LT1375/LT1376
APPLICATIONS INFORMATION
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 200mA
RMS
. The formula
to calculate this is:
Output Capacitor Ripple Current (RMS):
I
RIPPLE
(
RMS
)
=
0
.
29 V
OUT
V
IN
−
V
OUT
IN
( )(
(
L
)(
f
)(
V
)
)
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’s 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.
For further information on ceramic and other capacitor
types please refer to Design Note 95.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1376. Ripple voltage is determined by the high
frequency 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
P-P
(
V
)(
V
−
V
)
=
(
V
)(
L
)(
f
)
OUT
IN
OUT
IN
For high frequency switchers, the sum of ripple current
slew rates may also be relevant and can be calculated
from:
U
W
U
U
Σ
dI V
IN
=
dt L
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
RIPPLE
=
I
P-P
ESR
+
ESL
Σ
( )( ) ( )
(
5
)(
10
−
5
)
dI
dt
Example: with V
IN
=10V, V
OUT
= 5V, L = 10µH, ESR = 0.1Ω,
ESL = 10nH:
I
P-P
=
=
0.5A
10 • 10
−
6
500 • 10
3
10
10
dI
=
10
6
Σ =
dt 10 • 10
−
6
V
RIPPLE
=
0.5A 0.1
+
10 • 10
−
9
10
6
=
0.05
+
0.01
=
60mV
P-P
( )
( )( )
V
OUT
AT I
OUT
= 1A
20mV/DIV
V
OUT
AT I
OUT
= 50mA
INDUCTOR CURRENT
AT I
OUT
= 1A
0.5A/DIV
INDUCTOR CURRENT
AT I
OUT
= 50mA
0.5µs/DIV
1375/76 F03
Figure 3. LT1376 Ripple Voltage Waveform
CATCH DIODE
The suggested catch 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 is equal to
13
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