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LT1959 Datasheet(PDF) 12 Page - Linear Technology

Part No. LT1959
Description  4.5A, 500kHz Step-Down Switching Regulator
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT1959 Datasheet(HTML) 12 Page - Linear Technology

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12
LT1959
APPLICATIONS INFORMATION
Ceramic Capacitors
Higher value, lower cost ceramic capacitors are now
available in smaller case sizes. These are ideal for input
bypassing because of their high ripple rating and tolerance
to turn-on surges. As output capacitors, caution must be
used. Solid tantalum capacitor’s ESR generates a loop
“zero” at 5kHz to 50kHz that is beneficial in giving accept-
able loop phase margin. Ceramic capacitors remain ca-
pacitive to beyond 300kHz and usually resonate with their
ESL before ESR becomes effective. When using ceramic
output capacitors, the loop compensation pole frequency
must be reduced by a typical factor of 10.
OUTPUT RIPPLE VOLTAGE
Figure 3 shows a typical output ripple voltage waveform
for the LT1959. 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
VV
V
VL f
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.
V
I
ESR
ESL
dI
dt
RIPPLE =
()( )+()
P-P
Σ
Example: with VIN =10V, VOUT = 5V, L = 10µH, ESR = 0.1Ω,
ESL = 10nH:
IA
dI
dt
VA
mV
RIPPLE
P-P
P-P
=
()
()
()
=
==
=
()( )+
=+
=
510 5
10 10 10
500 10
05
10
10 10
10
05
01
10 10
10
0 05 0 01 60
63
6
6
96
••
.
..
..
Σ
VOUT AT IOUT = 1A
VOUT AT IOUT = 50mA
INDUCTOR CURRENT
AT IOUT = 1A
0.5
µs/DIV
1374 F03
INDUCTOR CURRENT
AT IOUT = 50mA
20mV/DIV
0.5A/DIV
Figure 3. LT1959 Ripple Voltage Waveform
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
IV
V
V
D AVG
OUT
IN
OUT
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


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