Datasheet

10
LT1956/LT1956-5
1956f
APPLICATIO S I FOR ATIO
WUUU
Peak-to-peak output ripple voltage is the sum of a triwave
(created by peak-to-peak ripple current (I
LP-P
) 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 LP P
=
()()
+
()
-
Σ
where:
ESR = equivalent series resistance of the output
capacitor
ESL = equivalent series inductance of the output
capacitor
dI/dt = slew rate of inductor ripple current = V
IN
/L
Peak-to-peak ripple current (I
LP-P
) through the inductor
and into the output capacitor is typically chosen to be
between 20% and 40% of the maximum load current. It is
approximated by:
I
VVV
VfL
LP P
OUT IN OUT
IN
-
=
()( )
()()()
–
Example: with V
IN
= 12V, V
OUT
= 5V, L = 15µH, ESR =
0.080Ω and ESL = 10nH, output ripple voltage can be
approximated as follows:
IA
dI
dt
V
mV
RIPPLE
LP-P
P-P
=
()
−
()
()
()( )
=
==
=
()()
+
()()
()
=+=
−
−
512 5
12 15 10 500 10
0 389
12
15 10
10 0 8
0 389 0 08 10 10 10 0 8
0 031 0 008 39
66
6
6
96
••
.
•
•.
.. • .
..
––
Σ
To reduce output ripple voltage further requires an in-
crease in the inductor value with the trade-off being a
physically larger inductor with the possibility of increased
component height and cost.
Ceramic Output Capacitor
An alternative way to further reduce output ripple voltage
is to reduce the ESR of the output capacitor by using a
ceramic capacitor. Although this reduction of ESR re-
moves a useful zero in the overall loop response, this zero
can be replaced by inserting a resistor (R
C
) in series with
the V
C
pin and the compensation capacitor C
C
. (See
Ceramic Capacitors in Applications Information.)
Peak Inductor Current and Fault Current
To ensure that the inductor will not saturate, the peak in-
ductor current should be calculated knowing the maximum
load current. An appropriate inductor should then be cho-
sen. In addition, a decision should be made whether or not
the inductor must withstand continuous fault conditions.
If maximum load current is 0.5A, for instance, a 0.5A
inductor may not survive a continuous 2A overload condi-
tion. Dead shorts will actually be more gentle on the
inductor because the LT1956 has frequency and current
limit foldback.
Peak inductor and switch current can be significantly
higher than output current, especially with smaller induc-
tors and lighter loads, so don’t omit this step. Powdered
Table 2
VENDOR/ VALUE I
DC(MAX)
DCR HEIGHT
PART NO. (
µ H) (Amps) (Ohms) (mm)
Coiltronics
UP1B-100 10 1.9 0.111 5.0
UP1B-220 22 1.2 0.254 5.0
UP2B-220 22 2.0 0.062 6.0
UP2B-330 33 1.7 0.092 6.0
UP1B-150 15 1.5 0.175 5.0
Coilcraft
D01813P-153HC 15 1.5 0.170 5.0
D01813P-103HC 10 1.9 0.111 5.0
D53316P-223 22 1.6 0.207 5.1
D53316P-333 33 1.4 0.334 5.1
LP025060B-682 6.8 1.3 0.165 1.65
Sumida
CDRH4D28-4R7 4.7 1.32 0.072 3.0
CDRH5D28-100 10 1.30 0.065 3.0
CDRH6D28-150 15 1.40 0.084 3.0
CDRH6D28-180 18 1.32 0.095 3.0
CDRH6D28-220 22 1.20 0.128 3.0
CDRH6D38-220 22 1.30 0.096 4.0