LM3477

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SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

It is recommended that:

Q(max) = 2, and

Q(min) = 0.15

Values for VSL can be found in the Electrical Characteristics.

Note: Adding slope compensation with RSL will decrease the current limit. An iterative process may be needed to

meet current limit and stability requirements, see PROGRAMMING THE CURRENT LIMIT/HYSTERETIC

THRESHOLD.

Output Capacitor Selection

A capacitance between 47µF - 100µF is typically used. Skip to CALCULATIONS FOR THE OUTPUT

CAPACITOR for minimum capacitance calculations.

TYPE OF OUTPUT CAPACITORS

Different type of capacitors often have different combinations of capacitance, equivalent series resistance (ESR),

and voltage ratings. High-capacitance multi-layer ceramic capacitors (MLCCs) have a very low ESR, typically

12m

Ω, but also relatively low capacitance and low voltage ratings. Tantalum capacitors can have fairly low ESR,

such as 18m

Ω, and high capacitance (up to 1mF) at higher voltage ratings than MLCCs. Aluminum capacitors

offer high capacitance and relatively low ESR and are available in high voltage ratings. OSCON capacitors can

achieve ESR values that are even lower than those of MLCCs and with higher capacitance, but the voltage

ratings are low. Other tradeoffs in capacitor technology include temperature stability, surge current capability, and

capacitance density (physical size vs. capacitance).

OUTPUT CAPACITOR CONSIDERATIONS

Skip to CALCULATIONS FOR THE OUTPUT CAPACITOR if a quick design is desired. While it is generally

desired to use as little output capacitance as possible to keep costs down, the output capacitor should be chosen

with care as it directly affects the ripple component of the output voltage as well as other components in the

design. The output voltage ripple is directly proportional to the ESR of the output capacitor (see POWER

INDUCTOR SECTION). Therefore, designs requiring low output voltage ripple should have an output capacitor

with low ESR. Choosing a capacitor with low ESR has the additional benefit of requiring one less component in

the compensation network, as discussed in Compensation.

In addition to the output voltage ripple, the output capacitor directly affects the output voltage overshoot in a load

transient. Two transients are possible: an unloading transient and a loading transient. An unloading transient

occurs when the load current transitions to a higher current, and charge is unloaded from the output capacitor. A

loading transient is when the load transitions to a lower current, and charge is loaded to the output capacitor.

How the output voltage reacts during these transitions is known as the transient response. Both the capacitance

and the ESR of the output capacitor will affect the transient response.

Figure 28. A Loading Transient

The control loop of the LM3477/A can be made fast enough to saturate the duty cycle when the worst case lode

transient occurs. This means the duty cycle jumps to DMIN or DMAX, depending on the type of load transient. In a

loading transient, as shown in Figure 28, the duty cycle drops to DMIN while the inductor current falls to match the

load current. During this time, the regulator is heavily dependent on the output capacitors to handle the load

transient. The initial overshoot is caused by the ESR of the output capacitors. How the output voltage recovers

after that initial excursion depends on how fast the inductor current falls and how large the output capacitance is.

See Figure 29.

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