

a wall adapter, connected through a long and thin wire.

When a load step occurs at the output, a sudden inrush

current will surge through the long inductive wire, which

can induce ringing at the device's power input and

potentially cause a very large voltage spike at the VIN pin

to damage the device. For applications where the input

power is located far from the device input, it may be required

that the low-ESR ceramic input capacitors be placed in

parallel with a bulk capacitor of other types, such as

tantalum, electrolytic, or polymer, to dampen the voltage

ringing and overshoot at the input, caused by the long

input power path and input ceramic capacitor.

It is suggested to choose capacitors with higher

temperature ratings than required. Several ceramic

capacitors may be parallel to meet application

requirements, such as the RMS current, size, and height.

The Typical Application Circuit can use one 10

μF and one

high-frequency- noise-filtering 0.1

μF low-ESR ceramic

capacitors at the input.

Output Capacitor Selection

Output capacitance affects the output voltage of the

converter, the response time of the output feedback loop,

and the requirements for output voltage sag and soar. The

sag occurs after a sudden load step current applied, and

the soar occurs after a sudden load removal. Increasing

the output capacitance reduces the output voltage ripple

and output sag and soar, while it increases the response

time that the output voltage feedback loop takes to respond

to step loads, Therefore, there is a tradeoff between output

capacitance and output response. It is recommended to

choose a minimum output capacitance to meet the output

voltage requirements of the converter, and have a quick

transient response to step loads.

The ESR of the output capacitor affects the damping of

the output filter and the transient response. In general,

low-ESR capacitors are good choices due to their

excellent capability in energy storage and transient

performance. The RT5758, therefore, is specially optimized

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