LT1977
11
1977fa
APPLICATIO S I FOR ATIO
WUUU
INPUT CAPACITOR
Step-down regulators draw current from the input supply
in pulses. The rise and fall times of these pulses are very
fast. The input capacitor is required to reduce the voltage
ripple this causes at the input of LT1977 and force the
switching current into a tight local loop, thereby minimiz-
ing EMI. The RMS ripple current can be calculated from:
I
I
V
VVV
RIPPLE RMS
OUT
IN
OUT IN OUT()
–=
()
Ceramic capacitors are ideal for input bypassing. At 500kHz
switching frequency input capacitor values in the range of
4.7µF to 20µF are suitable for most applications. If opera-
tion is required close to the minimum input required by the
LT1977 a larger value may be required. This is to prevent
excessive ripple causing dips below the minimum operat-
ing voltage resulting in erratic operation.
Input voltage transients caused by input voltage steps or
by hot plugging the LT1977 to a pre-powered source such
as a wall adapter can exceed maximum V
IN
ratings. The
sudden application of input voltage will cause a large
surge of current in the input leads that will store energy in
the parasitic inductance of the leads. This energy will
cause the input voltage to swing above the DC level of input
power source and it may exceed the maximum voltage
rating of the input capacitor and LT1977. All input voltage
transient sequences should be observed at the V
IN
pin of
the LT1977 to ensure that absolute maximum voltage
ratings are not violated.
The easiest way to suppress input voltage transients is to
add a small aluminum electrolytic capacitor in parallel with
the low ESR input capacitor. The selected capacitor needs
to have the right amount of ESR to critically damp the
resonant circuit formed by the input lead inductance and
the input capacitor. The typical values of ESR will fall in the
range of 0.5Ω to 2Ω and capacitance will fall in the range
of 5µF to 50µF.
If tantalum capacitors are used, values in the 22µF to
470µF range are generally needed to minimize ESR and
meet ripple current and surge ratings. Care should be
taken to ensure the ripple and surge ratings are not
exceeded. The AVX TPS and Kemet T495 series are surge
rated. AVX recommends derating capacitor operating
voltage by 2:1 for high surge applications.
OUTPUT CAPACITOR
The output capacitor is normally chosen by its effective
series resistance (ESR) because this is what determines
output ripple voltage. To get low ESR takes volume, so
physically smaller capacitors have higher ESR. The ESR
range for typical LT1977 applications is 0.05Ω to 0.2Ω. A
typical output capacitor is an AVX type TPS, 100µF at 10V,
with a guaranteed ESR less than 0.1Ω. This is a “D” size
surface mount solid tantalum capacitor. TPS capacitors
are specially constructed and tested for low ESR, so they
give the lowest ESR for a given volume. The value in
microfarads is not particularly critical and values from
22µF to greater than 500µF work well, but you cannot
cheat Mother Nature on ESR. If you find a tiny 22µF solid
tantalum capacitor, it will have high ESR and output ripple
voltage could be unacceptable. Table 3 shows some
typical solid tantalum surface mount capacitors.
Table 3. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
E CASE SIZE ESR MAX (Ω) RIPPLE CURRENT (A)
AVX TPS 0.1 to 0.3 0.7 to 1.1
D CASE SIZE
AVX TPS 0.1 to 0.3 0.7 to 1.1
C CASE SIZE
AVX TPS 0.2 0.5
Many engineers have heard that solid tantalum capacitors
are prone to failure if they undergo high surge currents.
This is historically true and type TPS capacitors are
specially tested for surge capability but surge ruggedness
is not a critical issue with the output capacitor. Solid
tantalum capacitors fail during very high turn-on surges
which do not occur at the output of regulators. High
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
Unlike the input capacitor RMS, ripple current in the
output capacitor is normally low enough that ripple cur-
rent rating is not an issue. The current waveform is
