LT1507
U
W U U
APPLICATIONS INFORMATION
Table 2. Surface Mount Solid Tantalum Capacitor ESR
and Ripple Current
appropriate for input bypassing because of their high
ripple current ratings and tolerance of turn-on surges.
E CASE SIZE
AVX TPS, Sprague 593D
AVX TAJ
ESR (MAX Ω)
0.1 to 0.3
RIPPLE CURRENT (A)
0.7 to 1.1
0.4
OUTPUT RIPPLE VOLTAGE
0.7 to 0.9
D CASE SIZE
AVX TPS, Sprague 593D
AVX TAJ
Ripplevoltageisdeterminedbythehighfrequencyimped-
anceoftheoutputcapacitorandripplecurrentthroughthe
inductor. Ripple current is triangular (continuous mode)
with a peak-to-peak value of:
0.1 to 0.3
0.9 to 2.0
0.7 to 1.1
0.36 to 0.24
C CASE SIZE
AVX TPS
0.2 (Typ)
1.8 to 3.0
0.5 (Typ)
(V )(V − V )
OUT
AVX TAJ
0.22 to 0.17
OUT IN
I
=
P-P
(V )(L)(f)
IN
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 rugged-
ness 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.
Output ripple voltage is also triangular with peak-to-peak
amplitude of:
VRIPPLE = (IP–P)(ESR) (peak-to-peak)
Example:withVIN =5V, VOUT =3.3V, L=5µH, ESR=0.1Ω;
(3.3)(5− 3.3)
I
=
= 0.45
P-P
P-P
−6
3
5 5 10
500 10
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
triangularwithatypicalvalueof200mARMS.Theformula
to calculate this is:
V
= (0.45A)(0.1Ω) = 45mV
RIPPLE
P-P
MAXIMUM OUTPUT LOAD CURRENT
Maximum load current will be less than the 1.5A rating of
the LT1507, especially with lower inductor values. Induc-
tor ripple current must be taken into account as well as
reduced switch current at high duty cycles. Maximum
switch current rating (IP) of the LT1507 is 1.5A up to 50%
duty cycle (DC), decreasing to 1.35A at 80% duty cycle,
shown graphically in Typical Performance Characteristics
and as a formula below. Current rating decreases with
duty cycle because the LT1507 has internal slope com-
pensation to prevent current mode subharmonic switch-
ing. For more details on subharmonic oscillation read
Application Note 19. Peak guaranteed switch current (IP)
is found from:
Output Capacitor Ripple Current (RMS)
0.29(V )(V – V
)
OUT IN
OUT
I
(RMS) =
RIPPLE
(L)(f)(V )
IN
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 when ceramic is used for the
output capacitor. Solid tantalum capacitor ESR generates
a loop “zero” at 5kHz to 50kHz that is instrumental in
giving acceptable loop phase margin. Ceramic capacitors
remain capacitive to beyond 300kHz and usually resonate
with their ESL before ESR becomes effective. They are
V
V
OUT
I = 1.5A for
≤ 0.5
P
IN
V
V
0.5(V
)
OUT
OUT
I = 1.75A−
for
≥ 0.5
P
V
IN
IN
11
Linear [ Linear ]
