LT1940/LT1940L
10
1940fa
is likely to see high surge currents when the input source
is applied, tantalum capacitors should be surge rated. The
manufacturer may also recommend operation below the
rated voltage of the capacitor. Be sure to place the 1µF
ceramic as close as possible to the V
IN
and GND pins on
the IC for optimal noise immunity.
A final caution is in order regarding the use of ceramic
capacitors at the input. A ceramic input capacitor can
combine with stray inductance to form a resonant tank
circuit. If power is applied quickly (for example by plug-
ging the circuit into a live power source) this tank can ring,
doubling the input voltage and damaging the LT1940. The
solution is to either clamp the input voltage or dampen the
tank circuit by adding a lossy capacitor in parallel with the
ceramic capacitor. For details, see AN88.
Output Capacitor Selection
For 5V and 3.3V outputs with greater than 1A output, a
10µF 6.3V ceramic capacitor (X5R or X7R) at the output
results in very low output voltage ripple and good transient
response. For lower voltages, 10µF is adequate but in-
creasing C
OUT
to 15µF or 22µF will improve transient
performance. Other types and values can be used; the
following discusses tradeoffs in output ripple and tran-
sient performance.
The output capacitor filters the inductor current to gener-
ate an output with low voltage ripple. It also stores energy
in order satisfy transient loads and to stabilize the LT1940’s
control loop. Because the LT1940 operates at a high
frequency, you don’t need much output capacitance. Also,
the current mode control loop doesn’t require the pres-
ence of output capacitor series resistance (ESR). For these
reasons, you are free to use ceramic capacitors to achieve
very low output ripple and small circuit size.
Estimate output ripple with the following equations:
V
RIPPLE
= ∆I
L
/(8f C
OUT
) for ceramic capacitors, and
V
RIPPLE
= ∆I
L
ESR for electrolytic capacitors (tantalum
and aluminum);
where ∆I
L
is the peak-to-peak ripple current in the induc-
tor. The RMS content of this ripple is very low, and the
RMS current rating of the output capacitor is usually not
of concern.
Another constraint on the output capacitor is that it must
have greater energy storage than the inductor; if the stored
energy in the inductor is transferred to the output, you
would like the resulting voltage step to be small compared
to the regulation voltage. For a 5% overshoot, this require-
ment becomes C
OUT
> 10L(I
LIM
/V
OUT
)^2.
Finally, there must be enough capacitance for good tran-
sient performance. The last equation gives a good starting
point. Alternatively, you can start with one of the designs
in this data sheet and experiment to get the desired
performance. This topic is covered more thoroughly in the
section on loop compensation.
The high performance (low ESR), small size and robust-
ness of ceramic capacitors make them the preferred type
for LT1940 applications. However, all ceramic capacitors
are not the same. As mentioned above, many of the higher
value capacitors use poor dielectrics with high tempera-
ture and voltage coefficients. In particular, Y5V and Z5U
types lose a large fraction of their capacitance with applied
voltage and temperature extremes. Because the loop
stability and transient response depend on the value of
C
OUT,
you may not be able to tolerate this loss. Use X7R
and X5R types.
You can also use electrolytic capacitors. The ESRs of most
aluminum electrolytics are too large to deliver low output
ripple. Tantalum and newer, lower ESR organic electro-
lytic capacitors intended for power supply use are suit-
able, and the manufacturers will specify the ESR. The
choice of capacitor value will be based on the ESR required
for low ripple. Because the volume of the capacitor deter-
mines its ESR, both the size and the value will be larger
than a ceramic capacitor that would give you similar ripple
performance. One benefit is that the larger capacitance
may give better transient response for large changes in
load current. Table 2 lists several capacitor vendors.
APPLICATIO S I FOR ATIO
WUU
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