LT1375/ LT1376
U
W
U U
APPLICATIONS INFORMATION
discharge surges, such as when the regulator output is
dead shorted, do not harm the capacitors.
dI V
IN
Σ
=
dt
L
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 200mARMS. The formula
to calculate this is:
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.
Output Capacitor Ripple Current (RMS):
dI
V
= I
ESR + ESL Σ
0.29 V
V −V
IN OUT
(
P-P)(
) (
)
RIPPLE
(
OUT)(
)
dt
IRIPPLE RMS
=
(
)
L f V
( )( )(
)
IN
Example: withV =10V,VOUT =5V,L=10µH,ESR=0.1Ω,
IN
ESL = 10nH:
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’s ESR
generates aloop“zero”at5kHzto50kHzthatis 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
highripplecurrentratings andtoleranceofturn-onsurges.
For further information on ceramic and other capacitor
types please refer to Design Note 95.
5 10 − 5
( )(
)
IP-P
=
= 0.5A
10 10 •10−6 500 •103
( )
dI
10
Σ
=
= 106
10 •10−6
dt
V
= 0.5A 0.1 + 10 •10−9 106
(
)( )
RIPPLE
= 0.05 + 0.01= 60mV
P-P
VOUT AT IOUT = 1A
20mV/DIV
VOUT AT IOUT = 50mA
OUTPUT RIPPLE VOLTAGE
INDUCTOR CURRENT
AT IOUT = 1A
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:
0.5A/DIV
INDUCTOR CURRENT
AT IOUT = 50mA
0.5µs/DIV
1375/76 F03
Figure 3. LT1376 Ripple Voltage Waveform
V
V −V
IN OUT
(
OUT)(
)
IP-P
=
CATCH DIODE
The suggested catch diode (D1) is a 1N5818 Schottky, or
V L f
( )( )( )
IN
For high frequency switchers, the sum of ripple current its Motorola equivalent, MBR130. It is rated at 1A average
slew rates may also be relevant and can be calculated forward current and 30V reverse voltage. Typical forward
from:
voltage is 0.42V at 1A. The diode conducts current only
during switch off time. Peak reverse voltage is equal to
13
Linear [ Linear ]
