Datasheet

LM3477
www.ti.com SNVS141K –OCTOBER 2000–REVISED MARCH 2013
It is recommended that:
Q(max) = 2, and
Q(min) = 0.15
Values for V
SL
can be found in the Electrical Characteristics.
Note: Adding slope compensation with R
SL
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
12mΩ, but also relatively low capacitance and low voltage ratings. Tantalum capacitors can have fairly low ESR,
such as 18mΩ, 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 D
MIN
or D
MAX
, depending on the type of load transient. In a
loading transient, as shown in Figure 28, the duty cycle drops to D
MIN
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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