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LM3551, LM3552

SNVS371D – AUGUST 2005 – REVISED MAY 2013

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DIODE SELECTION

The output diode for a boost regulator must be chosen correctly depending on the output voltage and output

current. The output diode must have a reverse voltage rating equal to or greater than the output voltage used.

The average current rating must be greater than the maximum load current expected, and the peak current rating

must be greater than the peak inductor current. Using Schottky diodes with lower forward voltage drop will

decrease power dissipation and increase efficiency.

CAPACITOR SELECTION

Input Capacitor

An input capacitor is required to reduce the input ripple and noise for proper operation of the regulator. The size

used is dependant on the application and board layout. If the regulator will be loaded uniformly, with very little

load changes, and at lower current outputs, the input capacitor size can often be reduced. The size can also be

reduced if the input of the regulator is very close to the source output. The size will generally need to be larger

for applications where the regulator is supplying nearly the maximum rated output or if large load steps are

expected. A minimum value of 10µF should be used under normal operating condtions while a 10-22µF capacitor

may be required for higher power and dynamic loads. Larger values and/or lower ESR may be needed if the

application requires very low ripple on the input source voltage.

Output Capacitor

A minimum output capacitor value of 4.7µF (Sharp LED) and 10µF (Lumiled) is recommended and may be

increased to a larger value. The ESR of the output capacitor is important because it determines the peak to peak

output voltage ripple according to the approximate equation:

(10)

After choosing the output capacitor you can determine a pole-zero pair introduced into the control loop by the

following equations:

(11)

The zero created by the ESR of the output capacitor is generally at a very high frequency if the ESR is small. If

low ESR capacitors are used it can be neglected. The output capacitor pole information is useful in selecting the

proper compensation components and is discussed in the Compensation Components section of the datasheet.

Capacitor Properties

Surface-mount multi-layer ceramic capacitors are recommended for both the input and output capacitors. These

capacitors are small, inexpensive and have very low equivalent series resistance (ESR <20m

Ω typ.). Tantalum

capacitors, OS-CON capacitors, and aluminum electrolytic capacitors are not recommended for use with the

LM3551/2 due to their high ESR, as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R temperature characteristic are preferred for use with

the LM3551/2. These capacitors have tight capacitance tolerance (as good as ±10%) and hold their value over

temperature (X7R: ±15% over -55°C to 125°C; X5R: ±15% over -55°C to 85°C).

Capacitors with Y5V or Z5U temperature characteristic are generally not recommended for use with the

LM3551/2. Capacitors with these temperature characteristics typically have wide capacitance tolerance (+80%, -

20%) and vary significantly over temperature (Y5V: +22%, -82% over -30°C to +85°C range; Z5U: +22%, -56%

over +10°C to +85°C range). Under some conditions, a nominal 1µF Y5V or Z5U capacitor could have a

capacitance of only 0.1µF. Such deviation is likely to cause Y5V and Z5U capacitors to fail to meet the minimum

capacitance requirements of the LM3551/2.

The minimum voltage rating acceptable for the input capacitor is 6.3V (10V recommended) and 16V for the

output capacitor. In applications that have DC operating points near the maximum voltage rating of the ceramic

capacitor, larger capacitor values may be required to compensate for capacitance loss due to capacitor voltage

coefficient. See the capacitor manufacturer's datasheet for DC bias performance.

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