LT1373
W U U
APPLICATIO S I FOR ATIO
U
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
speciallytestedforsurgecapability,butsurgeruggedness
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.
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
becomingavailableinsmallercasesizes.Thesearetempt-
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
generatesaloop“zero”at5kHzto50kHzthatisinstrumen-
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
highripplecurrentratingsandtoleranceofturn-onsurges.
Linear Technology plans to issue a Design Note on the use
of ceramic capacitors in the near future.
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
(RMS) = I
= I
1 – DC
RIPPLE
OUT
V
– V
IN
IN
OUT
OUT
Output Diode
V
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.
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 (effective series resistance) of
0.3Ω or less work well up to a full 1.5A switch current.
Higher ESR capacitors may be acceptable at low switch
currents. Input capacitor ripple current for boost con-
verter is:
Frequency Compensation
Loop frequency compensation is performed on the output
oftheerroramplifier(VC pin)withaseriesRC network.The
main pole is formed by the series capacitor and the output
impedance (≈1MΩ) of the error amplifier. The pole falls in
the range of 5Hz to 30Hz. The series resistor creates a
“zero”at2kHzto10kHz, whichimprovesloopstabilityand
transientresponse.Asecondcapacitor,typicallyonetenth
thesizeofthemaincompensationcapacitor,issometimes
used to reduce the switching frequency ripple on the VC
pin. VC pin ripple is caused by output voltage ripple
attenuatedbytheoutputdividerandmultipliedbytheerror
amplifier. Without the second capacitor, VC pin ripple is:
0.3(V )(V
– V )
IN
IN OUT
(f)(L)(V
I
=
RIPPLE
)
OUT
f = 250kHz switching frequency
Theinputcapacitorcanseeaveryhighsurgecurrentwhen
a battery or high capacitance source is connected “live”,
andsolidtantalumcapacitorscanfailunderthiscondition.
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-
torvoltageby2:1forhighsurgeapplications. Ceramicand
1.245(V
)(g )(R )
m C
OUT
RIPPLE
V
V Pin Ripple =
C
9
Linear [ Linear ]
