Datasheet

13
LT3437
3437fc
is high enough at the switching frequency, output ripple
voltage (although smaller for a ceramic output capacitor)
may still affect the proper operation of the regulator. A
filter capacitor C
F
in parallel with the R
C
/C
C
network, along
with a small feedforward capacitor C
FB
, is suggested to
control possible ripple at the V
C
pin. The LT3437 can be
stabilized using a 100µF ceramic output capacitor and V
C
component values of C
C
= 1500pF, R
C
= 25k, C
F
= 330pF
and C
FB
=27pF.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT3437. Ripple voltage is determined by the
impedance of the output capacitor and ripple current
through the inductor. Peak-to-peak ripple current through
the inductor into the output capacitor is:
I
VVV
VLf
OUT IN OUT
IN
P-P
=
()
()()()
–
For high frequency switchers the ripple current slew rate
is also relevant and can be calculated from:
di
dt
V
L
IN
=
Peak-to-peak output ripple voltage is the sum of a triwave
created by peak-to-peak ripple current times ESR and a
square wave created by parasitic inductance (ESL) and
ripple current slew rate. Capacitive reactance is assumed
to be small compared to ESR or ESL.
V I ESR ESL
di
dt
RIPPLE
=
()( )
+
()
P-P
Example: with V
IN
= 12V, V
OUT
= 3.3V, L = 100µH, ESR =
0.075Ω, ESL = 10nH:
I
P-P
=
()( )
()
−
()()
=
==
33 12 33
12 100 6 200 3
0 120
12
100 6
012 6
.–.
.
–
.
ee
A
di
dt e
e
V
RIPPLE
= (0.120A)(0.075) + (10e – 9)(0.12e6)
= 0.009 + 0.0012 = 10.2mV
P-P
voltage could be unacceptable. Table 2 shows some
typical solid tantalum surface mount capacitors.
Table 2. 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
triangular with a typical value of 30mA
RMS
. The formula to
calculate this is:
Output capacitor ripple current (RMS)
I
VVV
LfV
RIPPLE RMS
OUT IN OUT
IN
()
.–
=
()( )
()()( )
=
029
12
I
P-P
CERAMIC CAPACITORS
Higher value, lower cost ceramic capacitors are now
becoming available. They are generally chosen for their
good high frequency operation, small size and very low
ESR (effective series resistance). Low ESR reduces output
ripple voltage but also removes a useful zero in the loop
frequency response, common to tantalum capacitors. To
compensate for this, a resistor R
C
can be placed in series
with the V
C
compensation capacitor C
C
(Figure 10). Care
must be taken, however, since this resistor sets the high
frequency gain of the error amplifier, including the gain at
the switching frequency. If the gain of the error amplifier
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