LT1372/LT1377
APPLICATIO S I FOR ATIO
Table 1. Surface Mount Solid Tantalum Capacitor
ESR and Ripple Current
E CASE SIZE
AVX TPS, Sprague 593D
AVX TAJ
D CASE SIZE
AVX TPS, Sprague 593D
AVX TAJ
C CASE SIZE
AVX TPS
AVX TAJ
B CASE SIZE
AVX TAJ
2.5 to 10
0.2 (Typ)
1.8 to 3.0
0.1 to 0.3
0.9 to 2.0
ESR (MAX
Ω)
0.1 to 0.3
0.7 to 0.9
RIPPLE CURRENT (A)
0.7 to 1.1
0.4
0.7 to 1.1
0.36 to 0.24
0.5 (Typ)
0.22 to 0.17
0.16 to 0.08
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.
Single inductor boost regulators have large RMS ripple
current in the output capacitor, which must be rated to
handle the current. The formula to calculate this is:
Output Capacitor Ripple Current (RMS)
DC
I
RIPPLE
(RMS) = I
OUT
1 – DC
= I
OUT
V
OUT
– V
IN
V
IN
Input Capacitors
The input capacitor of a boost converter is less critical due
to the fact that the input current waveform is triangular and
does not contain large squarewave currents as is found in
the output capacitor. Capacitors in the range of 10µF to
100µF with an ESR of 0.3Ω or less work well up to full 1.5A
switch current. Higher ESR capacitors may be acceptable
at low switch currents. Input capacitor ripple current for
boost converter is :
U
I
RIPPLE
=
0.3(V
IN
)(V
OUT
– V
IN
)
(f)(L)(V
OUT
)
f = 500kHz Switching frequency (LT1372) or,
1MHz Switching frequency (LT1377)
W
U U
The input capacitor can see a very high surge current when
a battery or high capacitance source is connected “live”
and solid tantalum capacitors can fail under this condition.
Several manufacturers have developed a line of solid
tantalum capacitors specially tested for surge capability
(AVX TPS series, for instance), but even these units may
fail if the input voltage approaches the maximum voltage
rating of the capacitor. AVX recommends derating capaci-
tor voltage by 2:1 for high surge applications. Ceramic and
aluminum electrolytic capacitors may also be used and
have a high tolerance to turn-on surges.
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems. Solid tantalum capacitor ESR
generates a loop “zero” at 5kHz to 50kHz that is instrumen-
tal in giving acceptable loop phase margin. Ceramic ca-
pacitors remain capacitive to beyond 300kHz and usually
resonate with their ESL before ESR becomes effective.
They are appropriate for input bypassing because of their
high ripple current ratings and tolerance of turn-on surges.
Linear Technology plans to issue a Design Note on the use
of ceramic capacitors in the near future.
Output Diode
The suggested output diode (D1) is a 1N5818 Schottky or
its Motorola equivalent, MBR130. It is rated at 1A average
forward current and 30V reverse voltage. Typical forward
voltage is 0.42V at 1A. The diode conducts current only
during switch off time. Peak reverse voltage for boost
converters is equal to regulator output voltage. Average
forward current in normal operation is equal to output
current.
9
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