LM2641MTC-ADJ Datasheet, PDF (22/31 Pages) Texas Instruments – LM2641 Dual Adjustable Step-Down Switching Power Supply Controller

LM2641
SNVS040B – JANUARY 2000 – REVISED APRIL 2013
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Capacitors are specified with an RMS current rating. To determine the requirement for an application, the
following formula can be used:
(6)
It is also recommended that a 0.1µF ceramic capacitor be placed from V_IN to ground for high frequency
bypassing, located as close as possible to the V_IN pin.
OUTPUT CAPACITORS
The output capacitor(s) are critical in loop stability (covered in INPUT CAPACITORS section) and also output
voltage ripple.
The types best suited for use as output capacitors are aluminum electrolytics and solid Tantalum.
Aluminum Electrolytics
The primary advantage of aluminum electrolytics is that they typically give the maximum capacitance-to-size
ratio, and they are reasonably priced. However, it must be noted that aluminum electrolytics used in high-
performance switching regulator designs must be high frequency, low ESR types such as Sanyo OSCON or
Panasonic HFQ which are specifically designed for switching applications. Capacitors such as these with good
high frequency (≥ 100kHz) specifications are not cheap.
Aluminum electrolytic capacitors should generally not be used in switching regulator applications where the
ambient temperature goes below 0°C. A typical low-voltage aluminum electrolytic has an ESR vs. Temperature
curve that is fairly flat from 25°C to 125°C. However, a temperature change from 25°C to 0°C will approximately
double the ESR, and it will double again going from 0°C down to −20°C.
Tantalum
Solid Tantalum capacitors are best in applications which must operate over a wide temperature range. A good
quality Tantalum will typically exhibit less than 2:1 change in ESR over the temperature range of +125°C to
−40°C. Recommended types are Sprague 593D, Sprague 594D, and AVX TPS series.
Selecting An Output Capacitor
The required value of output capacitance is directly related to the specification for the maximum amount of output
voltage ripple allowed in the application. Since ESR effects the ripple voltage, it is important to have a guideline
for ESR. The maximum allowed ESR can be calculated as follows.
V_RIPPLE = I_RIPPLE *ESR(max)
Using V = Ldi/dt
V_OUT = L *I_RIPPLE/{(1−D)T_S} = L *I_RIPPLE *F_S/(1−D)
I_RIPPLE = V_OUT*(1−D)/)L *F_S)
ESR(max) = V_RIPPLE/I_RIPPLE
A reasonable value for C_OUT can be obtained by choosing capacitors with net ESR less than ½ of ESR(max).
Hence,
ESR(max) = V_RIPPLE*L* F_S/ {V_OUT(1−D)}
(7)
The value of C_OUT necessary to meet the voltage ripple specification can be found using the approximation:
(8)
Where:
I_RIPPLE is the inductor ripple current.
V_RIPPLE is the output ripple voltage.
ESR is the equivalent series resistance of the output capacitor.
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