LT1374
APPLICATIONS INFORMATION
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:
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
dI
10
=
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
1374 F03
Figure 3. LT1374 Ripple Voltage Waveform
U
W
U
U
CATCH DIODE
The suggested catch diode (D1) is a 1N5821 Schottky, or
its Motorola equivalent, MBR330. It is rated at 3A average
forward current and 30V reverse voltage. Typical forward
voltage is 0.5V at 3A. 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
D
(
AVG
)
=
I
OUT
V
IN
−
V
OUT
V
IN
(
)
This formula will not yield values higher than 3A with
maximum load current of 4.25A unless the ratio of input to
output voltage exceeds 3.4:1. The only reason to consider
a larger diode is the worst-case condition of a high input
voltage and
overloaded
(not shorted) output. Under short-
circuit conditions, foldback current limit will reduce diode
current to less than 2.6A, but if the output is overloaded
and does not fall to less than 1/3 of nominal output voltage,
foldback will not take effect. With the overloaded condi-
tion, output current will increase to a typical value of 5.7A,
determined by peak switch current limit of 6A. With
V
IN
= 15V, V
OUT
= 4V (5V overloaded) and I
OUT
= 5.7A:
I
D
(
AVG
)
=
5.7 15
−
4
15
(
)
=
4.18A
This is safe for short periods of time, but it would be
prudent to check with the diode manufacturer if continu-
ous operation under these conditions must be tolerated.
BOOST PIN CONSIDERATIONS
For most applications, the boost components are a 0.27µF
capacitor and a FMMD914 diode. The anode is connected
to the regulated output voltage and this generates a
voltage across the boost capacitor nearly identical to the
regulated output. In certain applications, the anode may
instead be connected to the unregulated input voltage.
This could be necessary if the regulated output voltage is
very low (< 3V) or if the input voltage is less than 5V.
Efficiency is not affected by the capacitor value, but the
capacitor should have an ESR of less than 1Ω to ensure
1374fb
13
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