13
LT1576/LT1576-5
APPLICATIONS INFORMATION
WUU
U
V I ESR ESL
dI
dt
RIPPLE
=
()( )
+
()
P-P
Σ
Example: with V
IN
=10V, V
OUT
= 5V, L = 30µH, ESR = 0.1Ω,
ESL = 10nH:
IA
dI
dt
VA
mV
RIPPLE
P-P
P-P
=
()
−
()
()
=
==
=
()()
+
=+=
−
−
−
510 5
10 30 10 200 10
042
10
30 10
033 10
0 42 0 1 10 10 0 33 10
0 042 0 003 45
63
6
6
96
••
.
•
.•
.. • .•
..
Σ
Output Capacitor Ripple Current (RMS):
I
VVV
LfV
RIPPLE RMS
OUT IN OUT
IN
(
)
=
()
−
()
()()( )
029.
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
capacitors remain capacitive to beyond 300kHz and usu-
ally 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.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1576. 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
VVV
VLf
P
OUT IN OUT
IN
-P
=
()
−
()
()()()
For high frequency switchers, the sum of ripple current
slew rates may also be relevant and can be calculated
from:
Σ
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.
2µs/DIV 1576 F03
V
OUT
AT
I
OUT
= 1A
INDUCTOR
CURRENT
AT I
OUT
= 1A
V
OUT
AT
I
OUT
= 50mA
INDUCTOR
CURRENT
AT I
OUT
= 50mA
20mV/DIV
200mA/DIV
20mV/DIV
200mA/DIV
Figure 3. LT1576 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
regulator input voltage. Average forward current in normal
operation can be calculated from:
I
IVV
V
D AVG
OUT IN OUT
IN
(
)
=
−
()
